Network Working Group S. Hartman
Internet-Draft Painless Security
Intended status: Informational August 15, 2008
Expires: February 16, 2009
Requirements for Web Authentication Resistant to Phishing
draft-hartman-webauth-phishing-08.txt
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Abstract
This memo proposes requirements for protocols between web browsers
and relying parties at websites; these requirements also impact third
parties involved in the authentication process. 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 authenticate the website
to the browser as part of authenticating the user to the website.
Browsers MUST flag situations when this authentication fails and flag
situations when the target website is not authorized to accept the
identity being offered as this is a strong indication of fraud.
These requirements may serve as a basis for requirements for
preventing fraud in environments other than the web.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Purpose of this Memo . . . . . . . . . . . . . . . . . . . 5
1.2. Passwords and Interface . . . . . . . . . . . . . . . . . 6
2. Requirements notation . . . . . . . . . . . . . . . . . . . . 7
3. Threat Model . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1. Capabilities of Attackers . . . . . . . . . . . . . . . . 9
3.2. Attacks of Interest . . . . . . . . . . . . . . . . . . . 10
4. Requirements for Authentication that Protects Credentials . . 11
4.1. Support for Passwords and OTher Methods . . . . . . . . . 11
4.2. Trusted UI . . . . . . . . . . . . . . . . . . . . . . . . 11
4.3. No Password Equivelents . . . . . . . . . . . . . . . . . 12
4.4. Mutual Authentication . . . . . . . . . . . . . . . . . . 13
4.5. Authentication Tied to Request and Response . . . . . . . 14
4.6. Restricted Identity Providers . . . . . . . . . . . . . . 15
4.7. Protecting Enrollment . . . . . . . . . . . . . . . . . . 15
5. Is it the right Server? . . . . . . . . . . . . . . . . . . . 17
6. Iana Considerations . . . . . . . . . . . . . . . . . . . . . 19
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20
8. Security Considerations . . . . . . . . . . . . . . . . . . . 21
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
9.1. Normative References . . . . . . . . . . . . . . . . . . . 23
9.2. Informative References . . . . . . . . . . . . . . . . . . 23
Appendix A. Trusted UI Mechanisms . . . . . . . . . . . . . . . . 25
Appendix B. Change History . . . . . . . . . . . . . . . . . . . 26
B.1. Changes since 07 . . . . . . . . . . . . . . . . . . . . . 26
B.2. Changes since 06 . . . . . . . . . . . . . . . . . . . . . 26
B.3. Changes since 05 . . . . . . . . . . . . . . . . . . . . . 27
B.4. Changes since 02 . . . . . . . . . . . . . . . . . . . . . 27
B.5. Changes since 01 . . . . . . . . . . . . . . . . . . . . . 27
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 29
Intellectual Property and Copyright Statements . . . . . . . . . . 30
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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 are often sent over a TLS [RFC4346]
encrypted connection. As a result of this plaintext password
protocol, the server learns the password and can pretend to be the
user to any other system where the user has used the same user name
and password. The security of passwords over TLS depends on making
sure that the password is sent to the right, trusted server and on
that server not exposing the password to third parties. HTTPs
[RFC2818] implementations typically confirm that the name entered by
the user in the URL corresponds to the certificate.
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.
Typically the user names and password are not directly valuable to
the phisher. However they can be used to access resources of value.
For example a bank password may permit money transfer or access to
information useful in identity theft.
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, such as bank account numbers. This memo
presents requirements that can be part of a solution to significantly
reduce the effectiveness of the first category of phishing: provided
that a user uses an authentication mechanism that meets these
requirements, 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 know how to do is help the user detect fraud before they
release confidential information.
The approach taken by this memo is to handle these two types of
phishing differently. The user is given new authentication
mechanisms. If the user uses these mechanisms,they have strong
assurances that their password has not been disclosed and that the
ensuing data returned from the server was generated by a party that
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either knows their password or who is authenticated by an identity
provider (a third party involved in the authentication exchange in
order to allow credentials to be used across a wider variety of
websites) who knows their password. The server can then use
confidential information known to the user and 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 authenticates to the wrong server but discovers this before they
give that server any other confidential information, then there
exposure is very limited. The success of this solution depends
heavily on whether the user uses the new authentication mechanisms;
designing ways for users to tell if they are using the authentication
mechanisms and encouraging users to use these mechanisms will be
critical to achieving any security benefit from these requirements.
