Re: [OAUTH-WG] Auth Code Swap Attack

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

Perfect. Thanks Phil.

EHL

On Sep 6, 2011, at 20:42, "Phil Hunt" <[e-mail]> wrote:

> Eran,
>
> Just took a look at the text. This new version looks much improved. I think this is a good compromise.
>
> Thanks,
>
> Phil
>
> @independentid
> www.independentid.com
> [e-mail]
>
>
>
>
>
> On 2011-09-04, at 4:25 PM, Eran Hammer-Lahav wrote:
>
>> The corresponding 'state' parameter definition:
>>
>>               RECOMMENDED. 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 parameter SHOULD be used for preventing
>>               cross-site request forgery as described in section 10.12.
>>
>> EHL
>>
>>> -----Original Message-----
>>> From: [e-mail] [mailto:[e-mail]] On Behalf
>>> Of Eran Hammer-Lahav
>>> Sent: Sunday, September 04, 2011 4:20 PM
>>> To: William J. Mills; Anthony Nadalin; Torsten Lodderstedt
>>> Cc: OAuth WG ([e-mail])
>>> Subject: Re: [OAUTH-WG] Auth Code Swap Attack
>>>
>>> This is my proposed text for -21 (based on Bill's text as a starting point):
>>>
>>> 10.12.  Cross-Site Request Forgery
>>>
>>>  Cross-site request forgery (CSRF) is an exploit in which an attacker
>>>  causes the user-agent of a victim end-user to follow a malicious URI
>>>  (e.g. provided to the user-agent as a misleading link, image, or
>>>  redirection) to a trusting server (usually established via the
>>>  presence of a valid session cookie).
>>>
>>>  A CSRF attack against the client's redirection URI allows an attacker
>>>  to 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 attacker).
>>>
>>>  The client MUST implement CSRF protection for its redirection URI.
>>>  This is typically accomplished by requiring any request sent to the
>>>  redirection URI endpoint to include a value that binds the request to
>>>  the user-agent's authenticated state (e.g. a hash of the session
>>>  cookie used to authentication the user-agent).  The client SHOULD
>>>  utilize the "state" request parameter to deliver this value to the
>>>  authorization server 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 validate the
>>>  validity of the request by matching the binding value to the user-
>>>  agent's authenticated state.  The binding value used for CSRF
>>>  protection MUST contain a non-guessable value, and the user-agent's
>>>  authenticated state (e.g. session cookie, HTML5 local storage) MUST
>>>  be kept in a location accessible only to the client and the user-
>>>  agent (i.e., protected by same-origin policy).
>>>
>>>  A CSRF attack against the against the authorization server's
>>>  authorization endpoint can result in an attacker obtaining end-user
>>>  authorization for a malicious client without involving or alerting
>>>  the end-user.
>>>
>>>  The authorization server MUST implement CSRF protection for its
>>>  authorization endpoint, and ensure that a malicious client cannot
>>>  obtain authorization without the awareness and explicit consent of
>>>  the resource owner.
>>>
>>> EHL
>>>
>>>
>>> From: William J. Mills [mailto:[e-mail]]
>>> Sent: Thursday, August 25, 2011 12:11 PM
>>> To: Anthony Nadalin; Eran Hammer-Lahav; Torsten Lodderstedt
>>> Cc: OAuth WG ([e-mail])
>>> Subject: Re: [OAUTH-WG] Auth Code Swap Attack
>>>
>>> I had proposed text, and I'll reprise it here with a modification to make the
>>> authorizaton server related explicit.
>>>
>>> 10.12.  Cross-Site Request Forgery
>>>
>>> Cross-site request forgery (CSRF) is an attack whereby malicious URLs are
>>> sent to the user-agent of an end user (generally as hidden links or images)
>>> and transmitted from the user-agent the server trusts or has authenticated.
>>> The most commonly exploited mechanism for this is credentials held in
>>> cookies automatically presented by a web browser.  CSRF attacks against the
>>> client's redirection URI allow an attacker to inject their own authorization
>>> code or access token, which can result in the client using an access token
>>> associated with the attacker's account rather than the victim's.  CSRF attacks
>>> are also possible against an authorization endpoint resulting in delivering a
>>> user credential to an attacker.
>>>
>>> Client applications MUST implement CSRF protection for the redirection
>>> URI.  CSRF protection for a request is data included in the request that ties
>>> that request to the user's authenticated state, i.e. a cryptographic signature
>>> of the user credential and the redirection URI path.  Upon receipt of a
>>> request the client application computes the CSRF data based on the
>>> presented credential and compares that to the CSRF protection data
>>> presented in the request.  CSRF protection data MUST contain a non-
>>> guessable value, and the client MUST keep it in a location accessible only by
>>> the client or the user-agent (i.e., protected by same-origin policy). The
>>> "state" redirection URI parameter is provided as one method of carrying
>>> CSRF protection data, and is RECOMMENDED to provide the greatest
>>> compatibility with systems implementing strong redirection URI validation.
>>>
>>> Authorization servers MUST implement CSRF protection for authorization
>>> requests, use of the "state" parameter is RECOMMENDED as the way to
