Web Authorization Protocol (oauth) T. Lodderstedt, Ed.
Internet-Draft Deutsche Telekom AG
Intended status: Informational M. McGloin
Expires: August 22, 2012 IBM
P. Hunt
Oracle Corporation
February 19, 2012
OAuth 2.0 Threat Model and Security Considerations
draft-ietf-oauth-v2-threatmodel-02
Abstract
This document gives security considerations based on a comprehensive
threat model for the OAuth 2.0 Protocol.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 22, 2012.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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publication of this document. Please review these documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 6
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2. Attack Assumptions . . . . . . . . . . . . . . . . . . . . 7
2.3. Architectural assumptions . . . . . . . . . . . . . . . . 7
2.3.1. Authorization Servers . . . . . . . . . . . . . . . . 8
2.3.2. Resource Server . . . . . . . . . . . . . . . . . . . 8
2.3.3. Client . . . . . . . . . . . . . . . . . . . . . . . . 8
3. Security Features . . . . . . . . . . . . . . . . . . . . . . 9
3.1. Tokens . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1.2. Expires_In . . . . . . . . . . . . . . . . . . . . . . 10
3.2. Access Token . . . . . . . . . . . . . . . . . . . . . . . 11
3.3. Refresh Token . . . . . . . . . . . . . . . . . . . . . . 11
3.4. Authorization Code . . . . . . . . . . . . . . . . . . . . 12
3.5. Redirection URI . . . . . . . . . . . . . . . . . . . . . 12
3.6. State parameter . . . . . . . . . . . . . . . . . . . . . 12
3.7. Client Identity . . . . . . . . . . . . . . . . . . . . . 12
4. Security Threat Model . . . . . . . . . . . . . . . . . . . . 14
4.1. Clients . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.1.1. Threat: Obtain Client Secrets . . . . . . . . . . . . 15
4.1.2. Threat: Obtain Refresh Tokens . . . . . . . . . . . . 16
4.1.3. Threat: Obtain Access Tokens . . . . . . . . . . . . . 18
4.1.4. Threat: End-user credentials phished using
compromised or embedded browser . . . . . . . . . . . 18
4.1.5. Threat: Open Redirectors on client . . . . . . . . . . 19
4.2. Authorization Endpoint . . . . . . . . . . . . . . . . . . 19
4.2.1. Threat: Password phishing by counterfeit
authorization server . . . . . . . . . . . . . . . . . 20
4.2.2. Threat: User unintentionally grants too much
access scope . . . . . . . . . . . . . . . . . . . . . 20
4.2.3. Threat: Malicious client obtains existing
authorization by fraud . . . . . . . . . . . . . . . . 21
4.2.4. Threat: Open redirector . . . . . . . . . . . . . . . 21
4.3. Token endpoint . . . . . . . . . . . . . . . . . . . . . . 21
4.3.1. Threat: Eavesdropping access tokens . . . . . . . . . 22
4.3.2. Threat: Obtain access tokens from authorization
server database . . . . . . . . . . . . . . . . . . . 22
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4.3.3. Threat: Obtain client credentials over non secure
transport . . . . . . . . . . . . . . . . . . . . . . 22
4.3.4. Threat: Obtain client secret from authorization
server database . . . . . . . . . . . . . . . . . . . 23
4.3.5. Threat: Obtain client secret by online guessing . . . 23
4.3.6. Threat: DoS on dynamic client secret creation . . . . 23
4.4. Obtaining Authorization . . . . . . . . . . . . . . . . . 23
4.4.1. Authorization Code . . . . . . . . . . . . . . . . . . 24
4.4.1.1. Threat: Eavesdropping or leaking authorization
codes . . . . . . . . . . . . . . . . . . . . . . 24
4.4.1.2. Threat: Obtain authorization codes from
authorization server database . . . . . . . . . . 25
4.4.1.3. Threat: Online guessing of authorization codes . . 25
4.4.1.4. Threat: Malicious client obtains authorization . . 26
4.4.1.5. Threat: Authorization code phishing . . . . . . . 27
4.4.1.6. Threat: User session impersonation . . . . . . . . 28
4.4.1.7. Threat: Authorization code leakage through
counterfeit client . . . . . . . . . . . . . . . . 28
4.4.1.8. Threat: CSRF attack against redirect-uri . . . . . 30
4.4.1.9. Threat: Clickjacking attack against
authorization . . . . . . . . . . . . . . . . . . 31
4.4.1.10. Threat: Resource Owner Impersonation . . . . . . . 31
4.4.1.11. Threat: DoS, Exhaustion of resources attacks . . . 33
4.4.1.12. Threat: DoS using manufactured authorization
codes . . . . . . . . . . . . . . . . . . . . . . 33
4.4.2. Implicit Grant . . . . . . . . . . . . . . . . . . . . 35
4.4.2.1. Threat: Access token leak in
transport/end-points . . . . . . . . . . . . . . . 35
4.4.2.2. Threat: Access token leak in browser history . . . 35
4.4.2.3. Threat: Malicious client obtains authorization . . 35
4.4.2.4. Threat: Manipulation of scripts . . . . . . . . . 36
4.4.2.5. Threat: CSRF attack against redirect-uri . . . . . 36
4.4.3. Resource Owner Password Credentials . . . . . . . . . 37
4.4.3.1. Threat: Accidental exposure of passwords at
client site . . . . . . . . . . . . . . . . . . . 38
4.4.3.2. Threat: Client obtains scopes without end-user
authorization . . . . . . . . . . . . . . . . . . 38
4.4.3.3. Threat: Client obtains refresh token through
automatic authorization . . . . . . . . . . . . . 38
4.4.3.4. Threat: Obtain user passwords on transport . . . . 39
4.4.3.5. Threat: Obtain user passwords from
authorization server database . . . . . . . . . . 39
4.4.3.6. Threat: Online guessing . . . . . . . . . . . . . 40
4.4.4. Client Credentials . . . . . . . . . . . . . . . . . . 40
4.5. Refreshing an Access Token . . . . . . . . . . . . . . . . 40
4.5.1. Threat: Eavesdropping refresh tokens from
authorization server . . . . . . . . . . . . . . . . . 40
4.5.2. Threat: Obtaining refresh token from authorization
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server database . . . . . . . . . . . . . . . . . . . 41
4.5.3. Threat: Obtain refresh token by online guessing . . . 41
4.5.4. Threat: Obtain refresh token phishing by
counterfeit authorization server . . . . . . . . . . . 41
4.6. Accessing Protected Resources . . . . . . . . . . . . . . 42
4.6.1. Threat: Eavesdropping access tokens on transport . . . 42
4.6.2. Threat: Replay authorized resource server requests . . 42
4.6.3. Threat: Guessing access tokens . . . . . . . . . . . . 42
4.6.4. Threat: Access token phishing by counterfeit
resource server . . . . . . . . . . . . . . . . . . . 43
4.6.5. Threat: Abuse of token by legitimate resource
server or client . . . . . . . . . . . . . . . . . . . 44
4.6.6. Threat: Leak of confidential data in HTTP-Proxies . . 44
4.6.7. Threat: Token leakage via logfiles and HTTP
referrers . . . . . . . . . . . . . . . . . . . . . . 44
5. Security Considerations . . . . . . . . . . . . . . . . . . . 45
5.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 45
5.1.1. Confidentiality of Requests . . . . . . . . . . . . . 45
5.1.2. Server authentication . . . . . . . . . . . . . . . . 46
5.1.3. Always keep the resource owner informed . . . . . . . 46
5.1.4. Credentials . . . . . . . . . . . . . . . . . . . . . 46
5.1.4.1. Credential Storage Protection . . . . . . . . . . 47
5.1.4.2. Online attacks on secrets . . . . . . . . . . . . 48
5.1.5. Tokens (access, refresh, code) . . . . . . . . . . . . 49
5.1.5.1. Limit token scope . . . . . . . . . . . . . . . . 49
5.1.5.2. Expiration time . . . . . . . . . . . . . . . . . 49
5.1.5.3. Short expiration time . . . . . . . . . . . . . . 49
5.1.5.4. Limit number of usages/ One time usage . . . . . . 50
5.1.5.5. Bind tokens to a particular resource server
(Audience) . . . . . . . . . . . . . . . . . . . . 50
5.1.5.6. Use endpoint address as token audience . . . . . . 51
5.1.5.7. Audience and Token scopes . . . . . . . . . . . . 51
5.1.5.8. Bind token to client id . . . . . . . . . . . . . 51
5.1.5.9. Signed tokens . . . . . . . . . . . . . . . . . . 51
5.1.5.10. Encryption of token content . . . . . . . . . . . 51
5.1.5.11. Random token value with high entropy . . . . . . . 51
5.1.5.12. Assertion formats . . . . . . . . . . . . . . . . 52
5.1.6. Access tokens . . . . . . . . . . . . . . . . . . . . 52
5.2. Authorization Server . . . . . . . . . . . . . . . . . . . 52
5.2.1. Authorization Codes . . . . . . . . . . . . . . . . . 52
5.2.1.1. Automatic revocation of derived tokens if
abuse is detected . . . . . . . . . . . . . . . . 52
5.2.2. Refresh tokens . . . . . . . . . . . . . . . . . . . . 52
5.2.2.1. Restricted issuance of refresh tokens . . . . . . 52
5.2.2.2. Binding of refresh token to client_id . . . . . . 53
5.2.2.3. Refresh Token Rotation . . . . . . . . . . . . . . 53
5.2.2.4. Refresh Token Revocation . . . . . . . . . . . . . 53
5.2.2.5. Device identification . . . . . . . . . . . . . . 54
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5.2.2.6. X-FRAME-OPTION header . . . . . . . . . . . . . . 54
5.2.3. Client authentication and authorization . . . . . . . 54
5.2.3.1. Don't issue secrets to public clients or
clients with inappropriate security policy . . . . 55
5.2.3.2. Public clients without secret require user
consent . . . . . . . . . . . . . . . . . . . . . 55
5.2.3.3. Client_id only in combination with redirect_uri . 55
5.2.3.4. Deployment-specific client secrets . . . . . . . . 56
5.2.3.5. Validation of pre-registered redirect_uri . . . . 56
5.2.3.6. Client secret revocation . . . . . . . . . . . . . 57
5.2.3.7. Use strong client authentication (e.g.
client_assertion / client_token) . . . . . . . . . 57
5.2.4. End-user authorization . . . . . . . . . . . . . . . . 58
5.2.4.1. Automatic processing of repeated
authorizations requires client validation . . . . 58
5.2.4.2. Informed decisions based on transparency . . . . . 58
5.2.4.3. Validation of client properties by end-user . . . 58
5.2.4.4. Binding of authorization code to client_id . . . . 59
5.2.4.5. Binding of authorization code to redirect_uri . . 59
5.3. Client App Security . . . . . . . . . . . . . . . . . . . 59
5.3.1. Don't store credentials in code or resources
bundled with software packages . . . . . . . . . . . . 59
5.3.2. Standard web server protection measures (for
config files and databases) . . . . . . . . . . . . . 60
5.3.3. Store secrets in a secure storage . . . . . . . . . . 60
5.3.4. Utilize device lock to prevent unauthorized device
access . . . . . . . . . . . . . . . . . . . . . . . . 60
5.3.5. Platform security measures . . . . . . . . . . . . . . 60
5.3.6. Link state parameter to user agent session . . . . . . 60
5.4. Resource Servers . . . . . . . . . . . . . . . . . . . . . 61
5.4.1. Authorization headers . . . . . . . . . . . . . . . . 61
5.4.2. Authenticated requests . . . . . . . . . . . . . . . . 61
5.4.3. Signed requests . . . . . . . . . . . . . . . . . . . 62
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 62
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 62
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 62
8.1. Normative References . . . . . . . . . . . . . . . . . . . 62
8.2. Informative References . . . . . . . . . . . . . . . . . . 63
Appendix A. Document History . . . . . . . . . . . . . . . . . . 63
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 65
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1. Introduction
This document gives security considerations based on a comprehensive
threat model for the OAuth 2.0 Protocol [I-D.ietf-oauth-v2]. It
contains the following content:
o Documents any assumptions and scope considered when creating the
threat model.
o Describes the security features in-built into the OAuth protocol
and how they are intended to thwart attacks.
o Gives a comprehensive threat model for OAuth and describes the
respective counter measures to thwart those threats.