The success of a solution to preventing the disclosure of other
confidential information based on giving users information about
whether they are authenticated to the right server depends on the
user being able to take advantage of this information and choosing to
do so.
The requirements presented in this memo are intended to be useful to
browser designers, designers of other HTTP applications and designers
of future HTTP authentication mechanisms.
These requirements and mechanisms that meet these requirements are
not sufficient to stop phishing; at best, they form part of a
solution. The World Wide Web Consortium proposes recommendations on
user interface guidelines for web security context [WSCUIG]. These
guidelines propose mechanisms that will make it more likely that
users will detect fraud before authentication. Efforts to limit the
effect of malicious software and to provide trustable software for
authentication are also important. Efforts to track known frauds and
alert users when they encounter fraudulent sites are also critical.
Together, all these efforts may significantly reduce phishing.
1.1. Purpose of this Memo
In publishing this memo, the IETF recommends the development of web
authentication mechanisms that meet the requirements outlined in
Section 4. It is hoped that developing these mechanisms will prove a
useful step in fighting phishing. However this memo does not
restrict work either in the IETF or any other organization. In
particular, new authentication efforts are not bound to meet the
requirements posed in this memo unless the charter for those efforts
chooses to make these binding requirements. Less formally, the IETF
presents this memo as an option to pursue while acknowledging that
there may be other promising paths both now and in the future.
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1.2. Passwords and Interface
There are two related concepts: the user interface of passwords and
plaintext password protocols. A plaintext password protocol is a
protocol where the server receives credentials sufficient to
impersonate a user to third parties. A password interface provides a
user experience where a user types a password into any computer,
including one they have never used before and that is sufficient to
authenticate. The requirements in this memo require support for
password user interfaces as one option for authentication. The
requirements of this memo are incompatible with plaintext password
protocols.
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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. Human factors issues contribute significantly
to these vulnerabilities. For example, security information
dialogues in web browsers can provide information on the subject of a
certificate. However, users rarely examine this information, so an
attacker could be successful even if examining the security dialogue
would show an attack. This threat model is intended to include these
sorts of attacks and so it is broader than the technical threats
against protocols. Efforts are under way to improve these human
factors issues [WSCUIG]. However these efforts only reduce the risk
that a user will be confused; even given improved user experience for
dealing with security context information, users will make mistakes
and believe that an attacker's site is the site they intended to
communicate with.
We assume that the implementations of authentication systems can be
trusted sufficiently to meet their design objectives. This does not
mean that the entire local system and browser need to be trusted.
However if there is a component that has access to users' passwords,
that component needs to be secure enough to be trusted not to divulge
passwords to attackers. Similarly in a system that used smart cards,
the smart cards would need to be trusted not to give attackers access
to private keys or other authentication material. Designing
implementations to limit the size and complexity of the most trusted
components and to layer trust will be important to the security of
implementations. Designing protocols to enable good implementation
will be critical to the utility of the protocols. As a consequence
of this assumption, these requirements are insufficient to provide
protection against phishing if malicious browser extensions, Trojan
software or other malicious software is installed into a sufficiently
trusted part of the local computer or authentication tokens.
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. As a consequence of this assumption,
users will likely be fooled by strings either in website names or
certificates that look visually similar but that are composed of
different code points.
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3.1. Capabilities of Attackers
We assume 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.
Mechanisms attackers use to accomplish this include links with a
misleading name or URI, which they may distribute in emails; attacks
against DNS; and man-in-the-middle attacks against a TLS handshake.
The former two attacks allow the attacker to pass authentication
because the victim user can be tricked into accepting the attacker's
certificate. The latter attack will typically create a warning on
the victim user's side, but many users do not make informed decisions
on how to respond to such a warning, making them inclined to accept
the bogus certificate.
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
sufficiently that a significant class of users will not treat the
spoofed security indicators as a problem. 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 because the attacker would not know what
accounts to list or approximate balances that will look convincing to
a user. Of course attackers may know some personal information about
a user. Websites that want to rely on attackers not knowing certain
information need to maintain the privacy of that information.
It's not clear how valuable this limitation on the attacker's ability
will prove in practice. Research into the effectiveness of security
indicators [SECIND] suggests that users do not pay attention to
security indicators. One difference between the security indicators
tested in today's research and using private information to detect
fraud is that the private information may be directly related to the
task the user is trying to perform. However the attacker can attempt
to come up with a convincing explanation such as a partial outage or
system upgrade for why the private information is not available.
