Re: [OAUTH-WG] Auth Code Swap Attack

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

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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