Threats include any intentional attacks on OAuth tokens and resources
protected by OAuth tokens as well as security risks introduced if the
proper security measures are not put in place. Threats are
structured along the lines of the protocol structure to aid
development teams implement each part of the protocol securely. For
example all threats for granting access or all threats for a
particular grant type or all threats for protecting the resource
server.
2. Overview
2.1. Scope
The security considerations document only considers clients bound to
a particular deployment as supported by [I-D.ietf-oauth-v2]. Such
deployments have the following characteristics:
o Resource server URLs are static and well-known at development
time, authorization server URLs can be static or discovered.
o Token scope values (e.g. applicable URLs and methods) are well-
known at development time.
o Client registration: Since registration of clients is out of scope
of the current core spec, this document assumes a broad variety of
options from static registration during development time to
dynamic registration at runtime.
The following are considered out of scope :
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o Communication between authorization server and resource server
o Token formats
o Except for "Resource Owner Password Credentials" (see
[I-D.ietf-oauth-v2], section 4.3), the mechanism used by
authorization servers to authenticate the user
o Mechanism by which a user obtained an assertion and any resulting
attacks mounted as a result of the assertion being false.
o Clients not bound to a specific deployment: An example could be a
mail client with support for contact list access via the portable
contacts API (see [portable-contacts]). Such clients cannot be
registered upfront with a particular deployment and should
dynamically discover the URLs relevant for the OAuth protocol.
2.2. Attack Assumptions
The following assumptions relate to an attacker and resources
available to an attacker:
o It is assumed the attacker has full access to the network between
the client and authorization servers and the client and the
resource server, respectively. The attacker may eavesdrop on any
communications between those parties. He is not assumed to have
access to communication between authorization and resource server.
o It is assumed an attacker has unlimited resources to mount an
attack.
o It is assumed that 2 of the 3 parties involved in the OAuth
protocol may collude to mount an attack against the 3rd party.
For example, the client and authorization server may be under
control of an attacker and collude to trick a user to gain access
to resources.
2.3. Architectural assumptions
This section documents the assumptions about the features,
limitations, and design options of the different entities of a OAuth
deployment along with the security-sensitive data-elements managed by
those entity. These assumptions are the foundation of the threat
analysis.
The OAuth protocol leaves deployments with a certain degree of
freedom how to implement and apply the standard. The core
specification defines the core concepts of an authorization server
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and a resource server. Both servers can be implemented in the same
server entity, or they may also be different entities. The later is
typically the case for multi-service providers with a single
authentication and authorization system, and are more typical in
middleware architectures.
2.3.1. Authorization Servers
The following data elements are stored or accessible on the
authorization server:
o user names and passwords
o client ids and secrets
o client-specific refresh tokens
o client-specific access tokens (in case of handle-based design)
o HTTPS certificate/key
o per-authorization process (in case of handle-based design):
redirect_uri, client_id, authorization code
2.3.2. Resource Server
The following data elements are stored or accessible on the resource
server:
o user data (out of scope)
o HTTPS certificate/key
o authorization server credentials (handle-based design), or
o authorization server shared secret/public key (assertion-based
design)
o access tokens (per request)
It is assumed that a resource server has no knowledge of refresh
tokens, user passwords, or client secrets.
2.3.3. Client
A full definition of different client types and profiles is given in
[I-D.ietf-oauth-v2], Section 2.1.
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The following data elements are stored or accessible on the client:
o client id (and client secret or corresponding client credential)
o one or more refresh tokens (persistent) and access tokens
(transient) per end-user or other security-context or delegation
context
o trusted CA certificates (HTTPS)
o per-authorization process: redirect_uri, authorization code
3. Security Features
These are some of the security features which have been built into
the OAuth 2.0 protocol to mitigate attacks and security issues.
3.1. Tokens
OAuth makes extensive use many kinds of tokens (access tokens,
refresh tokens, authorization codes). The information content of a
token can be represented in two ways as follows:
Handle (or artifact) a reference to some internal data structure
within the authorization server; the internal data structure
contains the attributes of the token, such as user id, scope, etc.
Handles enable simple revocation and do not require cryptographic
mechanisms to protect token content from being modified. On the
other hand, handles require communication between issuing and
consuming entity (e.g. authorization and resource server) in order
to validate the token and obtain token-bound data. This
communication might have an negative impact on performance and
scalability if both entities reside on different systems. Handles
are therefore typically used if the issuing and consuming entity
are the same. A 'handle' token is often referred to as an
'opaque' token because the resource server does not need to be
able to interpret the token directly, it simply uses the token.
Assertions (aka self-contained token) a parseable token. An
assertion typically has a duration, an audience, and is digitally
signed containing information about the user and the client.
Examples of assertion formats are SAML assertions and Kerberos
tickets. Assertions can typically directly be validated and used
by a resource server without interactions with the authorization
server. This results in better performance and scalability in
deployment where issuing and consuming entity reside on different
systems. Implementing token revocation is more difficult with
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assertions than with handles.
Tokens can be used in two ways to invoke requests on resource servers
as follows:
bearer token A 'bearer token' is a token that can be used by any
client who has received the token (e.g.
[I-D.ietf-oauth-v2-bearer]). Because mere possession is enough to
use the token it is important that communication between end-
points be secured to ensure that only authorized end-points may
capture the token. The bearer token is convenient to client
applications as it does not require them to do anything to use
them (such as a proof of identity). Bearer tokens have similar
characteristics to web single-sign-on (SSO) cookies used in
browsers.
proof token A 'proof token' is a token that can only be used by a
specific client. Each use of the token, requires the client to
perform some action that proves that it is the authorized user of
the token. Examples of this are MAC tokens, which require the
client to digitally sign the resource request with a secret
corresponding to the particular token send with the request
(e.g.[I-D.ietf-oauth-v2-http-mac]).
3.1.1. Scope
A Scope represents the access authorization associated with a
particular token with respect to resource servers, resources and
methods on those resources. Scopes are the OAuth way to explicitly
manage the power associated with an access token. A scope can be
controlled by the authorization server and/or the end-user in order
to limit access to resources for OAuth clients these parties deem
less secure or trustworthy. Optionally, the client can request the
scope to apply to the token but only for lesser scope than would
otherwise be granted, e.g. to reduce the potential impact if this
token is sent over non secure channels. A scope is typically
complemented by a restriction on a token's lifetime.
3.1.2. Expires_In
Expires_In allows an authorization server (based on its policies or
on behalf of the end-user) to limit the lifetime of the access token.
This mechanisms can be used to issue short-living tokens to OAuth
clients the authorization server deems less secure or where sending
tokens over non secure channels.
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3.2. Access Token
An access token is used by a client to access a resource. Access
tokens typically have short life-spans (minutes or hours) that cover
typical session lifetimes. An access token may be refreshed through
the use of a refresh token. The short lifespan of an access token in
combination with the usage of refresh tokens enables the possibility
of passive revocation of access authorization on the expiry of the
current access token.
3.3. Refresh Token
A refresh token represents a long-lasting authorization of a certain
client to access resources on behalf of a resource owner. Such
tokens are exchanged between client and authorization server, only.
Clients use this kind of token to obtain ("refresh") new access
tokens used for resource server invocations.
A refresh token, coupled with a short access token lifetime, can be
used to grant longer access to resources without involving end user
authorization. This offers an advantage where resource servers and
authorization servers are not the same entity, e.g. in a distributed
environment, as the refresh token is always exchanged at the
authorization server. The authorization server can revoke the
refresh token at any time causing the granted access to be revoked
once the current access token expires. Because of this, a short
access token lifetime is important if timely revocation is a high
priority.
The refresh token is also a secret bound to the client identifier and
client instance which originally requested the authorization and
representing the original resource owner grant. This is ensured by
the authorization process as follows:
1. The resource owner and user-agent safely deliver the
authorization code to the client instance in first place.
2. The client uses it immediately in secure transport-level
communications to the authorization server and then securely
stores the long-lived refresh token.
3. The client always uses the refresh token in secure transport-
level communications to the authorization server to get an access
token (and optionally rollover the refresh token).
So as long as the confidentiality of the particular token can be
ensured by the client, a refresh tokens can also be used as an
alternative mean to authenticate the client instance itself.
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3.4. Authorization Code
An Authorization Code represents the intermediate result of a
successful end-user authorization process and is used by the client
to obtain access and refresh token. Authorization codes are sent to
the client's redirection URI instead of tokens for two purposes.
1. Instead of (longer-lasting) tokens, the short-lived authorization
code is exposed to potential attackers via URI query parameters
(HTTP referrer), browser cache, or log file entries.
2. It is much simpler to authenticate clients during the direct
request between client and authorization server than in the
context of the indirect authorization request. The later would
require digital signatures.
3.5. Redirection URI
A redirection URI helps to detect malicious clients and prevents
phishing attacks from clients attempting to trick the user into
believing the phisher is the client. The value of the actual
redirection URI used in the authorization request has to be presented
and is verified when an authorization code is exchanged for tokens.
This helps to prevent attacks, where the authorization code is
revealed through redirectors and counterfeit web application clients.
The authorization server should require public clients and
confidential clients using implicit grant type to pre-register their
redirect URIs and validate against the registered redirection URI in
the authorization request.
3.6. State parameter
The state parameter is used to link requests and callbacks to prevent
CSRF attacks where an attacker authorizes access to his own resources
and then tricks a users into following a redirect with the attacker's
token. This parameter should bind to the authenticated state in a
user agent and, as per the core OAuth spec, the user agent must be
capable of keeping it in a location accessible only by the client and
user agent, i.e. protected by same-origin policy
3.7. Client Identity
Authentication protocols have typically not taken into account the
identity of the software component acting on behalf of the end-user.
OAuth does this in order to increase the security level in delegated
authorization scenarios and because the client will be able to act
without the user being present.
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OAuth uses the client identifier to collate associated request to the
same originator, such as
o a particular end-user authorization process and the corresponding
request on the token's endpoint to exchange the authorization code
for tokens or
o the initial authorization and issuance of a token by an end-user
to a particular client, and subsequent requests by this client to
obtain tokens without user consent (automatic processing of
repeated authorization)
The client identity may also be used by the authorization server to
display relevant registration information to a user when requesting
consent for scope requested by a particular client. The client
identity may be used to limit the number of request for a particular
client or to charge the client per request. Client Identity may
furthermore be useful to differentiate access by different clients,
e.g. in server log files.
OAuth defines two client types, confidential and public, based on
their ability to authenticate with the authorization server (i.e.
ability to maintain the confidentiality of their client credentials).
Confidential clients are capable of maintaining the confidentiality
of client credentials (i.e. a client secret associated with the
client identifier) or capable of secure client authentication using
other means, such as a client assertion (e.g. SAML) or key
cryptography. The latter is considered more secure.
The authorization server should determine whether the client is
capable of keeping its secret confidential or using secure
authentication. Alternatively, the end-user can verify the identity
of the client, e.g. by only installing trusted applications.The
redicrection URI can be used to prevent delivering credentials to a
counterfeit client after obtaining end-user authorization in some
cases, but can't be used to verify the client identity.
Clients can be categorized as follows based on the client type,
profile (e.g. native vs web application) and deployment model:
Deployment-independent client_id with pre-registered redirect_uri and
without client_secret Such an identity is used by multiple
installations of the same software package. The identity of such
a client can only be validated with the help of the end-user.
This is a viable option for native applications in order to
identify the client for the purpose of displaying meta information
about the client to the user and to differentiate clients in log
files. Revocation of such an identity will affect ALL deployments
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of the respective software.
Deployment-independent client_id with pre-registered redirect_uri and
with client_secret This is an option for native applications only,
since web application would require different redirect URIs. This
category is not advisable because the client secret cannot be
protected appropriately (see Section 4.1.1). Due to its security
weaknesses, such client identities have the same trust level as
deployment-independent clients without secret. Revocation will
affect ALL deployments.
Deployment-specific client_id with pre-registered redirect_uri and
with client_secret The client registration process ensures the
validation of the client's properties, such as redirection URI,
website address, web site name, contacts. Such a client identity
can be utilized for all relevant use cases cited above. This
level can be achieved for web applications in combination with a
manual or user-bound registration process. Achieving this level
for native applications is much more difficult. Either the
installation of the application is conducted by an administrator,
who validates the client's authenticity, or the process from
validating the application to the installation of the application
on the device and the creation of the client credentials is
controlled end-to-end by a single entity (e.g. application market
provider). Revocation will affect a single deployment only.
