Re: [OAUTH-WG] Call for Adoption: OAuth 2.0 Mix-Up Mitigation

John Bradley <> Mon, 25 January 2016 14:35 UTC

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From: John Bradley <>
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Date: Mon, 25 Jan 2016 11:35:20 -0300
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To: Vladimir Dzhuvinov <>
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Subject: Re: [OAUTH-WG] Call for Adoption: OAuth 2.0 Mix-Up Mitigation
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Using client secret JWT works as long as long as the attacker doesn’t know the client secret of the client_id associated with the “good AS”.  
The reason is that the attacker cannot replay a signed assertion because it contains a audience and client_id that would not match.

In the case where the “bad AS” gets the client to register and can proxy that registration and create a new client_id at the “good AS” with the client’s redirect URI, than this mitigation would not work.
The bad AS would just use the client credential stolen in the proxy registration to create a new authentication to the token endpoint.

While as a general principal not giving the attacker your credential unencrypted on the wire is a very good thing, if the attacker man in the middles the registration then this won’t help.

We did discuss this in Germany.  It was also felt that forcing clients to do crypto was overkill. 

Standardizing doing client secret JWT authentication in OAuth would be a good thing, but should be separate.

Based on the assertions draft you only need to define how to use the keys and what the values for aud is in the non connect case (I suspect the token endpoint URI would be the obvious choice but someone needs to write it down to be interoperable). 

John B.