>>> transmit the CSRF protection data.  The CSRF protection data MUST contain a
>>> non-guessable value, and MUST be presented as part of the authorization
>>> request data (e.g. not as a cookie).  Authorization servers MAY use proof of
>>> previous  authorization by a user for a client in lieu of explicit CSRF protection.
>>>
>>> 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 which MUST
>>> then validate the received value against the stored value, or by recomputing
>>> the expected value of the CSRF protection data and comparing that to the
>>> value presented.
>>>
>>>
>>>
>>> ________________________________________
>>> From: Anthony Nadalin <[e-mail]>
>>> To: Eran Hammer-Lahav <[e-mail]>; Torsten Lodderstedt
>>> <[e-mail]>
>>> Cc: "OAuth WG ([e-mail])" <[e-mail]>
>>> Sent: Thursday, August 25, 2011 8:11 AM
>>> Subject: Re: [OAUTH-WG] Auth Code Swap Attack I have not seen any
>>> updated text, so I don’t believe we have consensus. Also we have a flawed
>>> protocol and we are not providing a fix, suggest that MUST be on the state
>>> also unless someone has a better fix
>>>
>>> From: [e-mail] [mailto:[e-mail]] On Behalf
>>> Of Eran Hammer-Lahav
>>> Sent: Wednesday, August 24, 2011 7:54 AM
>>> To: Torsten Lodderstedt
>>> Cc: OAuth WG ([e-mail])
>>> Subject: Re: [OAUTH-WG] Auth Code Swap Attack
>>>
>>> I believe we have full consensus on this approach.
>>>
>>> EHL
>>>
>>> From: Torsten Lodderstedt [mailto:[e-mail]]
>>> Sent: Tuesday, August 23, 2011 11:06 PM
>>> To: Eran Hammer-Lahav
>>> Cc: OAuth WG ([e-mail])
>>> Subject: Re: [OAUTH-WG] Auth Code Swap Attack
>>>
>>> making CSRF prevention a MUST and recommending the state parameter as
>>> implementation pattern is ok with me.
>>>
>>> regards,
>>> Torsten.
>>>
>>> Am 21.08.2011 21:02, schrieb Eran Hammer-Lahav:
>>> I light to the recent discussion, do you still feel that changing ‘state’ from
>>> optional to required is the best approach?
>>>
>>> EHL
>>>
>>> From: Torsten Lodderstedt [mailto:[e-mail]]
>>> Sent: Sunday, August 21, 2011 11:04 AM
>>> To: Eran Hammer-Lahav
>>> Cc: OAuth WG ([e-mail])
>>> Subject: Re: [OAUTH-WG] Auth Code Swap Attack
>>>
>>> My intention is to require clients to implement CSRF prevention. I thought
>>> making the state parameter mandatory would be the straightforward way.
>>>
>>> regards,
>>> Torsten.
>>>
>>> Am 18.08.2011 08:04, schrieb Eran Hammer-Lahav:
>>> I would like to hear from the other 3 authors of the proposed change about
>>> their reasons for changing the use of ‘state’ from recommended to required
>>> for CSRF prevention. It would also help moving this issue forward if the 4
>>> authors can provide answers or clarifications on the issues raised below.
>>>
>>> Assuming we can count all 4 authors are in favor of making the change, I
>>> believe we have a tie (4:4) and therefore no consensus for making it (as of
>>> this point). However, we did identify issues with the section’s language and
>>> clarity which we should address either way.
>>>
>>> To clarify – I am not proposing we close this issue just yet.
>>>
>>> EHL
>>>
>>> From: [e-mail] [mailto:[e-mail]] On Behalf
>>> Of Eran Hammer-Lahav
>>> Sent: Monday, August 15, 2011 9:35 AM
>>> To: OAuth WG ([e-mail])
>>> Subject: Re: [OAUTH-WG] Auth Code Swap Attack
>>>
>>> To demonstrate why making state required as proposed isn’t very helpful,
>>> here is an incomplete list of other requirements needed to make an
>>> effective CSRF:
>>>
>>> * State value must not be empty (a common bug in many implementations
>>> using simple value comparison).
>>>
>>> * ‘Non-guessable’ isn’t sufficient as most developers will simply use a hash of
>>> the session cookie, with or without salt which isn’t sufficient. We use “cannot
>>> be generated, modified, or guessed to produce valid values” elsewhere in
>>> the document, but this is much easier to get right for access tokens and
>>> refresh tokens than CSRF tokens which are often just some algorithm on top
>>> of the session cookie.
>>>
>>> * State CSRF value should be short-lived or based on a short-lived session
>>> cookie to prevent the use of a leaked state value in multiple attacks on the
>>> same user session once the leak is no longer viable.
>>>
>>> In addition, this is not what “state” was originally intended for. If the working
>>> group decides to mandate a CSRF parameter, it should probably be a new
>>> parameter with a more appropriate name (e.g. ‘csrf’). By forcing clients to
>>> use “state” for this purpose, developers will need to use dynamic queries for
>>> other state information which further reduces the security of the protocol
>>> (as the draft recommends not using dynamic callback query components).
>>> Encoding both CSRF tokens and other state information can be non-intuitive
>>> or complicated for some developers/platforms.
>>>
>>> EHL
>>>
>>>
>>>
>>>
>>> From: Eran Hammer-Lahav
>>> Sent: Friday, August 12, 2011 2:53 PM
>>> To: Anthony Nadalin; OAuth WG ([e-mail])
>>> Subject: Re: [OAUTH-WG] Auth Code Swap Attack
>>>
>>> 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.
>>>
>>> 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
>>>
>>>
>>>
>>> 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"?
>>>
>>>
>>> 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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