Deployment-specific client_id with client_secret without validated
properties Such a client can be recognized by the authorization
server in transactions with subsequent requests (e.g.
authorization and token issuance, refresh token issuance and
access token refreshment). The authorization server cannot assure
any property of the client to end-users. Automatic processing of
re-authorizations could be allowed as well. Such client
credentials can be generated automatically without any validation
of client properties, which makes it another option especially for
native applications. Revocation will affect a single deployment
only.
4. Security Threat Model
This section gives a comprehensive threat model of OAuth 2.0.
Threats are grouped first by attacks directed against an OAuth
component, which are client, authorization server, and resource
server. Subsequently, they are grouped by flow, e.g. obtain token or
access protected resources. Every countermeasure description refers
to a detailed description in Section 5.
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4.1. Clients
This section describes possible threats directed to OAuth clients.
4.1.1. Threat: Obtain Client Secrets
The attacker could try to get access to the secret of a particular
client in order to:
o replay its refresh tokens and authorization codes, or
o obtain tokens on behalf of the attacked client with the privileges
of that client.
The resulting impact would be:
o Client authentication of access to authorization server can be
bypassed
o Stolen refresh tokens or authorization codes can be replayed
Depending on the client category, the following attacks could be
utilized to obtain the client secret.
Attack: Obtain Secret From Source Code or Binary:
This applies for all client types. For open source projects, secrets
can be extracted directly from source code in their public
repositories. Secrets can be extracted from application binaries
just as easily when published source is not available to the
attacker. Even if an application takes significant measures to
obfuscate secrets in their application distribution one should
consider that the secret can still be reverse-engineered by anyone
with access to a complete functioning application bundle or binary.
Countermeasures:
o Don't issue secrets to public clients or clients with
inappropriate security policy - Section 5.2.3.1
o Public clients require user consent - Section 5.2.3.2
o Use deployment-specific client secrets - Section 5.2.3.4
o Client secret revocation - Section 5.2.3.6
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Attack: Obtain a Deployment-Specific Secret:
An attacker may try to obtain the secret from a client installation,
either from a web site (web server) or a particular devices (native
application).
Countermeasures:
o Web server: apply standard web server protection measures (for
config files and databases) - Section 5.3.2
o Native applications: Store secrets in a secure local storage -
Section 5.3.3
o Client secret revocation - Section 5.2.3.6
4.1.2. Threat: Obtain Refresh Tokens
Depending on the client type, there are different ways refresh tokens
may be revealed to an attacker. The following sub-sections give a
more detailed description of the different attacks with respect to
different client types and further specialized countermeasures.
Before detailing those threats, here are some generally applicable
countermeasures:
o The authorization server should validate the client id associated
with the particular refresh token with every refresh request-
Section 5.2.2.2
o Limited scope tokens - Section 5.1.5.1
o Refresh token revocation - Section 5.2.2.4
o Client secret revocation - Section 5.2.3.6
o Refresh tokens can automatically be replaced in order to detect
unauthorized token usage by another party (Refresh Token Rotation)
- Section 5.2.2.3
Attack: Obtain Refresh Token from Web application:
An attacker may obtain the refresh tokens issued to a web application
by way of overcoming the web server's security controls. Impact:
Since a web application manages the user accounts of a certain site,
such an attack would result in an exposure of all refresh tokens on
that side to the attacker.
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Countermeasures:
o Standard web server protection measures - Section 5.3.2
o Use strong client authentication (e.g. client_assertion /
client_token), so the attacker cannot obtain the client secret
required to exchange the tokens - Section 5.2.3.7
Attack: Obtain Refresh Token from Native clients:
On native clients, leakage of a refresh token typically affects a
single user, only.
Read from local file system: The attacker could try get file system
access on the device and read the refresh tokens. The attacker could
utilize a malicious application for that purpose.
Countermeasures:
o Store secrets in a secure storage - Section 5.3.3
o Utilize device lock to prevent unauthorized device access -
Section 5.3.4
Attack: Steal device:
The host device (e.g. mobile phone) may be stolen. In that case, the
attacker gets access to all applications under the identity of the
legitimate user.
Countermeasures:
o Utilize device lock to prevent unauthorized device access -
Section 5.3.4
o Where a user knows the device has been stolen, they can revoke the
affected tokens - Section 5.2.2.4
Attack: Clone Device:
All device data and applications are copied to another device.
Applications are used as-is on the target device.
Countermeasures:
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o Utilize device lock to prevent unauthorized device access -
Section 5.3.4
o Combine refresh token request with device identification -
Section 5.2.2.5
o Refresh Token Rotation - Section 5.2.2.3
o Where a user knows the device has been cloned, they can use this
countermeasure - Refresh Token Revocation - Section 5.2.2.4
4.1.3. Threat: Obtain Access Tokens
Depending on the client type, there are different ways access tokens
may be revealed to an attacker. Access tokens could be stolen from
the device if the application stores them in a storage, which is
accessible to other applications.
Impact: Where the token is a bearer token and no additional mechanism
is used to identify the client, the attacker can access all resources
associated with the token and its scope.
Countermeasures:
o Keep access tokens in transient memory and limit grants:
Section 5.1.6
o Limited scope tokens - Section 5.1.5.1
o Keep access tokens in private memory or apply same protection
means as for refresh tokens - Section 5.2.2
o Keep access token lifetime short - Section 5.1.5.3
4.1.4. Threat: End-user credentials phished using compromised or
embedded browser
A malicious application could attempt to phish end-user passwords by
misusing an embedded browser in the end-user authorization process,
or by presenting its own user-interface instead of allowing trusted
system browser to render the authorization user interface. By doing
so, the usual visual trust mechanisms may be bypassed (e.g. TLS
confirmation, web site mechanisms). By using an embedded or internal
client application user interface, the client application has access
to additional information it should not have access to (e.g. uid/
password).
Impact: If the client application or the communication is
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compromised, the user would not be aware and all information in the
authorization exchange could be captured such as username and
password.
Countermeasures:
o The OAuth flow is designed so that client applications never need
to know user passwords. Client applications should avoid directly
asking users for the their credentials. In addition, end users
could be educated about phishing attacks and best practices, such
as only accessing trusted clients, as OAuth does not provide any
protection against malicious applications and the end user is
solely responsible for the trustworthiness of any native
application installed.
o Client applications could be validated prior to publication in an
application market for users to access. That validation is out of
scope for OAuth but could include validating that the client
application handles user authentication in an appropriate way.
o Client developers should not write client applications that
collect authentication information directly from users and should
instead delegate this task to a trusted system component, e.g. the
system-browser.
4.1.5. Threat: Open Redirectors on client
An open redirector is an endpoint using a parameter to automatically
redirect a user-agent to the location specified by the parameter
value without any validation. If the authorization server allows the
client to register only part of the redirection URI, an attacker can
use an open redirector operated by the client to construct a
redirection URI that will pass the authorization server validation
but will send the authorization code or access token to an endpoint
under the control of the attacker.
Impact: An attacker could gain access to authorization codes or
access tokens
Countermeasure
o require clients to register full redirection URI Section 5.2.3.5
4.2. Authorization Endpoint
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4.2.1. Threat: Password phishing by counterfeit authorization server
OAuth makes no attempt to verify the authenticity of the
Authorization Server. A hostile party could take advantage of this
by intercepting the Client's requests and returning misleading or
otherwise incorrect responses. This could be achieved using DNS or
ARP spoofing. Wide deployment of OAuth and similar protocols may
cause Users to become inured to the practice of being redirected to
websites where they are asked to enter their passwords. If Users are
not careful to verify the authenticity of these websites before
entering their credentials, it will be possible for attackers to
exploit this practice to steal Users' passwords.
Countermeasures:
o Authorization servers should consider such attacks when developing
services based on OAuth, and should require transport-layer
security for any requests where the authenticity of the
authorization server or of request responses is an issue (see
Section 5.1.2).
o Authorization servers should attempt to educate Users about the
risks phishing attacks pose, and should provide mechanisms that
make it easy for users to confirm the authenticity of their sites.
4.2.2. Threat: User unintentionally grants too much access scope
When obtaining end user authorization, the end-user may not
understand the scope of the access being granted and to whom or they
may end up providing a client with access to resources which should
not be permitted.
Countermeasures:
o Explain the scope (resources and the permissions) the user is
about to grant in a understandable way - Section 5.2.4.2
o Narrow scope based on client - When obtaining end user
authorization and where the client requests scope, the
authorization server may want to consider whether to honour that
scope based on who the client is. That decision is between the
client and authorization server and is outside the scope of this
spec. The authorization server may also want to consider what
scope to grant based on the client type, e.g. providing lower
scope to public clients. - Section 5.1.5.1
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4.2.3. Threat: Malicious client obtains existing authorization by fraud
Authorization servers may wish to automatically process authorization
requests from clients which have been previously authorized by the
user. When the user is redirected to the authorization server's end-
user authorization endpoint to grant access, the authorization server
detects that the user has already granted access to that particular
client. Instead of prompting the user for approval, the
authorization server automatically redirects the user back to the
client.
A malicious client may exploit that feature and try to obtain such an
authorization code instead of the legitimate client.
Countermeasures:
o Authorization servers should not automatically process repeat
authorizations to public clients or unless the client is validated
using a pre-registered redirect URI (Section 5.2.3.5 )
o Authorization servers can mitigate the risks associated with
automatic processing by limiting the scope of Access Tokens
obtained through automated approvals - Section 5.1.5.1
4.2.4. Threat: Open redirector
An attacker could use the end-user authorization endpoint and the
redirection URI parameter to abuse the authorization server as an
open redirector. An open redirector is an endpoint using a parameter
to automatically redirect a user-agent to the location specified by
the parameter value without any validation.
Impact: An attacker could utilize a user's trust in your
authorization server to launch a phishing attack.
Countermeasure
o require clients to register full redirection URI Section 5.2.3.5
o don't redirect to redirection URI, if client identity or
redirection URI can't be verified Section 5.2.3.5
4.3. Token endpoint
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4.3.1. Threat: Eavesdropping access tokens
Attackers may attempts to eavesdrop access token on transit from the
authorization server to the client.
Impact: The attacker is able to access all resources with the
permissions covered by the scope of the particular access token.
Countermeasures:
o As per the core OAuth spec, the authorization servers must ensure
that these transmissions are protected using transport-layer
mechanisms such as TLS or SSL (see Section 5.1.1).
o If end-to-end confidentiality cannot be guaranteed, reducing scope
(see Section 5.1.5.1) and expiry time (Section 5.1.5.3) for access
tokens can be used to reduce the damage in case of leaks.
4.3.2. Threat: Obtain access tokens from authorization server database
This threat is applicable if the authorization server stores access
tokens as handles in a database. An attacker may obtain access
tokens from the authorization server's database by gaining access to
the database or launching a SQL injection attack. Impact: disclosure
of all access tokens
Countermeasures:
o System security measures - Section 5.1.4.1.1
o Store access token hashes only - Section 5.1.4.1.3
o Standard SQL injection Countermeasures - Section 5.1.4.1.2
4.3.3. Threat: Obtain client credentials over non secure transport
An attacker could attempt to eavesdrop the transmission of client
credentials between client and server during the client
authentication process or during OAuth token requests. Impact:
Revelation of a client credential enabling the possibility for
phishing or imitation of a client service.
Countermeasures:
o Implement transport security through - Section 5.1.1
o Alternative authentication means, which do not require to send
plaintext credentials over the wire (Examples: Digest
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authentication)
4.3.4. Threat: Obtain client secret from authorization server database
An attacker may obtain valid client_id/secret combinations from the
authorization server's database by gaining access to the database or
launching a SQL injection attack. Impact: disclosure of all
client_id/secret combinations. This allows the attacker to act on
behalf of legitimate clients.
Countermeasures:
o Ensure proper handling of credentials as per Credential Storage
Protection.
4.3.5. Threat: Obtain client secret by online guessing
An attacker may try to guess valid client_id/secret pairs. Impact:
disclosure of single client_id/secret pair.
