Comments on draft-sheffer-oauth-jwt-bcp-00
1. Section 2 lists 7 known and possible threats and vulnerabilities with
JWT implementations and deployments.
In the OAuth Threat Model Document (RFC 6819) collusions between users
located inside of a system are not mentioned
but nevertheless need to be considered. One threat should be added in
this document: Client collusions
Text proposal:
2.8 Client collusions
RFC 6819 (OAuth 2.0 Threat Model and Security Considerations) issued
in January 2013 identifies security threats coming
from hostile parties but does not mention threats coming from
collaborating parties.
JWTs are issued to a token requestor and are normally intended to be
used by that token requestor. They are cases where
the forwarding of a JWT to another party is a desirable property so
that it can be used by that other party. This is typically the case
when a JWT is intended to be used for delegation purposes.
However, there are cases where a JWT does not contain a "sub" claim,
but other claims that do not allow to identify the token requestor.
This is typically the case when a JWT contains an attribute
specifying that the token holder is over 18. In such a case, the
forwarding of
such a JWT to another party would not be a desirable property and
should be detected by target servers.
Access token binding protection methods currently developed either
by the Token Binding WG [Token Binding WG] or by the OAuth WG [OAuth
WG]
do not allow to counter the forwarding of a JWT legitimately
obtained by a party to another party. Either the legitimate party
can provide keys
to the other party, or if it can't (because private keys are
protected by a secure element) it can send requests to his secure
element to perform
the cryptographic computations that the other party needs.
Whatever kind of cryptographic is being used, when two parties are
willing to collaborate, a software-only solution will be unable to
prevent the transfer of an attribute of a party that possesses it to
another party that does not possess it. The use of a secure element
will be mandatory.
However, the use of a secure element simply protecting the
confidentiality and the integrity of some secret key or private key
will be insufficient.
Additional functional and security properties will be required for
the secure elements.
The documents related to OAuth 2.0 do not currently specify how to
support secure elements so that the forwarding of a JWT legitimately
obtained
by a party to another party can be detected by the target server.
Unless some extensions are being defined, the OAuth 2.0 framework
will be unable
to support JWTs containing attributes that do not allow to fully
identify the token holder, typically a single attribute specifying
that the token holder
is over 18.
2. Section 3 lists some best practices. Section 3.8 is written as follows:
3.8. Use and Validate Audience
If the same issuer can issue JWTs that are intended for use by more
than one relying party or application, the JWT MUST contain an "aud"
(audience) claim that can be used to determine whether the JWT is
being used by an intended party or was substituted by an attacker at
an unintended party.
JWTs may be used in a number of contexts, in particular when the
resource owner and the target server are not collocated.
If the resource or the audience parameter is being used, the STS will
have the ability to know exactly which individual or entity
has accessed which target service and may keep a log of that activity.
It would thus be in a position to act as Big Brother.
Mandating implementations to have built-in Big Brother properties does
not seem to be a "good practice".
However, there is indeed the need to restrict the use of JWTs to
specific targets. The key point is that the target service must be able
to recognize itself that the token is indeed targeted to it. As an
example, a challenge may be requested to the target service and
that challenge may then be placed into a specific filed of the JWT. The
target service may then verify that the value included
in the JWT is the one that has been recently provided.
A parameter specifying the type of the control value and the value of
the control would need to be used.
This parameter would be called a target_id (tid). It would solve the Big
Brother case.
This parameter cannot be introduced in a BCP document, but this should
be done in another document.
When the resource owner and the target server are not collocated and
when privacy is a concern, the use of these parameters
should be deprecated and the use of a "tid" parameter should be
recommended.
In any case, stating that "the JWT MUST contain an "aud" (audience)
claim" is incorrect.
Denis
JSON Web Tokens (JWTs) and the JSON Object Signing and Encryption
(JOSE) functions underlying them are now being widely used in diverse
sets of applications. During IETF 98 in Chicago
<https://ietf.org/meeting/98/>, we discussed reports of people
implementing and using JOSE and JWTs insecurely, the causes of these
problems, and ways to address them. Part of this discussion was an
invited JOSE/JWT Security Update
<https://www.ietf.org/proceedings/98/slides/slides-98-oauth-sessb-jwt-security-update-00.pdf>
presentation that I gave to two working groups, which included links
to problem reports and describes mitigations. Citing the widespread
use of JWTs in new IETF applications, Security Area Director Kathleen
Moriarty suggested during these discussions that a Best Current
Practices (BCP) document be written for JSON Web Tokens (JWTs).
I’m happy to report that Yaron Sheffer, Dick Hardt, and myself have
produced an initial draft of a JWT BCP. Its abstract is:
JSON Web Tokens, also known as JWTs [RFC7519
<https://tools.ietf.org/html/rfc7519>], are URL-safe JSON-based
security tokens that contain a set of claims that can be signed and/or
encrypted. JWTs are being widely used and deployed as a simple
security token format in numerous protocols and applications, both in
the area of digital identity, and in other application areas. The goal
of this Best Current Practices document is to provide actionable
guidance leading to secure implementation and deployment of JWTs.
In Section 2, we describe threats and vulnerabilities. In Section 3,
we describe best practices addressing those threats and
vulnerabilities. We believe that the best practices in Sections 3.1
through 3.8 are ready to apply today. Section 3.9 (Use Mutually
Exclusive Validation Rules for Different Kinds of JWTs) describes
several possible best practices on that topic to serve as a starting
point for a discussion on which of them we want to recommend under
what circumstances.
We invite input from the OAuth Working Group and other interested
parties on what best practices for JSON Web Tokens and the JOSE
functions underlying them should be. We look forward to hearing your
thoughts and working on this specification together.
The specification is available at:
* https://tools.ietf.org/html/draft-sheffer-oauth-jwt-bcp-00
An HTML-formatted version is also available at:
* http://self-issued.info/docs/draft-sheffer-oauth-jwt-bcp-00.html
-- Mike
P.S. This notice was also posted at http://self-issued.info/?p=1690
<http://self-issued.info/?p=1690> and as @selfissued
<https://twitter.com/selfissued>.
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