Re: [OAUTH-WG] A Scope Attack against OAuth 2.0

[John Bradley <[e-mail]>]

On 2012-02-21, at 7:32 PM, Nicholas Devenish wrote:

> 
> On 21 Feb 2012, at 21:59, John Bradley wrote:
>> This 'attack'  is one that only works with badly designed clients that are making unwarranted assumptions about the behaviour of the Authorization server.  
> 
> Unwarranted assumptions? The spec in 3.3 says:
> 
> "If the issued access token scope is different from the one requested by the client, the authorization server MUST include the "scope" response parameter to inform the client of the actual scope granted."
> 
> - It says MUST; therefore any server that doesn't do this is non-compliant?
> - It says scope different from the one requested by the *client*. The possibility that the resource owner intercepts this request and changes it doesn't seem to be considered here (it is not strictly the clients request if that happens)
> - The purpose seems to be that the client should be told if the scope changes; this would be important if the client requires a certain scope level to work (though this could be solved more generally in many other ways that wouldn't be in the scope of the oauth spec)
> 
> Thus, assuming that the server is stating compliance, isn't the assumption completely warranted?

No the authorization server may at any time for any reason remove a scope from a granted access_token or refresh_token.   

Reporting back changes in scopes granted along with the access_token is a convenience not a security feature.  

Assuming it is a security feature and those scopes will continue to be valid for the token after granting is a bad design given the OAuth 2 spec.
> 
>> The only way for a client to know if a token has a scope it to try it, or use a introspection endpoint.  End of story.
> 
> An introspection endpoint obviously isn't part of the specification, so isn't relevant to the discussion (though it solves the discussed facebook issue).
> 
> You are right though, that the only way for a client to know for sure is to try to use it; Even if the spec mandated always returning the scope to the client, the user could always intercept the return redirection (assuming a non-confidential client) and change the scope there.
> 
> Perhaps MUST should change to SHOULD, given that this essentially seems unenforceable?

A SHOULD may lead people to the conclusion that it is secure.   I am happy with saying it is not secure the only want to know is to have the client be prepared to deal with tokens that do not contain the desired scope when used.   That is the only 100% solution.

John B.