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,
when passwords are used, if we want to avoid the user giving the
attacker their password, the web site must prove that it has an
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established authentic relationship with the user without requiring a
plaintext password protocol. One approach could be to transition to
a solution where the user could not give the real target site their
password if they are using a new mechanism. Instead they will need
to cryptographically prove that they know their password without
revealing it.
3.2. Attacks of Interest
The ultimate goal of these requirements is to provide protection
against disclosure of confidential information to unintended parties.
These requirements focus on two such disclosures and handle them
separately. The first category is disclosure of credentials that
could allow an unintended party to impersonate the user, possibly
gaining access to additional confidential information. The second
attack is disclosure of confidential information not directly related
to authentication. The second class of attack cannot be directly
defeated, but we can give information to users that they could use to
help know when they are communicating with an unintended party.
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 the first
class of attack. Solutions to these requirements with similar
facilities MUST discuss the security considerations surrounding use
of these facilities.
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 Authentication that Protects Credentials
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.
4.1. Support for Passwords and OTher Methods
The web authentication solution MUST support the password user
interface and MUST be secure even when the password interface is
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. The
smart card is costly to deploy because it requires a process for
replacing smart cards, requires support staff to be trained in
answering questions regarding smart cards and requires a smart card
to be issued when an identity is issued. Smart cards are less
convenient because users cannot gain access to protected resources
without having their card physically with them. Many public access
computers do not have smart cards available and do not provide access
to USB ports; when they do they tend not to support smart cards. It
is important not to underestimate the training costs (either in money
or user frustration) of teaching people used to remembering a user
name and password about a new security technology. Sites that
aggregate identity--for example allowing a user to log into an
identity provider and then gain access to other resources may be a
significant part of a solution. However we cannot assume that a
given user will have only one such website: there are valid and
common reasons a user (or the relying party) would not trust all
identity information to one such site.
A solution to these requirements MUST also support smart cards and
other authentication solutions. Some environments have security
requirements that are strong enough that passwords simply are not a
viable option. Many efforts are under way to reduce the deployment
costs of token-based authentication mechanisms and to address some of
the concerns that make passwords a requirement today.
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 device.
The primary concern is to make sure that the user knows any password
is being given to trusted software rather than being filled into an
HTML form element that will be sent to the server as part of a
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plaintext password protocol.
There are many approaches to establishing a trusted UI. One example
is to use a dynamic UI based on a secret shared by the user and the
local UI; the paper [ANTIPHISHING] recommends this approach. The W3C
recommends this approach for security indicators in section 7.1 of
its user interface guidelines [WSCUIG]. However, the W3C notes that
research suggests users may not pay attention to these trust
indicators. 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 Mac 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, another potential approach is to benefit from extensive
research in the multi-level security community 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. These approaches are not exhaustive and may not even
be good; they are provided to demonstrate that thought into how to
design trusted UIs is ongoing. However, designing a user interface
that allows users of the web to distinguish trusted components from
components potentially controlled by an attacker is an open problem.
It is likely that transitioning to many new security protocols will
depend on a solution to this problem.
4.3. No Password Equivelents
A critical requirement is that when a user authenticates to a
website, the website MUST NOT receive a strong password equivalent
[IABAUTH]. A strong password equivalent is anything that would allow
a phisher to authenticate as a user with a different website.
Consequently, plaintext password protocols are incompatible with
these requirements. Weak password equivalents (quantities that act
as a password for a given service but cannot be reused with other
services ) are problematic outside of the context of enrolling a user
or changing a password. The requirement for mutual authentication
Section 4.4 is incompatible with sending weak password equivalents in
every authentication. Even if that requirement is relaxed, the scope
of a particular weak password equivalent needs to be carefully
considered. Consider for example a protocol that hashes a password
and the host name component of a URI together to form a weak password
equivalent. The same password equivalent is used regardless of which
certificate authority certifies the public key of the website. If an
attacker mounted a man-in-the-middle attack, presenting a self-signed
certificate, and the user accepted the certificate when asked by the
browser, then the attacker would receive the same weak password
equivalent needed to access the legitimate website. Such a protocol
would not do a good job of addressing the threats outlined in the
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threat model. However if mutual authentication were not a
requirement, a protocol that hashed a password and the public key
from the TLS certificate of the website to form a weak password
equivalent might meet the other requirements. In any event, weak
password equivalents MUST NOT be sent without confidentiality
protection.