> On Jan 25, 2016, at 5:41 AM, Vladimir Dzhuvinov <> wrote:
> The token endpoint mix-up attack was originally discussed in the context of OIDC, in August 2015. Back then John Bradley noted that by simply replacing Basic client authentication with JWT (client_secret_jwt or private_key_jwt) the attack is successfully prevented. Yes, the authz code will be captured, but the bogus AS will not be able to replay it with the legitimate AS because the JWT used for client authentication is bound (via the "aud" claim) to the original token endpoint.
> <>
> <>
> Is this going to be added to the list of solutions? Its usefulness is limited to confidential clients, but it's a good solution nevertheless.
> JWT authentication helps in another case also - when the client by mistake sends out the token request over plain HTTP.
> Cheers,
> Vladimir
> On 23/01/16 22:52, Nat Sakimura wrote:
>> Since we have been discussing in another thread and you guys may be a aware
>> of it in this thread, here is one of the problem statement for the mix-up
>> draft explaining why it does not solve the problem as it is written now.
>> It can be fixed to solve the problem, but then the overhead size will be
>> much larger and involves an extra round trip compared to oauth-meta draft.
>> See
>> <>
>> for the details.
>> Nat
>> 2016年1月22日(金) 18:11 Hans Zandbelt <> <>:
>>> +1 to everything Mike stated
>>> Hans.
>>> On 1/22/16 2:04 AM, Mike Jones wrote:
>>>> I believe that it’s simpler and more elegant to return an issuer, from
>>>> which the discovery metadata document can be retrieved, which contains
>>>> **all** the configuration information about the authorization server,
>>>> than to return some of the configuration parameters but not most of them
>>>> (which is what the oauth-meta proposal does).  There’s lots more in a
>>>> typical discovery document than just a few endpoint addresses. For
>>>> instance, there are statements about what response types are supported,
>>>> other configuration choices, keys, etc.
>>>> I also think that returning the issuer is more future-proof than trying
>>>> to decide now what all the configuration information is that might want
>>>> to be verified by the client and hoping we get it right.  With the
>>>> issuer, assuming that discovery is supported, it can retrieve and check
>>>> all of it, should the client want/need to do so.  Even when discovery
>>>> isn’t supported, the issuer still provides a concrete identifier for the
>>>> authorization server that can be checked for equality.
>>>> We talked about the value of that approach in the side meetings in
>>>> Yokohama and people were supportive then.
>>>>                                                                  -- Mike
>>>> *From:*OAuth [ <>] *On Behalf Of *John Bradley
>>>> *Sent:* Thursday, January 21, 2016 4:48 PM
>>>> *To:* Nat Sakimura <> <>
>>>> *Cc:* <> WG <> <>
>>>> *Subject:* Re: [OAUTH-WG] Call for Adoption: OAuth 2.0 Mix-Up Mitigation
>>>> I will point out for clarification that this would be like IdP discovery
>>>> in openID 2 that everyone did.  I think IdP not doing RP discovery in
>>>> openID 2 is a weak argument.
>>>> There may be other evidence that RP will not do discovery, but if that
>>>> is the case why are we doing a OAuth discovery spec?
>>>> Many people see your spec as discovery as well just in a different way.
>>>> I think both ways can work, but doing both leaves too many options.
>>>> John B.
>>>>     On Jan 21, 2016, at 9:38 PM, Nat Sakimura < <>
>>>>     <> <>> wrote:
>>>>     Still doing the analysis. We spent 1.5 hours today with John,
>>>>     George, nov and me on the OpenID Connect WG call on this issue. John
>>>>     explained the mitigation, but none of us was convinced that it works.
>>>>     Then, after the call, I and nov went on with various scenarios. The
>>>>     interim conclusion is that:
>>>>       * client_id response parameter does not help. There are legitimate
>>>>         cases that client_id duplicates out there and we cannot ban that.
>>>>       * iss response parameter does not help unless the discovery is
>>>>         performed and the value of iss is checked against the value of
>>>>         OAuth issuer inside the document. (<- Discovery becomes
>>>>         mandatory. From our experience on RP discovery step in OpenID
>>>>         2.0, chance of this being done properly seems to be rather slim.)
>>>>       * sending the state to the token endpoint helps in the case code
>>>>         was stollen, but code should not be stolen to start with.
>>>>     Cheers,
>>>>     Nat
>>>>     2016年1月22日(金) 7:59 John Bradley < <>
>>>>     <> <>>:
>>>>         There have been a lot of discussions. I think Hannes posted some
>>>>         minutes of the F2F meeting we had with the security researchers.
>>>>         The problem can’t be mitigated without some action on the client
>>>>         side.  It boils down to the client making a request to AS1 (From
>>>>         it’s perspective) and getting back a response from AS 2 (that it
>>>>         thinks is AS1)
>>>>         This can be done if AS1 is a good AS but has it’s logs
>>>>         compromised so that an attacker can read them. Hans Zandbelt
>>>>         built a proof of concept for the attack.
>>>>         In some cases the attacker gets the code and the credential to
>>>>         use it at the good AS token endpoint and in others it just gets
>>>>         the code and can replay that at the client to extract
>>>>         information from the API through the client by binding the api
>>>>         access to a new account.
>>>>         PKCE unfortunately mitigates a different attack, where the
>>>>         client is impersonated and trys to replay a intercepted code.
>>>>         It however assumes the token endpoint is good, and is no help in
>>>>         the case of a compromised token endpoint.
>>>>         The client in these attacks is vulnerable because it relies on
>>>>         some local state, or the value of the state parameter to know
>>>>         who the response is coming from.  This allows a authorization
>>>>         request that is intercepted to be used to create a new request
>>>>         in that browser to a different AS, or trick the client into
>>>>         using a Authorization endpoint for a different authorization
>>> server.
>>>>         One problem is that OAuth doesn’t really have a unified concept
>>>>         of what a AS is.  Traditionally it is a Authorization endpoint
>>>>         URI, token end point URI and resource server URI that some one
>>>>         gives the client in some documentation.
>>>>         As ling as a client only talks to one AS then there is no
>>>>         problem.   However once a client is configured to talk to more
>>>>         than one AS, we have problems if one of those AS lies about it’s
>>>>         endpoints, or is compromised and used to attack another AS.   As
>>>>         a design goal you don’t want the overall  security to be limited
>>>>         by the weakest system.
>>>>         One approach as Nat promotes is to have the authorization
>>>>         endpoint return the next hop, token endpoint for code or RS URI
>>>>         for token. The token endpoint must also return the RS URI or the
>>>>         client must push the RS URI to the token endpoint or the
>>>>         attacker just replaces the RS URI in the config and captures the
