Network Working Group S. Hartman
Internet-Draft MIT
Expires: November 21, 2006 May 20, 2006
Requirements for Web Authentication Resistant to Phishing
draft-hartman-webauth-phishing-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This memo proposes requirements for protocols between web identity
providers and users and for requirements for protocols between
identity providers and relying parties. These requirements minimize
the likelihood that criminals will be able to gain the credentials
necessary to impersonate a user or be able to fraudulently convince
users to disclose personal information. To meet these requirements
browsers must change. Websites must never receive information such
as passwords that can be used to impersonate the user to third
parties. Browsers should perform mutual authentication and flag
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situations when the target website is not authorized to accept the
identity being offered as this is a strong indication of fraud.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements notation . . . . . . . . . . . . . . . . . . . . 5
3. Threat Model . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1 Capabilities of Attackers . . . . . . . . . . . . . . . . 6
3.2 Attacks of Interest . . . . . . . . . . . . . . . . . . . 7
4. Requirements for Preventing Phishing . . . . . . . . . . . . . 8
4.1 Support for Passwords . . . . . . . . . . . . . . . . . . 8
4.2 Trusted UI . . . . . . . . . . . . . . . . . . . . . . . . 8
4.3 No Password Equivelents . . . . . . . . . . . . . . . . . 9
4.4 Authenticating the Server . . . . . . . . . . . . . . . . 9
4.5 Protecting Enrollment . . . . . . . . . . . . . . . . . . 11
5. Security Considerations . . . . . . . . . . . . . . . . . . . 12
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6.1 Normative References . . . . . . . . . . . . . . . . . . . 14
6.2 Informative References . . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 14
Intellectual Property and Copyright Statements . . . . . . . . 15
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1. Introduction
Typically, web sites ask users to send a user name and password in
order to log in and authenticate their identity to the website. The
user name and plaintext password is often sent over a TLS [RFC4346]
encrypted connection. As a result of this, the server learns the
password and can pretend to be the user to any other system where the
user has used the same password. The security of passwords over TLS
depends on making sure that the password is sent to the right,
trusted server. TLS implementations typically confirm that the name
entered by the user in the URL corresponds to the certificate as
described in [RFC2818].
One serious security threat on the web today is phishing. Phishing
is a form of fraud where an attacker convinces a user to provide
confidential information to the attacker believing they are providing
the information to a party they trust with that information. For
example, an email claiming to be from a user's bank may direct the
user to go to a website and verify account information. The
attacker captures the user name and password and potentially other
sensitive information. Domain names that look like target websites,
links in email, and many other factors contribute to phishers'
ability to convince users to trust them.
It is useful to distinguish two targets of phishing. Sometimes
phishing is targeting web authentication credentials such as user
name and password. Sometimes phishing is targeting other
confidential information. This memo presents requirements that
significantly reduce the effectiveness of the first category of
phishing: these requirements guarantee that even if the user
authenticates to the wrong server, that server cannot impersonate the
user to a third party. However to combat phishing targeted at other
confidential information the best we can do is try to help the user
detect fraud before they release confidential information.
So, the approach taken by these requirements is to handle these two
types of phishing differently. Users are given some trusted
mechanism to determine whether they are typing their password into a
secure browser component that will authenticate them to the web
server or whether they are typing their password into a legacy
mechanism that will send their password to the server. If the user
types a password into the trusted browser component, they have strong
assurances that their password has not been disclosed and that the
page returned from the web server was generated by a party that
either knows their password or who is authenticated by an identity
provider who knows their password. The web server can then use
confidential information known to the user and web server to enhance
the user's trust in its identity beyond what is available given the
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social engineering attacks against TLS server authentication. If a
user enters their password into the wrong server but discovers this
before they give that server any other confidential information, then
there exposure is very limited.
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2. Requirements notation
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 [RFC2119].
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3. Threat Model
This section describes the assumed capabilities of phishers,
describes assumptions about web security and describes what
vulnerabilities exist.
We assume that the browser and operating system are secure and can be
trusted by the end user. There are many attacks against browsers and
operating systems. However without this assumption we cannot make
progress in securing web authentication. So we will assume that
browsers and operating systems are secure and note that other work to
improve the security of browsers and operating systems is critical to
the security of the entire web authentication system.