Countermeasures:
o High entropy of secrets - Section 5.1.4.2.2
o Lock accounts - Section 5.1.4.2.3
o Use Strong Client Authentication - Section 5.2.3.7
4.3.6. Threat: DoS on dynamic client secret creation
If an authorization servers includes a nontrivial amount of entropy
in client secrets and if the authorization server automatically
grants them, an attacker could exhaust the pool by repeatedly
applying for them.
Countermeasures:
o The authorization server should consider some verification step
for new clients. The authorization server should include a
nontrivial amount of entropy in client secrets.
4.4. Obtaining Authorization
This section covers threats which are specific to certain flows
utilized to obtain access tokens. Each flow is characterized by
response types and/or grant types on the end-user authorization and
tokens endpoint, respectively.
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4.4.1. Authorization Code
4.4.1.1. Threat: Eavesdropping or leaking authorization codes
An attacker could try to eavesdrop transmission of the authorization
code between authorization server and client. Furthermore,
authorization codes are passed via the browser which may
unintentionally leak those codes to untrusted web sites and attackers
by different ways:
o Referrer headers: browsers frequently pass a "referrer" header
when a web page embeds content, or when a user travels from one
web page to another web page. These referrer headers may be sent
even when the origin site does not trust the destination site.
The referrer header is commonly logged for traffic analysis
purposes.
o Request logs: web server request logs commonly include query
parameters on requests.
o Open redirectors: web sites sometimes need to send users to
another destination via a redirector. Open redirectors pose a
particular risk to web-based delegation protocols because the
redirector can leak verification codes to untrusted destination
sites.
o Browser history: web browsers commonly record visited URLs in the
browser history. Another user of the same web browser may be able
to view URLs that were visited by previous users.
Note: A description of a similar attacks on the SAML protocol can be
found at http://www.oasis-open.org/committees/download.php/3405/
oasis-sstc-saml-bindings-1.1.pdf (S.4.1.1.9.1), http://
www.thomasgross.net/publications/papers/
GroPfi2006-SAML2_Analysis_Janus.WSSS_06.pdf and http://
www.oasis-open.org/committees/download.php/11191/
sstc-gross-sec-analysis-response-01.pdf.
Countermeasures:
o As per the core OAuth spec, the authorization server as well as
the client must ensure that these transmissions are protected
using transport-layer mechanisms such as TLS or SSL (see
Section 5.1.1).
o The authorization server will require the client to authenticate
wherever possible, so the binding of the authorization code to a
certain client can be validated in a reliable way (see
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Section 5.2.4.4).
o Limited duration of authorization codes - Section 5.1.5.3
o The authorization server should enforce a one time usage
restriction (see Section 5.1.5.4).
o If an Authorization Server observes multiple attempts to redeem a
authorization code, the Authorization Server may want to revoke
all tokens granted based on the authorization code (see
Section 5.2.1.1).
o In the absence of these countermeasures, reducing scope
(Section 5.1.5.1) and expiry time (Section 5.1.5.3) for access
tokens can be used to reduce the damage in case of leaks.
o The client server may reload the target page of the redirection
URI in order to automatically cleanup the browser cache.
4.4.1.2. Threat: Obtain authorization codes from authorization server
database
This threat is applicable if the authorization server stores
authorization codes as handles in a database. An attacker may obtain
authorization codes from the authorization server's database by
gaining access to the database or launching a SQL injection attack.
Impact: disclosure of all authorization codes, most likely along with
the respective redirect_uri and client_id values.
Countermeasures:
o Best practices for credential storage protection should be
employed - Section 5.1.4.1
o System security measures - Section 5.1.4.1.1
o Store access token hashes only - Section 5.1.4.1.3
o Standard SQL injection countermeasures - Section 5.1.4.1.2
4.4.1.3. Threat: Online guessing of authorization codes
An attacker may try to guess valid authorization code values and send
it using the grant type "code" in order to obtain a valid access
token.
Impact: disclosure of single access token, probably also associated
refresh token.
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Countermeasures:
o Handle-based tokens must use high entropy: Section 5.1.5.11
o Assertion-based tokens should be signed: Section 5.1.5.9
o Authenticate the client, adds another value the attacker has to
guess - Section 5.2.3.4
o Binding of authorization code to redirection URI, adds another
value the attacker has to guess - Section 5.2.4.5
o Short expiration time - Section 5.1.5.3
4.4.1.4. Threat: Malicious client obtains authorization
A malicious client could counterfeit a valid client and obtain an
access authorization that way. The malicious client could even
utilize screen scraping techniques in order to simulate the user
consent in the authorization flow.
Assumption: It is not the task of the authorization server to protect
the end-user's device from malicious software. This is the
responsibility of the platform running on the particular device
probably in cooperation with other components of the respective
ecosystem (e.g. an application management infrastructure). The sole
responsibility of the authorization server is to control access to
the end-user's resources living in resource servers and to prevent
unauthorized access to them. Based on this assumption, the following
countermeasures are available to cope with the threat.
Countermeasures:
o The authorization server should authenticate the client, if
possible (see Section 5.2.3.4). Note: the authentication takes
place after the end-user has authorized the access.
o The authorization server should validate the client's redirection
URI against the pre-registered redirection URI, if one exists (see
Section 5.2.3.5). Note: An invalid redirect URI indicates an
invalid client whereas a valid redirect URI not neccesserily
indicates a valid client. The level of confidence depends on the
client type. For web applications, the confidence is high since
the redirect URI refers to the globally unique network endpoint of
this application whose address is also validated using HTTPS
server authentication by the user agent. In contrast for native
clients, the redirect URI typically refers to device local
resources, e.g. a custom scheme. So a malicious client on a
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particular device can use the valid redirect URI the legitimate
client uses on all other devices.
o After authenticating the end-user, the authorization server should
ask him/her for consent. In this context, the user should be
explained the purpose, scope, and duration of the authorization.
Moreover, the authorization server should show the user any
identity information it has for that client. It is up to the user
to validate the binding of this data to the particular application
(e.g. Name) and to approve the authorization request. (see
Section 5.2.4.3).
o The authorization server should not perform automatic re-
authorizations for clients it is unable to reliably authenticate
or validate (see Section 5.2.4.1).
o If the authorization server automatically authenticates the end-
user, it may nevertheless require some user input in order to
prevent screen scraping. Examples are CAPTCHAs or user-specific
secrets like PIN codes.
o The authorization server may also limit the scope of tokens it
issues to clients it cannot reliably authenticate (see
Section 5.1.5.1).
4.4.1.5. Threat: Authorization code phishing
A hostile party could impersonate the client site and get access to
the authorization code. This could be achieved using DNS or ARP
spoofing. This applies to clients, which are web applications, thus
the redirect URI is not local to the host where the user's browser is
running.
Impact: This affects web applications and may lead to a disclosure of
authorization codes and, potentially, the corresponding access and
refresh tokens.
Countermeasures:
It is strongly recommended that one of the following countermeasures
is utilized in order to prevent this attack:
o The redirection URI of the client should point to a HTTPS
protected endpoint and the browser should be utilized to
authenticate this redirection URI using server authentication (see
Section 5.1.2).
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o The authorization server should require the client to be
authenticated, i.e. confidential client, so the binding of the
authorization code to a certain client can be validated in a
reliable way (see Section 5.2.4.4).
4.4.1.6. Threat: User session impersonation
A hostile party could impersonate the client site and impersonate the
user's session on this client. This could be achieved using DNS or
ARP spoofing. This applies to clients, which are web applications,
thus the redirect URI is not local to the host where the user's
browser is running.
Impact: An attacker who intercepts the authorization code as it is
sent by the browser to the callback endpoint can gain access to
protected resources by submitting the authorization code to the
client. The client will exchange the authorization code for an
access token and use the access token to access protected resources
for the benefit of the attacker, delivering protected resources to
the attacker, or modifying protected resources as directed by the
attacker. If OAuth is used by the client to delegate authentication
to a social site (e.g. as in the implementation of the "Facebook
Login" button), the attacker can use the intercepted authorization
code to log in to the client as the user.
Note: Authenticating the client during authorization code exchange
will not help to detect such an attack as it is the legitimate client
that obtains the tokens.
Countermeasures:
o In order to prevent an attacker from impersonating the end-users
session, the redirection URI of the client should point to a HTTPS
protected endpoint and the browser should be utilized to
authenticate this redirection URI using server authentication (see
Section 5.1.2)
4.4.1.7. Threat: Authorization code leakage through counterfeit client
The attack leverages the authorization code grant type in an attempt
to get another user (victim) to log-in, authorize access to his/her
resources, and subsequently obtain the authorization code and inject
it into a client application using the attacker's account. The goal
is to associate an access authorization for resources of the victim
with the user account of the attacker on a client site.
The attacker abuses an existing client application and combines it
with his own counterfeit client web site. The attack depends on the
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victim expecting the client application to request access to a
certain resource server. The victim, seeing only a normal request
from an expected application, approves the request. The attacker
then uses the victim's authorization to gain access to the
information unknowingly authorized by the victim.
The attacker conducts the following flow:
1. The attacker accesses the client web site (or application) and
initiates data access to a particular resource server. The
client web site in turn initiates an authorization request to the
resource server's authorization server. Instead of proceeding
with the authorization process, the attacker modifies the
authorization server end-user authorization URL as constructed by
the client to include a redirection URI parameter referring to a
web site under his control (attacker's web site).
2. The attacker tricks another user (the victim) to open that
modified end-user authorization URI and to authorize access (e.g.
an email link, or blog link). The way the attacker achieve that
goal is out of scope.
3. Having clicked the link, the victim is requested to authenticate
and authorize the client site to have access.
4. After completion of the authorization process, the authorization
server redirects the user agent to the attacker's web site
instead of the original client web site.
5. The attacker obtains the authorization code from his web site by
means out of scope of this document.
6. He then constructs a redirection URI to the target web site (or
application) based on the original authorization request's
redirection URI and the newly obtained authorization code and
directs his user agent to this URL. The authorization code is
injected into the original client site (or application).
7. The client site uses the authorization code to fetch a token from
the authorization server and associates this token with the
attacker's user account on this site.
8. The attacker may now access the victims resources using the
client site.
Impact: The attackers gains access to the victim's resources as
associated with his account on the client site.
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Countermeasures:
o The attacker will need to use another redirection URI for its
authorization process than the target web site because it needs to
intercept the flow. So if the authorization server associates the
authorization code with the redirection URI of a particular end-
user authorization and validates this redirection URI with the
redirection URI passed to the token's endpoint, such an attack is
detected (see Section 5.2.4.5).
o The authorization server may also enforce the usage and validation
of pre-registered redirect URIs (see Section 5.2.3.5). This will
allow for an early recognition of session fixation attempts.
o For native applications, one could also consider to use
deployment-specific client ids and secrets (see Section 5.2.3.4,
along with the binding of authorization code to client_id (see
Section 5.2.4.4), to detect such an attack because the attacker
does not have access the deployment-specific secret. Thus he will
not be able to exchange the authorization code.
o The client may consider using other flows, which are not
vulnerable to this kind of attacks such as "Implicit Grant" or
"Resource Owner Password Credentials" (see Section 4.4.2 or
Section 4.4.3).
4.4.1.8. Threat: CSRF attack against redirect-uri
Cross-Site Request Forgery (CSRF) is a web-based attack whereby HTTP
requests are transmitted from a user that the website trusts or has
authenticated (e.g., via HTTP redirects or HTML forms). CSRF attacks
on OAuth approvals can allow an attacker to obtain authorization to
OAuth protected resources without the consent of the User.
This attack works against the redirection URI used in the
authorization code flow. An attacker could authorize an
authorization code to their own protected resources on an
authorization server. He then aborts the redirect flow back to the
client on his device and tricks the victim into executing the
redirect back to the client. The client receives the redirect,
fetches the token(s) from the authorization server and associates the
victim's client session with the resources accessible using the
token.
Impact: The user accesses resources on behalf of the attacker. The
effective impact depends on the type of resource accessed. For
example, the user may upload private items to an attacker's
resources. Or when using OAuth in 3rd party login scenarios, the
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user may associate his client account with the attacker's identity at
the external identity provider. This way the attacker could easily
access the victim's data at the client by logging in from another
device with his credentials at the external identity provider.