There are two implications of this requirement. First, a 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 at a first glance. However, 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 bind it to either the
website that will receive the token or to a key known only to the
user. Binding could include cryptographic binding or mechanisms such
as issuing a one-time password for use with a specific website. 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 or server.
4.4. Mutual Authentication
[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 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. When mutual authentication fails, there is a strong
indication of a problem: either the user supplied the wrong
credential or the website is not the one the user intended to
communicate with.
Requiring mutual authentication excludes a class of mechanisms where
a weak password equivalent is generated for the server and is sent.
One prominent member of this class is [PWDHASH]; this mechanism has
the desirable property that it requires no change to the server and
can be implemented locally on the browser. These mechanisms provide
better security than plaintext password protocols. However attacks
where the server ignores authentication in order to obtain
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confidential information are important enough that it is desirable to
develop mechanisms that provide this assurance. The desire to
develop these new mechanisms is not intended to discourage the
deployment of mechanisms like Pwdhash that improve security today.
Typically one protocol performs authentication of both parties.
There tend to be opportunities for a man-in-the-middle attack when
one protocol authenticates one direction and another protocol
authenticates the opposite direction. Sometimes, as in the case of
TLS and plaintext password protocols, the opportunity for attacks
depends on human factors issues or certificate management. In other
cases, attacks may be more direct. Authentication of the server and
client at the TLS level is sufficient to meet the requirement of
mutual authentication. 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
confidence that the right server is being contacted than if public
key credentials are used in their typical mode. 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. For public key
protocols, the mutual authentication requirement would mean that the
server typically needs to sign an assertion of what identity it
authenticated or of the request as a whole.
4.5. Authentication Tied to Request and Response
Users expect that whatever party they authenticate to will be the
party that generates the content they see. One possible phishing
attack is to insert the phisher between the user and the real site as
a man-in-the-middle. On today's websites, the phisher typically
gains the user's user name and password. Even if the other
requirements of this specification are met, the phisher could gain
access to the user's session on the target site. This attack is of
particular concern to the banking industry. A man-in-the-middle may
gain access to the session which may give the phisher confidential
information or the ability to execute transactions on the user's
behalf.
The authentication system MUST guarantee to the user and the target
server that the request was generated by the authenticated user and
the response is generated by the target server . This can be done in
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several ways including:
1. Assuming that only certificates from trusted CAs are accepted and
the user has not bypassed server certificate validation, it is
sufficient to confirm that the identity of the server at the TLS
level is the same at the HTTP authentication level. In the case
of TLS client authentication this is trivially true. Note
however that [WSCUIG] recommends accepting self-signed
certificates in some cases, so relying on this approach for cases
other than TLS authentication may be problematic.
2. Another alternative is to bind the authentication exchange to the
channel created by the TLS session. The general concept behind
channel binding is discussed in [RFC5056]. Channel binding has
been added to HTTP authentication mechanisms based on digest
authentication and on GSS-API, suggesting that support for
channel binding is workable for future HTTP authentication
mechanisms.
4.6. Restricted Identity Providers
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
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. For such identities, the
user MUST be assured that the target server is authorized by the
identity provider to accept identities from that identity provider.
Several mechanisms could be used to accomplish this:
1. The target server can provide a certificate issued by the
identity provider as part of the authentication.
2. The identity provider can explicitly approve the target server.
For example in a redirect-based scheme the identity provider
knows the identity of the relying party before providing claims
of identity to that party. A similar situation happens with
Kerberos or Digest Authentication in a AAA infrastructure
[RFC5090].
4.7. Protecting Enrollment
One area of particular vulnerability to phishing is enrollment of a
new identity in an authentication system. Protecting against
phishing targeted at obtaining other confidential information as a
new service is established is outside the scope of this document. If
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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 or other security credentials
are associated with an account. The process of establishing a
password MUST NOT provide a strong password equivalent (a quantity
such as the password itself that could be used to log into another
service where the same password is used as the user). That is,
parties other than the user and web browser MUST NOT gain enough
information to impersonate the user to a third party while
establishing a password.
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5. Is it the right Server?
