Re: [OAUTH-WG] Auth Code Swap Attack

[Eran Hammer-Lahav <[e-mail]>]

Again, the idea that you can produce a comprehensive description of
security threat is impractical if you are going to include every
browser-based attack and its remedies. OAuth use of browser redirection
imports almost every possible attack vector on the web. That's my point.
People constantly bring up these attack vectors, and in multiple flavors
and variations, as if *anyone* can produce a complete list of these issues.

Now, this doesn't mean we should not try to be comprehensive but this can
go on forever.

The changes to 10.12 seem reasonable with the exception of the new MUSTs.
I disagree that we should mandate clients to use the state parameter as
the only CSRF protection vector, especially in an evolving web
environment. We can still include a MUST for verifying that the user
redirected to the authorization server is the same user coming back, and
highlight the state parameter as one way to accomplish that.

How about:


state: OPTIONAL. An opaque value used by the client to maintain state
between the request and redirection. The authorization server includes
this value when redirecting the user-agent back to the client. Use of the
"state" parameter is RECOMMENDED for preventing cross-site request forgery
as described in Section 10.12.
 



Section 10.12 Cross-Site Request Forgery
 

Cross-site request forgery (CSRF) is a web-based attack whereby HTTP
requests are transmitted from the user-agent of an end-user the server
trusts or has authenticated. CSRF attacks enable the attacker to intermix
the attacker's security context with that of the resource owner resulting
in a compromise of either the resource server or of the client application
itself.

In the OAuth context, such attacks allow an attacker to inject their own
authorization code or access token into the client, which can result in
the client associating the attacker's protected resources to the victim's
account on the client. Depending on the nature of the client and the
protected resources, this can have undesirable and damaging effects. In
order to prevent such attacks, the client MUST employ CSRF protection.

It is strongly RECOMMENDED for the client to utilize the "state" request
parameter with authorization requests to the authorization server. When
used for CSRF prevention, the "state" request parameter MUST contain a
non-guessable value, which the client MUST also store with the resource
owner's user-agent in a location accessible only to the client or the
resource owner's user-agent (i.e., protected by same-origin policy). For
example, the client can using a DOM variable, HTTP cookie, or HTML5
client-side storage.


When redirecting the resource owner's user-agent back to the client, the
authorization server includes the value of the "state" parameter. Upon
receiving the redirection request, the client MUST confirm that returned
value of the "state" parameter matches the value stored with the resource
owner's user-agent.


EHL




From:  Torsten Lodderstedt <[e-mail]>
Date:  Fri, 12 Aug 2011 23:58:02 -0700
To:  Eran Hammer-lahav <[e-mail]>
Cc:  Anthony Nadalin <[e-mail]>, "OAuth WG ([e-mail])"
<[e-mail]>
Subject:  Re: [OAUTH-WG] Auth Code Swap Attack


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>    Am 12.08.2011 23:52, schrieb Eran Hammer-Lahav:
>    
>      This is really just a flavor of CSRF attacks. I have no
>        objections to better documenting it (though I feel the current
>        text is already sufficient), but we can't realistically expect
>        to identify and close every possible browser-based attack. A new
>        one is invented every other week.
>      
>      
>      The problem with this text is that developers who do no
>        understand CSRF attacks are not likely to implement it correctly
>        with this information. Those who understand it do not need the
>        extra verbiage which is more confusing than helpful.
>    
>
>    
>    We are constantly facing the fact that a comprehensive description
>    of security threats needs more space than we have in the core draft.
>    That's the reason why the security document has 63 pages and that's
>    also the reason why we decided to let the core spec refer to this
>    document for in-depth explanations. This holds true for this threat
>    as well.
>    
>    regards,
>    Torsten. 
>    
>    
>      
>      
>      As for the new requirements, they are insufficient to
>        actually accomplish what the authors propose without additional
>        requirements on state local storage and verification to complete
>        the flow. Also, the proposed text needs clarifications as noted
>        below.
>      
>      
>      
>      
>      
>        From:  Anthony Nadalin <[e-mail]>
>          Date:  Fri, 12 Aug 2011
>          12:06:36 -0700
>          To:  "OAuth WG ([e-mail])"
>          <[e-mail]>
>          Subject:  [OAUTH-WG]
>          Auth Code Swap Attack
>        
>        
>        
>        >
>>          
>>            
>>        
>      
>      
>      
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>        >
>>          
>>            
>>              
>>                Recommended
>>                      Changes to draft-ietf-oauth-v2
>>                 
>>                In section 4, request options (e.g.
>>                      4.1.1) featuring "state" should change from:
>>                 
>>                state
>>                OPTIONAL.
>>                    An opaque value used by the client to maintain state
>>                    between the request and callback. The authorization
>>                    server includes this value when redirecting the
>>                    user-agent back to the client.
>>                 
>>                to:
>>                 
>>                state
>>                REQUIRED.
>>                    An opaque value used by the client to maintain state
>>                    between the request and callback. The authorization
>>                    server includes this value when redirecting the
>>                    user-agent back to the client. The encoded value
>>                    SHOULD enable the client application to determine
>>                    the user-context that was active at the time of the
>>                     request (see section 10.12). The value MUST NOT be
>>                    guessable or predictable, and MUST be kept
>>                    confidential.
>>                 
>>              
>>            
>>          
>>        
>      
>      
>      
>      Making the parameter required without making its usage
>        required (I.e. "value SHOULD enable") accomplishes nothing.
>        Also, what does "MUST be kept confidential" mean? Confidential
>        from what? Why specify an "encoded value"?
>      
>      
>      
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>        >
>>          
>>            
>>              
>>                Section 10.12 Cross-Site Request
>>                      Forgery
>>                 
>>                Change to:
>>                 
>>                Cross-site
>>                    request forgery (CSRF) is a web-based attack whereby
>>                    HTTP requests are transmitted from the user-agent of
>>                    an end-user the server trusts or has authenticated.
>>                    CSRF attacks enable the attacker to intermix the
>>                    attacker's security context with that of
>>                    the resource owner resulting in a compromise of
>>                    either the resource server or of the client
>>                    application itself. In the OAuth context,
>>                    such attacks allow an attacker to inject their own
>>                    authorization code or access token into a client,
>>                    which can result in the client using an access token
>>                    associated with the attacker's account rather than
>>                    the victim's. Depending on the nature of the client
>>                    and the protected resources, this can have
>>                    undesirable and damaging effects.
>>                 
>>                    In order to prevent such attacks, the client
>>                    application MUST encode a non-guessable,
>>                    confidential end-user artifact and submit as the
>>                    "state" parameter to authorization and access token
>>                    requests to the authorization server. The client
>>                    MUST keep the state value in a location accessible
>>                    only by the client or the user-agent (i.e.,
>>                    protected by same-origin policy), for example, using
>>                    a DOM variable, HTTP cookie, or HTML5 client-side
>>                    storage.
>>                 
>>                    The authorization server includes the value of the
>>                    "state" parameter when redirecting the user-agent
>>                    back to the client. Upon receiving a redirect, the
>>                    client application MUST confirm that returned value
>>                    of "state" corresponds to the state value of the
>>                    user-agent's user session. If the end-user session
>>                    represents an authenticated user-identity, the
>>                    client MUST ensure that the user-identity has NOT
>>                    changed.
>>                 
>>              
>>            
>>          
>>        
>      
>      
>      
>      The above text uses 'user-context' and this 'user-identity'.
>        Neither term is defined.
>      
>      
>      EHL
>      
>      
>      
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