Countermeasures:
o The state parameter should be used to link the authorization
request with the redirection URI used to deliver the access token.
Section 5.3.6
o Client developers and end-user can be educated not follow
untrusted URLs.
4.4.1.9. Threat: Clickjacking attack against authorization
With Clickjacking, a malicious site loads the target site in a
transparent iframe overlaid on top of a set of dummy buttons which
are carefully constructed to be placed directly under important
buttons on the target site. When a user clicks a visible button,
they are actually clicking a button (such as an "Authorize" button)
on the hidden page.
Impact: An attacker can steal a user's authentication credentials and
access their resources
Countermeasure
o Native applications should use external browsers instead of
embedding browsers in a web view when requesting end-user
authorization
o For newer browsers, avoidance of iFrames can be enforced server
side by using the X-FRAME-OPTION header - Section 5.2.2.6
o For older browsers, javascript framebusting techniques can be used
but may not be effective in all browsers.
4.4.1.10. Threat: Resource Owner Impersonation
When a client requests access to protected resources, the
authorization flow normally involves the resource owner's explicit
response to the access request, either granting or denying access to
the protected resources. A malicious client can exploit knowledge of
the structure of this flow in order to gain authorization without the
resource owner's consent, by transmitting the necessary requests
programmatically, and simulating the flow against the authorization
server. That way, the client may gain access to the victims
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resources without her approval. An authorization server will be
vulnerable to this threat, if it uses non-interactive authentication
mechanisms or split the authorization flow across multiple pages.
The malicious client might embed a hidden HTML user agent, interpret
the HTML forms sent by the authorization server, and automatically
answer with the corresponding form post requests. As a pre-
requisite, the attacker must be able to execute the authorization
process in the context of an already authenticated session of the
resource owner with the authorization server. There are different
ways to achieve this:
o The malicious client could abuse an existing session in an
external browser or cross-browser cookies on the particular
device.
o It could also request authorization for a particular scope and
silently abuse the resulting session in his browser instance to
"silently" request another scope.
o Alternatively, the attacker might exploit an authorization
server's ability to authenticate the resource owner automatically
and without user interactions, e.g. based on certificates.
In all cases, such an attack is limited to clients running on the
victim's device, within the user agent or as native app.
Please note: Such attacks cannot be prevented using CSRF
countermeasures, since the attacker just "executes" the URLs as
prepared by the authorization server including any nonce etc.
Countermeasures:
Authorization servers should decide, based on an analysis of the risk
associated with this threat, whether to assume, detect, or to prevent
this threat.
In order to prevent such an attack, the authorization server may
force an user interaction based on non-predictable input values as
part of the user consent approval. The authorization server could
o combine password authentication and user consent in a single form,
o make use of CAPTCHAs, or
o or use one-time secrets send out of bound to the resource owner
(e.g. via text or instance message).
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Alternatively in order to allow the resource owner to detect abuse,
the authorization server could notify the resource owner of any
approval by appropriate means, e.g. text or instant message or
e-Mail.
4.4.1.11. Threat: DoS, Exhaustion of resources attacks
If an authorization server includes a nontrivial amount of entropy in
authorization codes or access tokens (limiting the number of possible
codes/tokens) and automatically grants either without user
intervention and has no limit on code or access tokens per user, an
attacker could exhaust the pool by repeatedly directing user(s)
browser to request code or access tokens. This is because more
entropy means a larger number of tokens can be issued.
Countermeasures:
o The authorization server should consider limiting the number of
access tokens granted per user. The authorization server should
include a nontrivial amount of entropy in authorization codes.
4.4.1.12. Threat: DoS using manufactured authorization codes
An attacker who owns a botnet can locate the redirect URIs of clients
that listen on HTTP, access them with random authorization codes, and
cause a large number of HTTPS connections to be concentrated onto the
authorization server. This can result in a DoS attack on the
authorization server.
This attack can still be effective even when CSRF defense/the 'state'
parameter are deployed on the client side. With such a defense, the
attacker might need to incur an additional HTTP request to obtain a
valid CSRF code/ state parameter. This apparently cuts down the
effectiveness of the attack by a factor of 2. However, if the HTTPS/
HTTP cost ratio is higher than 2 (the cost factor is estimated to be
around 3.5x at
<http://www.semicomplete.com/blog/geekery/ssl-latency.html>) the
attacker still achieves a magnification of resource utilization at
the expense of the authorization server.
Impact: There are a few effects that the attacker can accomplish with
this OAuth flow that they cannot easily achieve otherwise.
1. Connection laundering: With the clients as the relay between the
attacker and the authorization server, the authorization server
learns little or no information about the identity of the
attacker. Defenses such rate limiting on the offending attacker
machines are less effective due to the difficulty to identify the
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attacking machines. Although an attacker could also launder its
connections through an anonymizing systems such as Tor, the
effectiveness of that approach depends on the capacity of the
annoying system. On the other hand, a potentially large number
of OAuth clients could be utilized for this attack.
2. Asymmetric resource utilization: The attacker incurs the cost of
an HTTP connection and causes an HTTPS connection to be made on
the authorization server; and the attacker can co-ordinate the
timing of such HTTPS connections across multiple clients
relatively easily. Although the attacker could achieve something
similar, say, by including an iframe pointing to the HTTPS URL of
the authorization server in an HTTP web page and lure web users
to visit that page, timing attacks using such a scheme may be
more difficult as it seems nontrivial to synchronize a large
number of users to simultaneously visit a particular site under
the attacker's control.
Countermeasures
o Though not a complete countermeasure by themselves, CSRF defense
and the 'state' parameter created with secure random codes should
be deployed on the client side. The client should forward the
authorization code to the authorization server only after both the
CSRF token and the 'state' parameter are validated.
o If the client authenticates the user, either through a single sign
on protocol ( such as OpenID / Facebook Connect ) or through local
authentication, the client should suspend the access by a user
account if the number of invalid authorization codes submitted by
this user exceeds a certain threshold.
o The authorization server should send an error response to the
client reporting an invalid authorization code and rate limit or
disallow connections from clients whose number of invalid requests
exceeds a threshold.
o The authorization server may in addition sign the authorization
code using the public key from its SSL certificate, and require
the client to validate the signature. To enhance interoperability
between multiple clients and authorization servers, a standard
procedure to create and validate the signature (including what
attributes to sign) may be developed and agreed between the
clients and the servers.
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4.4.2. Implicit Grant
In the implicit grant type flow, the access token is directly
returned to the client as a fragment part of the redirection URI. It
is assumed that the token is not sent to the redirection URI target
as HTTP user agents do not send the fragment part of URIs to HTTP
servers. Thus an attacker cannot eavesdrop the access token on this
communication path and It cannot leak through HTTP referee headers.
4.4.2.1. Threat: Access token leak in transport/end-points
This token might be eavesdropped by an attacker. The token is sent
from server to client via a URI fragment of the redirection URI. If
the communication is not secured or the end-point is not secured, the
token could be leaked by parsing the returned URI.
Impact: the attacker would be able to assume the same rights granted
by the token.
Countermeasures:
o The authorization server should ensure confidentiality of the
response from the authorization server to the client (see
Section 5.1.1).
4.4.2.2. Threat: Access token leak in browser history
An attacker could obtain the token from the browser's history. Note
this means the attacker needs access to the particular device.
Countermeasures:
o Shorten token duration (see Section 5.1.5.3) and reduced scope of
the token may reduce the impact of that attack (see
Section 5.1.5.1).
o Make these requests non-cachable
o Native applications can directly embed a browser widget and
therewith gain full control of the cache. So the application can
cleanup browser history after authorization process.
4.4.2.3. Threat: Malicious client obtains authorization
An malicious client could attempt to obtain a token by fraud.
The same countermeasures as for Section 4.4.1.4 are applicable,
except client authentication.
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4.4.2.4. Threat: Manipulation of scripts
A hostile party could act as the client web server and replace or
modify the actual implementation of the client (script). This could
be achieved using DNS or ARP spoofing. This applies to clients
implemented within the Web Browser in a scripting language.
Impact: The attacker could obtain user credential information and
assume the full identity of the user.
Countermeasures:
o The authorization server should authenticate the server from which
scripts are obtained (see Section 5.1.2).
o The client should ensure that scripts obtained have not been
altered in transport (see Section 5.1.1).
o Introduce one time per-use secrets (e.g. client_secret) values
that can only be used by scripts in a small time window once
loaded from a server. The intention would be to reduce the
effectiveness of copying client-side scripts for re-use in an
attackers modified code.
4.4.2.5. Threat: CSRF attack against redirect-uri
Cross-Site Request Forgery (CSRF) is a web-based attack whereby HTTP
requests are transmitted from a user that the website trusts or has
authenticated (e.g., via HTTP redirects or HTML forms). CSRF attacks
on OAuth approvals can allow an attacker to obtain authorization to
OAuth protected resources without the consent of the User.
This attack works against the redirection URI used in the implicit
grant flow. An attacker could acquire an access token to their own
protected resources. He could then construct a redirection URI and
embed their access token in that URI. If he can trick the user into
following the redirection URI and the client does not have protection
against this attack, the user may have the attacker's access token
authorized within their client.
Impact: The user accesses resources on behalf of the attacker. The
effective impact depends on the type of resource accessed. For
example, the user may upload private items to an attacker's
resources. Or when using OAuth in 3rd party login scenarios, the
user may associate his client account with the attacker's identity at
the external identity provider. This way the attacker could easily
access the victim's data at the client by logging in from another
device with his credentials at the external identity provider.
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Countermeasures:
o The state parameter should be used to link the authorization
request with the redirection URI used deliver the access token.
This will ensure the client is not tricked into completing any
redirect callback unless it is linked to an authorization request
the client initiated. The state parameter should be unguessable
and the client should be capable of keeping the state parameter
secret.
o Client developers and end-user can be educated not follow
untrusted URLs.
4.4.3. Resource Owner Password Credentials
The "Resource Owner Password Credentials" grant type (see
[I-D.ietf-oauth-v2], Section 4.3), often used for legacy/migration
reasons, allows a client to request an access token using an end-
users user-id and password along with its own credential. This grant
type has higher risk because it maintains the uid/password anti-
pattern. Additionally, because the user does not have control over
the authorization process, clients using this grant type are not
limited by scope, but instead have potentially the same capabilities
as the user themselves. As there is no authorization step, the
ability to offer token revocation is bypassed.
Impact: The resource server can only differentiate scope based on the
access token being associated with a particular client. The client
could also acquire long-living tokens and pass them up to a attacker
web service for further abuse. The client, eavesdroppers, or end-
points could eavesdrop user id and password.
Countermeasures:
o Except for migration reasons, minimize use of this grant type
o The authorization server should validate the client id associated
with the particular refresh token with every refresh request -
Section 5.2.2.2
o As per the core Oauth spec, the authorization server must ensure
that these transmissions are protected using transport-layer
mechanisms such as TLS or SSL (see Section 5.1.1).
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4.4.3.1. Threat: Accidental exposure of passwords at client site
If the client does not provide enough protection, an attacker or
disgruntled employee could retrieve the passwords for a user.
Countermeasures:
o Use other flows, which do not rely on the client's cooperation for
secure resource owner credential handling
o Use digest authentication instead of plaintext credential
processing
o Obfuscation of passwords in logs
4.4.3.2. Threat: Client obtains scopes without end-user authorization
All interaction with the resource owner is performed by the client.
Thus it might, intentionally or unintentionally, happen that the
client obtains a token with scope unknown for or unintended by the
resource owner. For example, the resource owner might think the
client needs and acquires read-only access to its media storage only
but the client tries to acquire an access token with full access
permissions.
Countermeasures:
o Use other flows, which do not rely on the client's cooperation for
resource owner interaction
o The authorization server may generally restrict the scope of
access tokens (Section 5.1.5.1) issued by this flow. If the
particular client is trustworthy and can be authenticated in a
reliable way, the authorization server could relax that
restriction. Resource owners may prescribe (e.g. in their
preferences) what the maximum scope is for clients using this
flow.
o The authorization server could notify the resource owner by an
appropriate media, e.g. e-Mail, of the grant issued (see
Section 5.1.3).
4.4.3.3. Threat: Client obtains refresh token through automatic
authorization
All interaction with the resource owner is performed by the client.