In Section 4, requirements were presented for web authentication
solutions to minimize the risk of phishing targeted at web access
information. This section discusses in a non-normative manner
various mechanisms for determining that the right server has been
contacted. Authenticating to the right party is an important part of
reducing the risk of phishing targeted at other confidential
information.
Validation of the certificates used in TLS and verification that the
name in the URI maps to these certificates can be useful. As
discussed in Section 3, attackers can spoof the name in the URI.
However the TLS checks do defeat some attacks. The W3C user
interface guidelines may significantly increase the value of these
checks [WSCUIG]. As discussed in Section 4.5, TLS validation may be
important to higher-level checks.
A variety of initiatives propose to assign trust to servers. This
includes proposals to allow users to indicate certain servers are
trusted based on information they enter. Also, proposals to allow
third parties including parties established for this purpose and
existing certificate authorities to indicate trust have been made.
These proposals will almost certainly make phishing more difficult.
In the case where there is an existing relationship, these
requirements provide a way that information about the relationship
can be used to provide assurance that the right party has been
contacted.
In Section 4.2, we discuss how a secret between the user and their
local computer can be used to let the user know when a password will
be handled securely. A similar mechanism can be used to help the
user once they are authenticated to the website. The website can
present information based on a secret shared between the user and
website to convince the user that they have authenticated to the
correct site. This depends critically on the requirements of
Section 4.5 to guarantee that the phisher cannot obtain the secret.
Various schemes have used a secret shared between the server and the
web browser before authentication. Cookies or some other state
management mechanism are used to select the right secret to display
as the user logs into the site. Unfortunately these schemes have
proven ineffective in practice [SECIND]. However, the set of
information that can be used as contextual clues to evaluate whether
the right server has been reached after authentication is much
greater. For example, a bank server knows what accounts a user has
and knows their balances. A business partner may have information
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about past transactions or the current state of transactions. If
this information is related to the task that the user is trying to
perform, they may be more likely to evaluate it and notice problems
than they are to notice a missing security indicator before login.
Strong authentication mechanisms enable this type of evaluation after
the user has logged in. However it is not known how effective this
will be in practice.
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6. Iana Considerations
This document requests no action of IANA.
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7. Acknowledgments
I'd like to thank the MIT Kerberos Consortium for its funding of work
on this memo prior to April 2008.
I'd like to thank Nicolas Williams, Matt Knopp and David Blumenthal
for helping me walk through these requirements and make sure that if
a solution met them it would actually protect against the real world
attacks consumers of our technology are facing. I was particularly
focusing on attacks that financial institutions are seeing and their
help with this was greatly appreciated.
I'd like to thank Eric Rescorla and Ben Laurie for their significant
comments on this draft.
Eliot Lear provided many last call comments and helped work through
several long standing issues with the document.
Christian Vogt provided text and review comments.
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. It seems good that these requirements
are similar to the principles outlined by someone facing phishing as
an operational reality.
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8. 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--a component that presents a password interface--or whether
they are typing their password into a legacy mechanism that will send
their password to the server as part of a plaintext password
protocol. 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 parts of the browser and operating system
with access to passwords or other long-term credentials are trusted
software. As discussed in Section 3, there are numerous attacks
against host security. Appropriate steps should be taken to minimize
these risks. If the security of the trusted software 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. It is not
clear that we have yet developed a solution to this user interface
problem. 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; the W3C user interface guidelines [WSCUIG] provides
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requirements that are designed to prevent security sensitive user
interface from being spoofed by attacker-supplied content. The W3C
guidelines provide requirements for a more limited context focused
around security context but not authentication information. However
starting from these requirements may be a successful approach.
In addition, a significant risk that users will type their password
into legacy HTML forms comes from the incremental deployment of any
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 the same 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
of offline dictionary attack can be removed once a password is
already established by using a zero-knowledge password protocol. The
risk of online dictionary attack is always present. The risk of
offline 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 can minimize this risk. When zero-
knowledge password protocols are used, being extra careful to make
sure the right server is used when establishing a password can
significantly reduce this risk.
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9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[WSCUIG] Roessler , T. and A. Saldhana , "Web Security Context:
User Interface Guidelines", W3C Working Draft, July 2008,
<http://www.w3.org/TR/wsc-ui/>.
Publication of this draft needs to block until and unless
this references is approved as some form of W3C
recommendation.