We assume that users have limited motivation to combat phishing.
Users cannot be expected to read the source of web pages, understand
how DNS works well enough to look out for spoofed domains, or
understand URI encoding. Users do not typically understand
certificates and cannot make informed decisions about whether the
subject name in a certificate corresponds to the entity they are
attempting to communicate with.
3.1 Capabilities of Attackers
Attackers can convince the user to go to a website of their choosing.
Since the attacker controls the web site and since the user chose to
go to the website the TLS certificate will verify and the website
will appear to be secure. The certificate will typically not be
issued to the entity the user thinks they are communicating with, but
as discussed above, the user will not notice this.
The attacker can convincingly replicate any part of the UI of the
website being spoofed. The attacker can also spoof trust markers
such as the security lock, URL bar and other parts of the browser UI.
There is one limitation to the attacker's ability to replicate UI.
The attacker cannot replicate a UI that depends on information the
attacker does not know. For example, an attacker could generally
replicate the UI of a banking site's login page. However the
attacker probably could not replicate the account summary page until
the attacker learned the user name and password.
The attacker can convince the user to do anything with the phishing
site that they would do with the real target site. As a consequence,
if we want to avoid the user giving the attacker their password, we
must transition to a solution where the user would not give the real
target site their password. Instead they will need to
cryptographically prove that they know their password without
revealing it.
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3.2 Attacks of Interest
The primary attack of interest is an attack in which the user sends
confidential information to an unintended recipient.
Another significant attack is an attack in which a recipient gains
sufficient credentials to impersonate the user to other recipients.
The obvious version of this attack is when the recipient learns the
password of the user. However even giving the recipient a time-
limited token that can be used to impersonate the user would be an
instance of this attack. Note that some authentication systems such
as Kerberos [RFC4120] provide a facility to delegate the ability to
act as the user to the target of the authentication. Such a facility
when used with an inappropriately trusted target would be an instance
of this attack.
Of less serious concerns at the present time are attacks on data
integrity where a phisher provides false or misleading information to
a user.
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4. Requirements for Preventing Phishing
This section describes requirements for web authentication solutions.
These solutions are intended to prevent phishing targeted at
obtaining web authentication credentials. These requirements will
make it more difficult for phishers to target other confidential
information.
The requirements discussed here are similar to the principles
outlined in "Limits to Anti-Phishing" [ANTIPHISHING]. Most of this
work was discovered independently but work from that paper has been
integrated where appropriate. Google's perspective on phishing is
very interesting because of their operational experience.
4.1 Support for Passwords
The web authentication solution MUST support passwords and MUST be
secure even when passwords are commonly used. In many environments,
users need the ability to walk up to a computer they have never used
before and log in to a website. Carrying a smart card or USB token
significantly increases the deployment cost of the website and
decreases user convenience.
IT is desirable that a solution support other forms of authentication
such as smart cards and one-time passwords as these are useful in
some environments.
4.2 Trusted UI
Users need the ability to trust components of the UI in order to know
that the UI is being presented by a trusted component of the
browser. The primary concern is to make sure that the user knows
the password is being given to trusted software rather than being
filled into an HTML form element that will be sent to the server.
There are three basic approaches to establishing a trusted UI. The
first is to use a dynamic UI based on a secret known by the UI; the
Google paper [ANTIPHISHING] recommends this approach. A second
approach is to provide a UI action that highlights trusted or non-
trusted components in some way. This could work similarly to the
Expose feature in Apple's OS X where a keystroke visually
distinguishes structural elements of the UI. Of course such a
mechanism would only be useful if users actually used it. Finally,
the multi-level security community has extensive research in
designing UIs to display classified, compartmentalized information.
It is critical that these UIs be able to label information and that
these labels not be spoofable.