Thus it might, intentionally or unintentionally, happen that the
client obtains a long-term authorization represented by a refresh
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token even if the resource owner did not intend so.
Countermeasures:
o Use other flows, which do not rely on the client's cooperation for
resource owner interaction
o The authorization server may generally refuse to issue refresh
tokens in this flow (see Section 5.2.2.1). If the particular
client is trustworthy and can be authenticated in a reliable way
(see client authentication), the authorization server could relax
that restriction. Resource owners may allow or deny (e.g. in
their preferences) to issue refresh tokens using this flow as
well.
o The authorization server could notify the resource owner by an
appropriate media, e.g. e-Mail, of the refresh token issued (see
Section 5.1.3).
4.4.3.4. Threat: Obtain user passwords on transport
An attacker could attempt to eavesdrop the transmission of end-user
credentials with the grant type "password" between client and server.
Impact: disclosure of a single end-users password.
Countermeasures:
o Confidentiality of Requests - Section 5.1.1
o alternative authentication means, which do not require to send
plaintext credentials over the wire (Examples: Digest
authentication)
4.4.3.5. Threat: Obtain user passwords from authorization server
database
An attacker may obtain valid username/password combinations from the
authorization server's database by gaining access to the database or
launching a SQL injection attack.
Impact: disclosure of all username/password combinations. The impact
may exceed the domain of the authorization server since many users
tend to use the same credentials on different services.
Countermeasures:
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o Credential storage protection can be employed - Section 5.1.4.1
4.4.3.6. Threat: Online guessing
An attacker may try to guess valid username/password combinations
using the grant type "password".
Impact: Revelation of a single username/password combination.
Countermeasures:
o Password policy - Section 5.1.4.2.1
o Lock accounts - Section 5.1.4.2.3
o Tar pit - Section 5.1.4.2.4
o CAPTCHA - Section 5.1.4.2.5
o Abandon on grant type "password"
o Client authentication (see Section 5.2.3) will provide another
authentication factor and thus hinder the attack.
4.4.4. Client Credentials
Client credentials (see [I-D.ietf-oauth-v2], Section 3) consist of an
identifier (not secret) combined with an additional means (such as a
matching client secret) of authenticating a client. The threats to
this grant type are similar to Section 4.4.3.
4.5. Refreshing an Access Token
4.5.1. Threat: Eavesdropping refresh tokens from authorization server
An attacker may eavesdrop refresh tokens when they are transmitted
from the authorization server to the client.
Countermeasures:
o As per the core OAuth spec, the Authorization servers must ensure
that these transmissions are protected using transport-layer
mechanisms such as TLS or SSL (see Section 5.1.1).
o If end-to-end confidentiality cannot be guaranteed, reducing scope
(see Section 5.1.5.1) and expiry time (see Section 5.1.5.3) for
issued access tokens can be used to reduce the damage in case of
leaks.
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4.5.2. Threat: Obtaining refresh token from authorization server
database
This threat is applicable if the authorization server stores refresh
tokens as handles in a database. An attacker may obtain refresh
tokens from the authorization server's database by gaining access to
the database or launching a SQL injection attack.
Impact: disclosure of all refresh tokens
Countermeasures:
o Credential storage protection - Section 5.1.4.1
o Bind token to client id, if the attacker cannot obtain the
required id and secret - Section 5.1.5.8
4.5.3. Threat: Obtain refresh token by online guessing
An attacker may try to guess valid refresh token values and send it
using the grant type "refresh_token" in order to obtain a valid
access token.
Impact: exposure of single refresh token and derivable access tokens.
Countermeasures:
o For handle-based designs - Section 5.1.5.11
o For assertion-based designs - Section 5.1.5.9
o Bind token to client id, because the attacker would guess the
matching client id, too (see Section 5.1.5.8)
o Authenticate the client, adds another element the attacker has to
guess (see Section 5.2.3.4)
4.5.4. Threat: Obtain refresh token phishing by counterfeit
authorization server
An attacker could try to obtain valid refresh tokens by proxying
requests to the authorization server. Given the assumption that the
authorization server URL is well-known at development time or can at
least be obtained from a well-known resource server, the attacker
must utilize some kind of spoofing in order to succeed.
Countermeasures:
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o Server authentication (as described in Section 5.1.2)
4.6. Accessing Protected Resources
4.6.1. Threat: Eavesdropping access tokens on transport
An attacker could try to obtain a valid access token on transport
between client and resource server. As access tokens are shared
secrets between authorization and resource server, they should be
treated with the same care as other credentials (e.g. end-user
passwords).
Countermeasures:
o Access tokens sent as bearer tokens, should not be sent in the
clear over an insecure channel. As per the core OAuth spec,
transmission of access tokens must be protected using transport-
layer mechanisms such as TLS or SSL (see Section 5.1.1).
o A short lifetime reduces impact in case tokens are compromised
(see Section 5.1.5.3).
o The access token can be bound to a client's identity and require
the client to prove legitimate ownership of the token to the
resource server (see Section 5.4.2).
4.6.2. Threat: Replay authorized resource server requests
An attacker could attempt to replay valid requests in order to obtain
or to modify/destroy user data.
Countermeasures:
o The resource server should utilize transport security measure in
order to prevent such attacks (see Section 5.1.1). This would
prevent the attacker from capturing valid requests.
o Alternatively, the resource server could employ signed requests
(see Section 5.4.3) along with nounces and timestamps in order to
uniquely identify requests. The resource server should detect and
refuse every replayed request.
4.6.3. Threat: Guessing access tokens
Where the token is a handle, the attacker may use attempt to guess
the access token values based on knowledge they have from other
access tokens.
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Impact: Access to a single user's data.
Countermeasures:
o Handle Tokens should have a reasonable entropy (see
Section 5.1.5.11) in order to make guessing a valid token value
difficult.
o Assertion (or self-contained token ) tokens contents should be
protected by a digital signature (see Section 5.1.5.9).
o Security can be further strengthened by using a short access token
duration (see Section 5.1.5.2 and Section 5.1.5.3).
4.6.4. Threat: Access token phishing by counterfeit resource server
An attacker may pretend to be a particular resource server and to
accept tokens from a particular authorization server. If the client
sends a valid access tokens to this counterfeit resource server, the
server in turn may use that token to access other services on behalf
of the resource owner.
Countermeasures:
o Clients should not make authenticated requests with an access
token to unfamiliar resource servers, regardless of the presence
of a secure channel. If the resource server address is well-known
to the client, it may authenticate the resource servers (see
Section 5.1.2).
o Associate the endpoint address of the resource server the client
talked to with the access token (e.g. in an audience field) and
validate association at legitimate resource server. The endpoint
address validation policy may be strict (exact match) or more
relaxed (e.g. same host). This would require to tell the
authorization server the resource server endpoint address in the
authorization process.
o Associate an access token with a client and authenticate the
client with resource server requests (typically via signature in
order to not disclose secret to a potential attacker). This
prevents the attack because the counterfeit server is assumed to
miss the capabilities to correctly authenticate on behalf of the
legitimate client to the resource server (Section 5.4.2).
o Restrict the token scope (see Section 5.1.5.1) and or limit the
token to a certain resource server (Section 5.1.5.5).
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4.6.5. Threat: Abuse of token by legitimate resource server or client
A legitimate resource server could attempt to use an access token to
access another resource servers. Similarly, a client could try to
use a token obtained for one server on another resource server.
Countermeasures:
o Tokens should be restricted to particular resource servers (see
Section 5.1.5.5).
4.6.6. Threat: Leak of confidential data in HTTP-Proxies
The HTTP Authorization scheme (OAuth HTTP Authorization Scheme) is
optional. However, [RFC2616](Fielding, R., Gettys, J., Mogul, J.,
Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1," .) relies on the Authorization and
WWW-Authenticate headers to distinguish authenticated content so that
it can be protected. Proxies and caches, in particular, may fail to
adequately protect requests not using these headers. For example,
private authenticated content may be stored in (and thus retrievable
from) publicly-accessible caches.
Countermeasures:
o Resource servers not using the HTTP Authorization scheme (OAuth
HTTP Authorization Scheme - see Section 5.4.1) should take care to
use other mechanisms, such as the Cache-Control header, to
minimize the risk that authenticated content is not protected.
o Reducing scope (see Section 5.1.5.1) and expiry time
(Section 5.1.5.3) for access tokens can be used to reduce the
damage in case of leaks.
4.6.7. Threat: Token leakage via logfiles and HTTP referrers
If access tokens are sent via URI query parameters, such tokens may
leak to log files and HTTP referrers.
Countermeasures:
o Use authorization headers or POST parameters instead of URI
request parameters (see Section 5.4.1).
o Set logging configuration appropriately
o Prevent unauthorized persons from access to system log files (see
Section 5.1.4.1.1)
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o HTTP referrers can be prevented by reloading the target page again
without URI parameters
o Abuse of leaked access tokens can be prevented by enforcing
authenticated requests (see Section 5.4.2).
o The impact of token leakage may be reduced by limiting scope (see
Section 5.1.5.1) and duration (see Section 5.1.5.3) and enforcing
one time token usage (see Section 5.1.5.4).
5. Security Considerations
This section describes the countermeasures as recommended to mitigate
the threats as described in Section 4.
5.1. General
The general section covers consideratios that apply generally across
all OAuth components (client, resource server, token server, and
user-agents).
5.1.1. Confidentiality of Requests
This is applicable to all requests sent from client to authorization
server or resource server. While OAuth provides a mechanism for
verifying the integrity of requests, it provides no guarantee of
request confidentiality. Unless further precautions are taken,
eavesdroppers will have full access to request content and may be
able to mount interception or replay attacks through using content of
request, e.g. secrets or tokens.
Attacks can be mitigated by using transport-layer mechanisms such as
TLS or SSL. VPN may considered as well.
This is a countermeasure against the following threats:
o Replay of access tokens obtained on tokens endpoint or resource
server's endpoint
o Replay of refresh tokens obtained on tokens endpoint
o Replay of authorization codes obtained on tokens endpoint
(redirect?)
o Replay of user passwords and client secrets
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5.1.2. Server authentication
HTTPS server authentication or similar means can be used to
authenticate the identity of a server. The goal is to reliably bind
the DNS name of the server to the public key presented by the server
during connection establishment.
The client should validate the binding of the server to its domain
name. If the server fails to prove that binding, it is considered a
man-in-the-middle attack. The security measure depends on the
certification authorities the client trusts for that purpose.
Clients should carefully select those trusted CAs and protect the
storage for trusted CA certificates from modifications.
This is a countermeasure against the following threats:
o Spoofing
o Proxying
o Phishing by counterfeit servers
5.1.3. Always keep the resource owner informed
Transparency to the resource owner is a key element of the OAuth
protocol. The user should always be in control of the authorization
processes and get the necessary information to meet informed
decisions. Moreover, user involvement is a further security
countermeasure. The user can probably recognize certain kinds of
attacks better than the authorization server. Information can be
presented/exchanged during the authorization process, after the
authorization process, and every time the user wishes to get informed
by using techniques such as:
o User consent forms
o Notification messages (e-Mail, SMS, ...)
o Activity/Event logs
o User self-care applications or portals
5.1.4. Credentials
This sections describes countermeasures used to protect all kinds of
credentials from unauthorized access and abuse. Credentials are long
term secrets, such as client secrets and user passwords as well as
all kinds of tokens (refresh and access token) or authorization
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codes.
5.1.4.1. Credential Storage Protection
Administrators should undertake industry best practices to protect
the storage of credentials. Such practices may include but are not
limited to the following sub-sections.
5.1.4.1.1. Standard System Security Means
A server system may be locked down so that no attacker may get access
to sensible configuration files and databases.
5.1.4.1.2. Standard SQL Injection Countermeasures
If a client identifier or other authentication component is queried
or compared against a SQL Database it may become possible for an
injection attack to occur if parameters received are not validated
before submission to the database.
o Ensure that server code is using the minimum database privileges
possible to reduce the "surface" of possible attacks.
o Avoid dynamic SQL using concatenated input. If possible, use
static SQL.
o When using dynamic SQL, parameterize queries using bind arguments.
Bind arguments eliminate possibility of SQL injections.
o Filter and sanitize the input. For example, if an identifier has
a known format, ensure that the supplied value matches the
identifier syntax rules.