9.2. 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/'
[IABAUTH] Rescorla, E., "A Survey of Authentication Mechanisms",
draft-iab-auth-mech-05.txt (work in progress),
February 2006.
[PWDHASH] Ross, B., Jackson, C., Miyake, N., Boneh, D., and J.
Mitchell, "Stronger Password Authentication Using Browser
Extensions", Proceedings 14th Usenix Security Symposium,
2005, <http://crypto.stanford.edu/PwdHash/pwdhash.pdf>.
[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.
[RFC5056] Williams, N., "On the Use of Channel Bindings to Secure
Channels", RFC 5056, November 2007.
[RFC5090] Sterman, B., Sadolevsky, D., Schwartz, D., Williams, D.,
and W. Beck, "RADIUS Extension for Digest Authentication",
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RFC 5090, February 2008.
[SECIND] Schechter , S., Dhamija , R., Ozment, A., and I. Fischer,
"The Emperor's New Security Indicators: An evaluation of
website authentication and the effect of role playing on
usability studies", IEEE Symposium on Security and
Privacy, May 2007, <http://www.deas.harvard.edu/~rachna/
papers/
emperor-security-indicators-bank-sitekey-phishing-
study.pdf>.
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Appendix A. Trusted UI Mechanisms
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 user;
[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.
See Section 5 for another case where confidential information in a UI
can be used to build trust.
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Appendix B. Change History
B.1. Changes since 07
Reword the abstract not to talk about identity providers
Define identity provider. I'm moving away from using it except
where necessary, but I think that there a couple of cases where
the term is helpful rather than confusing.
Add a paragraph to the introduction helping to define how this
work fits in with other work.
Significantly rework the mutual authentication requirement to
describe why pwdhash is excluded, to give more motivation and to
try and clarify that authentication at different layers is
problematic
Rework the requirement for binding authentication to requests and
responses. The discussion of channel binding was obsolete and has
been updated based on advances in that area. Drop the comment
about redirect based schemes, because that depends on certificate
validation and the W3C guidelines recommend accepting self-signed
certificates in some cases.
Remove most references to identity providers from restricted
identities section and protecting enrollment section. The
concepts don't actually depend on whether an identity provider is
used.
Rework the section on finding the right server to provide a more
accurate description of image hints prior to login and to discuss
the uncertainty surrounding the effectiveness of strategies
discussed.
Rephrase terminology in security considerations to be consistent
with changes throughout the rest of the document. Refer to the
W3C guidelines as appropriate.
B.2. Changes since 06
Much expanded description of concerns about weak password
equivalents. They are not excluded except by the mutual
authentication requirement. However there are significant scoping
issues with them.
Clarify that the effectiveness of confidential information being
used to strengthen mutual authentication depends on users taking
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advantage of that.
Continue to clarify differences between plaintext password
protocols and the password user interface
Reduce the use of the term identity provider; it's not entirely
clear that concept needs to be worked in here and right now
identity provider is an undefined term
The text on how to make trusted UIs sounded very authoritative;
that was not the intent, so rework that text.
B.3. Changes since 05
Clarified introduction to distinguish what happens at the TLS
layer and what at the HTTP layer. Discuss motivation of phishing
more.
In the introduction, restate claims to be more accurate. These
requirements are useful if users actually use the authentication
mechanisms; convincing them to do so and making it obvious whether
they are is a significant risk. Also, we may give them the
theoretical information necessary to detect fraud, but whether
they act on that is open.
Add a purpose of this memo section. Whatever text ends up there
after community discussion needs to be called out in the last
call.
Add a section calling out the difference between plaintext
password protocols and password interface. This needs to be
worked into the rest of the document.
Update the threat model. Significant hopefully clarifying
changes.
B.4. Changes since 02
Updated discussion of TLS authentication to point out that it does
meet the requirement of mutual authentication.
Added pointer to HTTP TLS channel bindings work
B.5. Changes since 01
Updated threat model to give examples of attacks that are in scope
and to more clearly discuss host security based on comments from
Chris Drake.
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Clarify attacks of interest to be consistent with the
introduction.
Fix ups regarding one-time passwords. I'm not sure that OTPs can
meet all the requirements but clean things up where they clearly
can meet a requirement.
Clarify that in the mutual authentication case I'm concerned about
authentication of client to the server.
Clean up bugs in security considerations
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Author's Address
Sam Hartman
Painless Security, LLC
Email: hartmans-ietf@mit.edu
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