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In addition to making sure that passwords are only given to trusted
components, trusted UI will play another important role in the
overall solution to phishing. Once the user is authenticated to the
website then the website can use trusted UI based on a secret shared
between the user and website to convince the user that they have
authenticated to the correct site. This use of trusted UI dependes
critically on the requirements of Section 4.4 to guarantee that the
phisher cannot obtain the secret. It is tempting to use this form of
trusted UI before authentication. For example, a website could
request a user name and then display information based on a secret
for that user before accepting a password. The problem with this
approach is that phishers can obtain this information, because it can
be obtained without knowing the password. However if the trusted UI
is displayed after authentication then phishers could not obtain the
trusted UI. This is one of the many reasons why it is important to
prevent phishing targeted at authentication credentials.
4.3 No Password Equivelents
A critical requirement is that when a user authenticates to a
website, the website MUST NOT receive a password equivalent. A
password equivalent is anything that would allow a phisher to
authenticate to a third party as the user.
There are two implications of this requirement. First, strong
cryptographic authentication protocol needs to be used instead of
sending the password encrypted over TLS. The zero-knowledge class
of password protocols such as those discussed in section 8 of the
IAB authentication mechanisms document [IABAUTH] seem potentially
useful in this case. Note that mechanisms in this space tend to have
significant deployment problems because of intellectual property
issues.
The second implication of this requirement is that if an
authentication token is presented to a website, the website MUST NOT
be able to modify the token to authenticate as the user to a third
party. The party generating the token must cryptographically bind it
to either the website that will receive the token or to a key known
only to the user. If tokens are bound to keys, the user MUST prove
knowledge of this key as part of the authentication process. The key
MUST not be disclosed to the server unless the token is bound to the
server and the key is only used with that token.
4.4 Authenticating the Server
The Google paper [ANTIPHISHING] describes a requirement for mutual
authentication. A common phishing practice is to accept a user name
and password as part of an attempt to make the phishing site
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authentic. The real target is some other confidential information.
The user name and password are captured, but are not verified. After
the user name and password are entered, the phishing site collects
other confidential information.
Authentication of the server at the TLS level and authentication of
the client within the TLS session is not sufficient. AS discussed
previously the attacker can direct the user to a site that the
attacker controls so the TLS authentication will succeed. Then the
attacker can either ignore the authentication or attempt to tunnel
the authentication exchange back to the real site. If successful
such tunneling could allow the attacker to access the real site as
the user.
If authentication is based on a shared secret such as a password,
then the authentication protocol MUST prove that the secret or a
suitable verifier is known by both parties. Interestingly the
existence of a shared secret will provide better protection that the
right server is being contacted than if public key credentials are
used. By their nature, public key credentials allow parties to be
contacted without a prior security association. In protecting
against phishing targeted at obtaining other confidential
information, this may prove a liability. However public key
credentials provide strong protection against phishing targeted at
obtaining authentication credentials because they are not vulnerable
to dictionary attacks. Such dictionary attacks are a significant
weakness of shared secrets such as passwords intended to be
remembered by humans.
In situations where there is an identity provider that is separate
from the website as a relying party, additional requirements are
needed. The identity provider MUST verify that the website is a
valid relying party for this identity. Some identity providers will
allow anyone to accept their identity. However particularly for
financial institutions and large service providers it will be common
that only authorized business partners will be able to accept the
identity. The confirmation that the the relying party is such a
business partner will often be a significant part of the value
provided by the identity provider, so it is important that the
protocol enable this. The relying party MUST prove its identity is
the one expected by the identity provider.
This set of requirements is incompatible with a model where the
identity of the relying party is hidden from the identity provider.
Sites that need this level of privacy may wish to provide their own
identities or to provide aggregation points that can separate the
identity provider from the site. The aggregation point needs to know
the relying party, but the identity provider only needs to know the
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aggregation point.
To prevent attacks where the authentication is tunneled, the protocol
between the web browser and website MUST bind the authentication
exchange to the channel created by the TLS session. The general
concept behind channel binding is discussed in section 2.2.2 of
[BTNS]. This paragraph needs to be expanded to point to proposals
for doing channel binding with TLS. xxx
4.5 Protecting Enrollment
One area of particular vulnerability to phishing is enrollment.
Protecting against phishing targeted at obtaining other confidential
information as a new service is established is outside the scope of
this document. If confidential information such as credit card
numbers are provided as part of account setup, then this may be a
target for phishing.