5.1.4.1.3. No cleartext storage of credentials
The authorization server should not store credential in clear text.
Typical approaches are to store hashes instead. If the credential
lacks a reasonable entropy level (because it is a user password) an
additional salt will harden the storage to prevent offline dictionary
attacks. Note: Some authentication protocols require the
authorization server to have access to the secret in the clear.
Those protocols cannot be implemented if the server only has access
to hashes.
5.1.4.1.4. Encryption of credentials
For client applications, insecurely persisted client credentials are
easy targets for attackers to obtain. Store client credentials using
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an encrypted persistence mechanism such as a keystore or database.
Note that compiling client credentials directly into client code
makes client applications vulnerable to scanning as well as difficult
to administer should client credentials change over time.
5.1.4.1.5. Use of asymmetric cryptography
Usage of asymmetric cryptography will free the authorization server
of the obligation to manage credentials.
5.1.4.2. Online attacks on secrets
5.1.4.2.1. Password policy
The authorization server may decide to enforce a complex user
password policy in order to increase the user passwords' entropy.
This will hinder online password attacks.
5.1.4.2.2. High entropy of secrets
When creating token handles or other secrets not intended for usage
by human users, the authorization server should include a reasonable
level of entropy in order to mitigate the risk of guessing attacks.
The token value should be constructed from a cryptographically strong
random or pseudo-random number sequence [RFC1750] generated by the
Authorization Server. The probability of any two Authorization Code
values being identical should be less than or equal to 2^(-128) and
should be less than or equal to 2^(-160).
5.1.4.2.3. Lock accounts
Online attacks on passwords can be mitigated by locking the
respective accounts after a certain number of failed attempts.
Note: This measure can be abused to lock down legitimate service
users.
5.1.4.2.4. Tar pit
The authorization server may react on failed attempts to authenticate
by username/password by temporarily locking the respective account
and delaying the response for a certain duration. This duration may
increase with the number of failed attempts. The objective is to
slow the attackers attempts on a certain username down.
Note: this may require a more complex and stateful design of the
authorization server.
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5.1.4.2.5. Usage of CAPTCHAs
The idea is to prevent programs from automatically checking huge
number of passwords by requiring human interaction.
Note: this has a negative impact on user experience.
5.1.5. Tokens (access, refresh, code)
5.1.5.1. Limit token scope
The authorization server may decide to reduce or limit the scope
associated with a token. Basis of this decision is out of scope,
examples are:
o a client-specific policy, e.g. issue only less powerful tokens to
public clients,
o a service-specific policy, e.g. it a very sensible service,
o a resource-owner specific setting, or
o combinations of such policies and preferences.
The authorization server may allow different scopes dependent on the
grant type. For example, end-user authorization via direct
interaction with the end-user (authorization code) might be
considered more reliable than direct authorization via grant type
username/password. This means will reduce the impact of the
following threats:
o token leakage
o token issuance to malicious software
o unintended issuance of to powerful tokens with resource owner
credentials flow
5.1.5.2. Expiration time
Tokens should generally expire after a reasonable duration. This
complements and strengthens other security measures (such as
signatures) and reduces the impact of all kinds of token leaks.
5.1.5.3. Short expiration time
A short expiration time for tokens is a protection means against the
following threats:
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o replay
o reduce impact of token leak
o reduce likelihood of successful online guessing
Note: Short token duration requires preciser clock synchronisation
between authorization server and resource server. Furthermore,
shorter duration may require more token refreshments (access token)
or repeated end-user authorization processes (authorization code and
refresh token).
5.1.5.4. Limit number of usages/ One time usage
The authorization server may restrict the number of requests or
operations which can be performed with a certain token. This
mechanism can be used to mitigate the following threats:
o replay of tokens
o reduce likelihood of successful online guessing
For example, if an Authorization Server observes more than one
attempt to redeem a authorization code, the Authorization Server may
want to revoke all access tokens granted based on the authorization
code as well as reject the current request.
As with the authorization code, access tokens may also have a limited
number of operations. This forces client applications to either re-
authenticate and use a refresh token to obtain a fresh access token,
or it forces the client to re-authorize the access token by involving
the user.
5.1.5.5. Bind tokens to a particular resource server (Audience)
Authorization servers in multi-service environments may consider
issuing tokens with different content to different resource servers
and to explicitly indicate in the token the target server a token is
intended to be sent to (see Audience in SAML Assertions). This
countermeasure can be used in the following situations:
o It reduces the impact of a successful replay attempt, since the
token is applicable to a single resource server, only.
o It prevents abuse of a token by a rough resource server or client,
since the token can only be used on that server. It is rejected
by other servers.
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o It reduces the impact of a leakage of a valid token to a
counterfeit resource server.
5.1.5.6. Use endpoint address as token audience
This may be used to indicate to a resource server, which endpoint
address has been used to obtain the token. This measure will allow
to detect requests from a counterfeit resource server, since such
token will contain the endpoint address of that server.
5.1.5.7. Audience and Token scopes
Deployments may consider only using tokens with explicitly defined
scope, where every scope is associated with a particular resource
server. This approach can be used to mitigate attacks, where a
resource server or client uses a token for a different then the
intended purpose.
5.1.5.8. Bind token to client id
An authorization server may bind a token to a certain client
identity. This identity should be validated for every request with
that token. This means can be used, to
o detect token leakage and
o prevent token abuse.
Note: Validating the client identity may require the target server to
authenticate the client's identity. This authentication can be based
on secrets managed independent of the token (e.g. pre-registered
client id/secret on authorization server) or sent with the token
itself (e.g. as part of the encrypted token content).
5.1.5.9. Signed tokens
Self-contained tokens should be signed in order to detect any attempt
to modify or produce faked tokens.
5.1.5.10. Encryption of token content
Self-contained may be encrypted for privacy reasons or to protect
system internal data.
5.1.5.11. Random token value with high entropy
When creating token handles, the authorization server should include
a reasonable level of entropy in order to mitigate the risk of
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guessing attacks. The token value should be constructed from a
cryptographically strong random or pseudo-random number sequence
[RFC1750] generated by the Authorization Server. The probability of
any two token values being identical should be less than or equal to
2^(-128) and should be less than or equal to 2^(-160).
5.1.5.12. Assertion formats
For service providers intending to implement an assertion-based token
design it is highly recommended to adopt a standard assertion format
(such as SAML or JWT) that implements [draft-ietf-oauth-assertions].
5.1.6. Access tokens
The following measures should be used to protect access tokens
o keep them in transient memory (accessible by the client
application only)
o protect from exposure to 3rd parties (malicious application)
o limit number of access tokens granted to a user
5.2. Authorization Server
This section describes considerations related to the OAuth
Authorization Server end-point.
5.2.1. Authorization Codes
5.2.1.1. Automatic revocation of derived tokens if abuse is detected
If an Authorization Server observes multiple attempts to redeem an
authorization grant (e.g. such as an authorization code), the
Authorization Server may want to revoke all tokens granted based on
the authorization grant.
5.2.2. Refresh tokens
5.2.2.1. Restricted issuance of refresh tokens
The authorization server may decide based on an appropriate policy
not to issue refresh tokens. Since refresh tokens are long term
credentials, they may be subject theft. For example, if the
authorization server does not trust a client to securely store such
tokens, it may refuse to issue such a client a refresh token.
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5.2.2.2. Binding of refresh token to client_id
The authorization server should bind every refresh token to the id of
the client such a token was originally issued to and validate this
binding for every request to refresh that token. If possible (e.g.
confidential clients), the authorization server should authenticate
the respective client.
This is a countermeasure against refresh token theft or leakage.
Note: This binding should be protected from unauthorized
modifications.
5.2.2.3. Refresh Token Rotation
Refresh token rotation is intended to automatically detect and
prevent attempts to use the same refresh token in parallel from
different apps/devices. This happens if a token gets stolen from the
client and is subsequently used by the attacker and the legitimate
client. The basic idea is to change the refresh token value with
every refresh request in order to detect attempts to obtain access
tokens using old refresh tokens. Since the authorization server
cannot determine whether the attacker or the legitimate client is
trying to access, in case of such an access attempt the valid refresh
token and the access authorization associated with it are both
revoked.
The OAuth specification supports this measure in that the tokens
response allows the authorization server to return a new refresh
token even for requests with grant type "refresh_token".
Note: this measure may cause problems in clustered environments since
usage of the currently valid refresh token must be ensured. In such
an environment, other measures might be more appropriate.
5.2.2.4. Refresh Token Revocation
The authorization server may allow clients or end-users to explicitly
request the invalidation of refresh tokens.
This is a countermeasure against:
o device theft,
o impersonation of resource owner, or
o suspected compromised client applications.
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5.2.2.5. Device identification
The authorization server may require to bind authentication
credentials to a device identifier. The IMEI is one example of such
an identifier, there are also operating system specific identifiers.
The authorization server could include such an identifier when
authenticating user credentials in order to detect token theft from a
particular device.
5.2.2.6. X-FRAME-OPTION header
For newer browsers, avoidance of iFrames can be enforced server side
by using the X-FRAME-OPTION header. This header can have two values,
deny and same origin, which will block any framing or framing by
sites with a different origin, respectively.
This is a countermeasure against the following threats:
o Clickjacking attacks
5.2.3. Client authentication and authorization
As described in Section 3 (Security Features), clients are
identified, authenticated and authorized for several purposes, such
as a
o Collate sub-sequent requests to the same client,
o Indicate the trustworthiness of a particular client to the end-
user,
o Authorize access of clients to certain features on the
authorization or resource server, and
o Log a client identity to log files for analysis or statics.
Due to the different capabilities and characteristics of the
different client types, there are different ways to support achieve
objectives, which will be described in this section. Authorization
server providers should be aware of the security policy and
deployment of a particular clients and adapt its treatment
accordingly. For example, one approach could be to treat all clients
as less trustworthy and unsecure. On the other extreme, a service
provider could activate every client installation by hand of an
administrator and that way gain confidence in the identity of the
software package and the security of the environment the client is
installed in. And there are several approaches in between.
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5.2.3.1. Don't issue secrets to public clients or clients with
inappropriate security policy
Authorization servers should not issue secrets to "public" clients
that cannot protect secrets. This prevents the server from
overestimating the value of a successful authentication of the
client.
For example, it is of limited benefit to create a single client id
and secret which is shared by all installations of a native
application. Such a scenario requires that this secret must be
transmitted from the developer via the respective distribution
channel, e.g. an application market, to all installations of the
application on end-user devices. A secret, burned into the source
code of the application or a associated resource bundle, cannot be
entirely protected from reverse engineering. Secondly, such secrets
cannot be revoked since this would immediately put all installations
out of work. Moreover, since the authorization server cannot really
trust on the client's identity, it would be dangerous to indicate to
end-users the trustworthiness of the client.
There are other ways to achieve a reasonable security level, as
described in the following sections.
5.2.3.2. Public clients without secret require user consent
Authorization servers should not allow automatic authorization for
public clients. The authorization may issue a client id, but should
require that all authorizations are approved by the end-user. This
is a countermeasure for clients without secret against the following
threats:
o Impersonation of public client applications
5.2.3.3. Client_id only in combination with redirect_uri
The authorization may issue a client_id and bind the client_id to a
certain pre-configured redirect_uri. Any authorization request with
another redirection URI is refused automatically. Alternatively, the
authorization server should not accept any dynamic redirection URI
for such a client_id and instead always redirect to the well-known
pre-configured redirection URI. This is a countermeasure for clients
without secrets against the following threats:
o Cross-site scripting attacks
o Impersonation of public client applications
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5.2.3.4. Deployment-specific client secrets
A authorization server may issue separate client identifiers and
corresponding secrets to the different deployments of a client. The
effect of such an approach would be to turn otherwise "public"
clients back into "confidential" clients.
For web applications, this could mean to create one client_id and
client_secret per web site a software package is installed on. So
the provider of that particular site could request client id and
secret from the authorization server during setup of the web site.
This would also allow to validate some of the properties of that web
site, such as redirection URI, address, and whatever proofs useful.
The web site provider has to ensure the security of the client secret
on the site.
For native applications, things are more complicated because every
installation of the application on any device is another deployment.