>>>>         token that way.
>>>>         The other way is to provide a name for each AS as part of
>>>>         registration and the client not allow duplicate registrations
>>>>         with the same name.  When the response comes back the client
>>>>         checks the name against the one it made the request to. If the
>>>>         name dereferences to a discovery document then the client can
>>>>         check that name at registration or runtime to validate the net
>>> hops.
>>>>         I think the two approaches mitigate the attack in similar ways.
>>>>         Nat’s is more REST friendly and returns the next hop as a
>>>>         parameter of header.
>>>>         The one Mike and I wrote up based on the meeting in Germany
>>>>         provides identifiers (iss and client_id) that can be checked for
>>>>         validity in the simple case and be dereferenced via .well-known
>>>>         to get the information Nat’s proposal is providing.
>>>>         Perhaps the main difference is Nat is using the token endpoint
>>>>         as the identifier for the AS and Mike’s is using location for a
>>>>         discovery document as the identifier.
>>>>         I don’t recall the reasons using the token endpoint as the
>>>>         identifier was rejected at the meeting.  Perhaps others can post
>>>>         the reasons.
>>>>         We need to close on this quickly, otherwise if we are indecisive
>>>>         fixes will not go into products for another year or so until
>>>>         this is a RFC.
>>>>         That is my main concern.
>>>>         John B.
>>>>             On Jan 21, 2016, at 11:50 AM, Josh Mandel < <>
>>>>             <> <>> wrote:
>>>>             Thanks Nat - that's helpful. If both mitigations *can* work
>>>>             effectively, then I would like to see this group consider
>>>>             the decision between them carefully (if that hasn't happened
>>>>             yet). Again, don't hesitate to let me know if this is the
>>>>             wrong place/time for such discussion.
>>>>             At a high level, I would rather ask server developers to do
>>>>             some "coding", for two reasons:
>>>>             1. Most OAuth servers talk to many, many clients. So
>>>>             consolidating the security critical work in one place
>>>>             (server) is a net savings of work (rather than asking each
>>>>             client to implement these checks.
>>>>             2. OAuth server developers are typically more sophisticated
>>>>             than client developers, and therefore more likely to
>>>>             understand the implications and more likely to get these
>>>>             critical details correct. Asking each client developer to do
>>>>             something right is likely to result in heterogenius
>>>>             implementation and persistent security holes. But if the
>>>>             server does the heavy lifting, and clients just have to pass
>>>>             along an extra parameter, this is more likely to see
>>>>             consistent implementation (for example, clients will fail to
>>>>             work if misconfigured, which will prompt developers to fix
>>>>             them).
>>>>             On Jan 21, 2016 09:40, "Nat Sakimura" < <>
>>>>             <> <>> wrote:
>>>>                 Hi Josh,
>>>>                 It is similar but slightly different IMHO.
>>>>                 Section 4.6.4 of the RFC6819 is the access token
>>>>                 phishing by a counterfeit resource server.
>>>>                 The mix-up attack described here is the code phishing by
>>>>                 a counterfeit token endpoint.
>>>>                 In my view, both can be mitigated by the server
>>>>                 returning the next step: i.e., authorization endpoint
>>>>                 returning the legitimate token endpoint uri, and token
>>>>                 endpoint returning legitimate resource endpoint uris.
>>>>                 This involves no discovery endpoint, which is good.
>>>>                 Your way also works. It is just the reverse of my
>>>>                 proposal. The difference being that my proposal does not
>>>>                 need any coding on the server but just configuration,
>>>>                 and it can return more metadata if needed.
>>>>                 Cheers,
>>>>                 Nat
>>>>                 2016年1月21日(木) 23:04 Josh Mandel < <>
>>>>                 <> <>>:
>>>>                     Apologies if this is the wrong forum for my comment
>>>>                     (and please direct me to the appropriate place in
>>>>                     that case), but I have two questions about the
>>>>                     propose mitigation (and the thinking behind it) that
>>>>                     I think the write-up could address:
>>>>                     1. Could the writeup clarify whether/how the primary
>>>>                     "mixup" threat differs from what RFC6819 identifies
>>>>                     as in section 4.6.4?
>>>>                     2. Has the workgroup considered a mitigation that
>>>>                     puts more responsibility on the authorization
>>>>                     server, and less on the client? For example, if
>>>>                     would be helpful for the writeup to clarify why
>>>>                     having the client send an "audience field" (in the
>>>>                     terminology of RFC6819) to the authorization
>>>>                     endpoint would not mitigate the threat. (In that
>>>>                     scenario, the authorization server can recognize
>>>>                     that the audience does not correspond to a resource
>>>>                     server it knows, rather than asking clients to make
>>>>                     this check). I assume this approach has been
>>>>                     considered and rejected as an incomplete mitigation,
>>>>                     but I don't have visibility into where/how that
>>>>                     discussion went.
>>>>                     Thanks,
>>>>                     Josh
>>>>                     Hi all,
>>>>                     this is the call for adoption of OAuth 2.0 Mix-Up
>>>>                     Mitigation, see
>>> <>
>>>>                     Please let us know by Feb 9th whether you accept /
>>>>                     object to the
>>>>                     adoption of this document as a starting point for
>>>>                     work in the OAuth
>>>>                     working group.
>>>>                     Note: This call is related to the announcement made
>>>>                     on the list earlier
>>>>                     this month, see
>>> <>.
>>>>                     More
>>>>                     time for analysis is provided due to the complexity
>>>>                     of the topic.
>>>>                     Ciao
>>>>                     Hannes & Derek
>>>>                     _______________________________________________
>>>>                     OAuth mailing list
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>>> --
>>> Hans Zandbelt              | Sr. Technical Architect
>>> <> | Ping Identity
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> -- 
> Vladimir Dzhuvinov :: <>_______________________________________________
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