However there is one critical aspect in which enrollment impacts the
security of authentication. During enrollment, a password is
typically established for an account at an identity provider. The
process of establishing a password MUST NOT provide a password
equivalent to the identity provider. That is, the identity provider
MUST NOT gain enough information to impersonate the user to a third
party while establishing a password.
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5. Security Considerations
This memo discusses the security of web authentication and how to
minimize the risk of phishing in web authentication systems. This
section discusses the security of the overall system and discusses
how components of the system that are not directly within the scope
of this document affect the security of web transactions. This
section also discusses residual risks that remain even when the
requirements proposed here are implemented.
The approach taken here is to separate the problem of phishing into
phishing targeted at web authentication credentials and phishing
targeted at other information. Users are given some trusted
mechanism to determine whether they are typing their password into a
secure browser component that will authenticate them to the web
server or whether they are typing their password into a legacy
mechanism that will send their password to the server. If the user
types a password into the trusted browser component, they have strong
assurances that their password has not been disclosed and that the
page returned from the web server was generated by a party that
either knows their password or who is authenticated by an identity
provider who knows their password. The web server can then use
confidential information known to the user and web server to enhance
the user's trust in its identity beyond what is available given the
social engineering attacks against TLS server authentication. If a
user enters their password into the wrong server but discovers this
before they give that server any other confidential information, then
there exposure is very limited.
This model assumes that the browser and operating system are a
trusted component. As discussed in Section 3, there are numerous
attacks against host security. Appropriate steps should be taken to
minimize these risks. If the host security is compromised, the
password can be captured as it is typed by the user.
This model assumes that users will only enter their passwords into
trusted browser components. There are several potential problems
with this assumption. First, users need to understand the UI
distinction and know what it looks like when they are typing into a
trusted component and what a legacy HTML form looks like. Users must
care enough about the security of their passwords to only type them
into trusted components. The browser must be designed in such a way
that the server cannot create a UI component that appears to be a
trusted component but is actually a legacy HTML form; Section 4.2
discusses this requirement.
IN addition, a significant risk that users will type their password
into legacy HTML forms comes from the incremental deployment of any
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web authentication technology. Websites will need a way to work with
older web browsers that do not yet support mechanisms that meet these
requirements. Not all websites will immediately adopt these
mechanisms. Users will sometimes browse from computers that have
mechanisms meeting these requirements and sometimes from older
browsers. They only gain protection from phishing when they type
passwords into trusted components. If a password is sometimes used
with websites that meet these requirements and sometimes with legacy
websites, and if the password is captured by a phisher targeting a
legacy website, then that captured password can be used even on
websites meeting these requirements. Similarly, if a user is tricked
into using HTML forms when they should not, passwords can be exposed.
Users can significantly reduce this risk by using different passwords
for websites that use trusted browser authentication than for those
that still use HTML forms.
The risk of dictionary attack is always a significant concern for
password systems. Users can (but typically do not) minimize this
risk by choosing long, hard to guess phrases for passwords. The risk
can be removed once a password is already established by using a
zero-knowledge password protocol. However the risk of dictionary
attack is always present when setting up a new password or changing
a password. Minimizing the number of services that use the same
password and being extra careful to make sure the right server is
used when establishing a password can minimize this risk.
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6. References
6.1 Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
6.2 Informative References
[ANTIPHISHING]
Nelson, J. and D. Jeske, "Limits to Anti Phishing",
January 2006.
Proceedings of the W3c Security and Usability Workshop; ht
tp://www.w3.org/2005/Security/usability-ws/papers/
37-google/'
[BTNS] Touch, J., "Problem and Applicability Statement for Better
Than Nothing Security",
draft-ietf-btns-prob-and-applic-02.txt (work in progress),
February 2006.
[IABAUTH] Rescorla, E., "A Survey of Authentication Mechanisms",
draft-iab-auth-mech-05.txt (work in progress),
February 2006.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
July 2005.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006.
Author's Address
Sam Hartman
Massachusetts Institute of Technology
Email: hartmans-ietf@mit.edu
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