Deployment specific secrets will require
1. Either to obtain a client_id and client_secret during download
process from the application market, or
2. During installation on the device.
Either approach will require an automated mechanism for issuing
client ids and secrets, which is currently not defined by OAuth.
The first approach would allow to achieve a level where the client is
authenticated and identified, whereas the second option only allows
to authenticate the client but not to validate properties of the
client. But this would at least help to prevent several replay
attacks. Moreover, deployment-specific client_id and secret allow to
selectively revoke all refresh tokens of a specific deployment at
once.
5.2.3.5. Validation of pre-registered redirect_uri
An authorization server should require all clients to register their
redirect_uri and the redirect_uri should be the full URI as defined
in [I-D.ietf-oauth-v2]. The way this registration is performed is
out of scope of this document. As per the core spec, every actual
redirection URI sent with the respective client_id to the end-user
authorization endpoint must match the registered redirection URI.
Where it does not match, the authorization server should assume the
inbound GET request has been sent by an attacker and refuse it.
Note: the authorization server should not redirect the user agent
back to the redirection URI of such an authorization request.
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o Authorization code leakage through counterfeit web site: allows to
detect attack attempts already after first redirect to end-user
authorization endpoint (Section 4.4.1.7).
o For clients with validated properties, this measure also helps to
detect malicious applications early in the end-user authorization
process. This reduces the need for a interactive validation by
the user (Section 4.4.1.4, Section 4.4.2.3).
o Open Redirector attack via client redirection endpoint. (
Section 4.1.5. )
o Open Redirector phishing attack via authorization server
redirection endpoint ( Section 4.2.4 )
The underlying assumption of this measure is that an attacker will
need to use another redirection URI in order to get access to the
authorization code. Deployments might consider the possibility of an
attacker using spoofing attacks to a victims device to circumvent
this security measure.
Note: Pre-registering clients might not scale in some deployments
(manual process) or require dynamic client registration (not
specified yet). With the lack of dynamic client registration, it
only works for clients bound to certain deployments at development/
configuration time. As soon as dynamic resource server discovery
gets involved, that's no longer feasible.
5.2.3.6. Client secret revocation
An authorization server may revoke a client's secret in order to
prevent abuse of a revealed secret.
Note: This measure will immediately invalidate any authorization code
or refresh token issued to the respective client. This might be
unintentionally for client identifiers and secrets used across
multiple deployments of a particular native or web application.
This a countermeasure against:
o Abuse of revealed client secrets for private clients
5.2.3.7. Use strong client authentication (e.g. client_assertion /
client_token)
By using an alternative form of authentication such as client
assertion [draft-ietf-oauth-assertions], the need to distribute
client_secret is eliminated. This may require the use of a secure
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private key store or other supplemental authentication system as
specified by the client assertion issuer in its authentication
process.
5.2.4. End-user authorization
This secion involves considerations for authorization flows involving
the end-user.
5.2.4.1. Automatic processing of repeated authorizations requires
client validation
Authorization servers should NOT automatically process repeat
authorizations where the client is not authenticated through a client
secret or some other authentication mechanism such as signing with
security certificates (5.7.2.7. Use strong client authentication
(e.g. client_assertion / client_token)) or validation of a pre-
registered redirect URI (5.7.2.5. Validation of pre-registered
redirection URI ).
5.2.4.2. Informed decisions based on transparency
The authorization server should clearly explain to the end-user what
happens in the authorization process and what the consequences are.
For example, the user should understand what access he is about to
grant to which client for what duration. It should also be obvious
to the user, whether the server is able to reliably certify certain
client properties (web site address, security policy).
5.2.4.3. Validation of client properties by end-user
In the authorization process, the user is typically asked to approve
a client's request for authorization. This is an important security
mechanism by itself because the end-user can be involved in the
validation of client properties, such as whether the client name
known to the authorization server fits the name of the web site or
the application the end-user is using. This measure is especially
helpful in all situation where the authorization server is unable to
authenticate the client. It is a countermeasure against:
o Malicious application
o A client application masquerading as another client
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5.2.4.4. Binding of authorization code to client_id
The authorization server should bind every authorization code to the
id of the respective client which initiated the end-user
authorization process. This measure is a countermeasure against:
o replay of authorization codes with different client credentials
since an attacker cannot use another client_id to exchange an
authorization code into a token
o Online guessing of authorization codes
Note: This binding should be protected from unauthorized
modifications.
5.2.4.5. Binding of authorization code to redirect_uri
The authorization server should bind every authorization code to the
actual redirection URI used as redirect target of the client in the
end-user authorization process. This binding should be validated
when the client attempts to exchange the respective authorization
code for an access token. This measure is a countermeasure against
authorization code leakage through counterfeit web sites since an
attacker cannot use another redirection URI to exchange an
authorization code into a token.
5.3. Client App Security
This section deals with considerations for client applications.
5.3.1. Don't store credentials in code or resources bundled with
software packages
Because of the numbers of copies of client software, there is limited
benefit to create a single client id and secret which is shared by
all installations of an application. Such an application by itself
would be considered a "public" client as it cannot be presumed to be
able to keep client secrets. A secret, burned into the source code
of the application or a associated resource bundle, cannot be
entirely protected from reverse engineering. Secondly, such secrets
cannot be revoked since this would immediately put all installations
out of work. Moreover, since the authorization server cannot really
trust on the client's identity, it would be dangerous to indicate to
end-users the trustworthiness of the client.
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5.3.2. Standard web server protection measures (for config files and
databases)
Use standard web server protection measures - Section 5.3.2
5.3.3. Store secrets in a secure storage
The are different way to store secrets of all kinds (tokens, client
secrets) securely on a device or server.
Most multi-user operation systems segregate the personal storage of
the different system users. Moreover, most modern smartphone
operating systems even support to store app-specific data in separate
areas of the file systems and protect it from access by other
applications. Additionally, applications can implements confidential
data itself using a user-supplied secret, such as PIN or password.
Another option is to swap refresh token storage to a trusted backend
server. This mean in turn requires a resilient authentication
mechanisms between client and backend server. Note: Applications
should ensure that confidential data is kept confidential even after
reading from secure storage, which typically means to keep this data
in the local memory of the application.
5.3.4. Utilize device lock to prevent unauthorized device access
On a typical modern phone, there are many "device lock" options which
can be utilized to provide additional protection where a device is
stolen or misplaced. These include PINs, passwords and other
biomtric featres such as "face recognition". These are not equal in
their level of security they provide.
5.3.5. Platform security measures
o Validation process
o software package signatures
o Remote removal
5.3.6. Link state parameter to user agent session
The state parameter is used to link client requests and prevent CSRF
attacks, for example against the redirection URI. An attacker could
inject their own authorization code or access token, which can result
in the client using an access token associated with the attacker's
protected resources rather than the victim's (e.g. save the victim's
bank account information to a protected resource controlled by the
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attacker).
The client should utilize the "state" request parameter to send the
authorization server a value that binds the request to the user-
agent's authenticated state (e.g. a hash of the session cookie used
to authenticate the user-agent) when making an authorization request.
Once authorization has been obtained from the end-user, the
authorization server redirects the end-user's user-agent back to the
client with the required binding value contained in the "state"
parameter.
The binding value enables the client to verify the validity of the
request by matching the binding value to the user- agent's
authenticated state.
5.4. Resource Servers
The following section details security considerations for resource
servers.
5.4.1. Authorization headers
Authorization headers are recognized and specially treated by HTTP
proxies and servers. Thus the usage of such headers for sending
access tokens to resource servers reduces the likelihood of leakage
or unintended storage of authenticated requests in general and
especially Authorization headers.
5.4.2. Authenticated requests
An authorization server may bind tokens to a certain client identity
and encourage resource servers to validate that binding. This will
require the resource server to authenticate the originator of a
request as the legitimate owner of a particular token. There are a
couple of options to implement this countermeasure:
o The authorization server may associate the distinguished name of
the client with the token (either internally or in the payload of
an self-contained token). The client then uses client
certificate-based HTTP authentication on the resource server's
endpoint to authenticate its identity and the resource server
validates the name with the name referenced by the token.
o same as before, but the client uses his private key to sign the
request to the resource server (public key is either contained in
the token or sent along with the request)
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o Alternatively, the authorization server may issue a token-bound
secret, which the client uses to sign the request. The resource
server obtains the secret either directly from the authorization
server or it is contained in an encrypted section of the token.
That way the resource server does not "know" the client but is
able to validate whether the authorization server issued the token
to that client
This mechanisms is a countermeasure against abuse of tokens by
counterfeit resource servers.
5.4.3. Signed requests
A resource server may decide to accept signed requests only, either
to replace transport level security measures or to complement such
measures. Every signed request should be uniquely identifiable and
should not be processed twice by the resource server. This
countermeasure helps to mitigate:
o modifications of the message and
o replay attempts
6. IANA Considerations
This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an
RFC.
7. Acknowledgements
We would like to thank Hui-Lan Lu, Francisco Corella, Peifung E Lam,
Shane B Weeden, Skylar Woodward, Niv Steingarten, Tim Bray, and James
H. Manger for their comments and contributions.
8. References
8.1. Normative References
[I-D.ietf-oauth-v2]
Hammer, E., Recordon, D., and D. Hardt, "The OAuth 2.0
Authorization Protocol", draft-ietf-oauth-v2-23 (work in
progress), January 2012.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
8.2. Informative References
[I-D.ietf-oauth-v2-bearer]
Jones, M., Hardt, D., and D. Recordon, "The OAuth 2.0
Authorization Protocol: Bearer Tokens",
draft-ietf-oauth-v2-bearer-17 (work in progress),
February 2012.
[I-D.ietf-oauth-v2-http-mac]
Hammer-Lahav, E., "HTTP Authentication: MAC Access
Authentication", draft-ietf-oauth-v2-http-mac-01 (work in
progress), February 2012.
[I-D.lodderstedt-oauth-revocation]
Lodderstedt, T., Dronia, S., and M. Scurtescu, "Token
Revocation", draft-lodderstedt-oauth-revocation-03 (work
in progress), September 2011.
[portable-contacts]
Smarr, J., "Portable Contacts 1.0 Draft C", August 2008.
Appendix A. Document History
[[ to be removed by RFC editor before publication as an RFC ]]
draft-lodderstedt-oauth-security-01
o section 4.4.1.2 - changed "resource server" to "client" in
countermeasures description.
o section 4.4.1.6 - changed "client shall authenticate the server"
to "The browser shall be utilized to authenticate the redirection
URI of the client"
o section 5 - general review and alignment with public/confidential
client terms
o all sections - general clean-up and typo corrections
draft-ietf-oauth-v2-threatmodel-00
o section 3.4 - added the purposes for using authorization codes.
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o extended section 4.4.1.1
o merged 4.4.1.5 into 4.4.1.2
o corrected some typos
o reformulated "session fixation", renamed respective sections into
"authorization code disclosure through counterfeit client"
o added new section "User session impersonation"
o worked out or reworked sections 2.3.3, 4.4.2.4, 4.4.4, 5.1.4.1.2,
5.1.4.1.4, 5.2.3.5
o added new threat "DoS using manufactured authorization codes" as
proposed by Peifung E Lam
o added XSRF and clickjacking (incl. state parameter explanation)
o changed sub-section order in section 4.4.1
o incorporated feedback from Skylar Woodward (client secrets) and
Shane B Weeden (refresh tokens as client instance secret)
o aligned client section with core draft's client type definition
o converted I-D into WG document
draft-ietf-oauth-v2-threatmodel-01
o Alignment of terminology with core draft 22 (private/public
client, redirect URI validation policy, replaced definition of the
client categories by reference to respective core section)
o Synchronisation with the core's security consideration section
(UPDATE 10.12 CSRF, NEW 10.14/15)
o Added Resource Owner Impersonation
o Improved section 5
o Renamed Refresh Token Replacement to Refresh Token Rotation
draft-ietf-oauth-v2-threatmodel-02
o Incoporated Tim Bray's review comments (e.g. removed all normative
language)
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Authors' Addresses
Torsten Lodderstedt (editor)
Deutsche Telekom AG
Email: torsten@lodderstedt.net
Mark McGloin
IBM
Email: mark.mcgloin@ie.ibm.com
Phil Hunt
Oracle Corporation
Email: phil.hunt@yahoo.com
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