Hi Andre,

how do you think differs the threat you descibed from 
http://tools.ietf.org/html/draft-ietf-oauth-v2-threatmodel-01#section-4.4.1.4?

regards,
Torsten.
Am 26.10.2011 22:44, schrieb André DeMarre:
> Should a brief explanation of this be added to the Threat Model and
> Security Considerations document? Or does anyone even agree that this
> can be a problem?
>
> Regards,
> Andre DeMarre
>
> On Tue, Oct 4, 2011 at 11:32 AM, André DeMarre<andredemarre@gmail.com>  wrote:
>> I've not seen this particular variant of phishing and client
>> impersonation discussed. A cursory search revealed that most of the
>> related discussion centers around either (a) client impersonation with
>> stolen client credentials or (b) phishing by malicious clients
>> directing resource owners to spoofed authorization servers. This is
>> different.
>>
>> This attack exploits the trust a resource owner has for an OAuth
>> authorization server so as to lend repute to a malicious client
>> pretending to be from a trustworthy source. This is not necessarily a
>> direct vulnerability of OAuth; rather, it shows that authorization
>> servers have a responsibility regarding client application names and
>> how they present resource owners with the option to allow or deny
>> authorization.
>>
>> A key to this exploit is the process of client registration with the
>> authorization server. A malicious client developer registers his
>> client application with a name that appears to represent a legitimate
>> organization which resource owners are likely to trust. Resource
>> owners at the authorization endpoint may be misled into granting
>> authorization when they see the authorization server asserting "<some
>> trustworthy name>  is requesting permission to..."
>>
>> Imagine someone registers a client application with an OAuth service,
>> let's call it Foobar, and he names his client app "Google, Inc.". The
>> Foobar authorization server will engage the user with "Google, Inc. is
>> requesting permission to do the following." The resource owner might
>> reason, "I see that I'm legitimately on the https://www.foobar.com
>> site, and Foobar is telling me that Google wants permission. I trust
>> Foobar and Google, so I'll click Allow."
>>
>> To make the masquerade act even more convincing, many of the most
>> popular OAuth services allow app developers to upload images which
>> could be official logos of the organizations they are posing as. Often
>> app developers can supply arbitrary, unconfirmed URIs which are shown
>> to the resource owner as the app's website, even if the domain does
>> not match the redirect URI. Some OAuth services blindly entrust client
>> apps to customize the authorization page in other ways.
>>
>> This is hard to defend against. Authorization server administrators
>> could police client names, but that approach gives them a burden
>> similar to certificate authorities to verify organizations before
>> issuing certificates. Very expensive.
>>
>> A much simpler solution is for authorization servers to be careful
>> with their wording and educate resource owners about the need for
>> discretion when granting authority. Foobar's message above could be
>> changed: "An application calling itself Google, Inc. is requesting
>> permission to do the following" later adding, "Only allow this request
>> if you are sure of the application's source." Such wording is less
>> likely to give the impression that the resource server is vouching for
>> the application's identity.
>>
>> Authorization servers would also do well to show the resource owner
>> additional information about the client application to help them make
>> informed decisions. For example, it could display all or part of the
>> app's redirect URI, saying, "The application is operating on
>> example.com" or "If you decide to allow this application, your browser
>> will be directed to http://www.example.com/." Further, if the client
>> app's redirect URI uses TLS (something authorization servers might
>> choose to mandate), then auth servers can verify the certificate and
>> show the certified organization name to resource owners.
>>
>> This attack is possible with OAuth 1, but OAuth 2 makes successful
>> exploitation easier. OAuth 1 required the client to obtain temporary
>> credentials (aka access tokens) before sending resource owners to the
>> authorization endpoint. Now with OAuth 2, this attack does not require
>> resource owners to interact with the client application before
>> visiting the authorization server. The malicious client developer only
>> needs to distribute links around the web to the authorization server's
>> authorization endpoint. If the HTTP service is a social platform, the
>> client app might distribute links using resource owners' accounts with
>> the access tokens it has acquired, becoming a sort of worm. Continuing
>> the Google/Foobar example above, it might use anchor text such as "I
>> used Google Plus to synchronize with my Foobar account." Moreover, if
>> the app's redirect URI bounces the resource owner back to the HTTP
>> service after acquiring an authorization code, the victim will never
>> see a page rendered at the insidious app's domain.
>>
>> This is especially dangerous because the public is not trained to
>> defend against it. Savvy users are (arguably) getting better at
>> protecting themselves from traditional phishing by verifying the
>> domain in the address bar, and perhaps checking TLS certificates, but
>> such defenses are irrelevent here. Resource owners now need to verify
>> not only that they are on the legitimate authorization server, but to
>> consider the trustworthyness of the link that referred them there.
>>
>> I'm not sure what can or should be done, but I think it's important
>> for authorization server implementers to be aware of this attack. If
>> administrators are not able to authenticate client organizations, then
>> they are shifting this burden to resource owners. They should do all
>> they can to educate resource owners and help them make informed
>> decisions before granting authorization.
>>
>> Regards,
>> Andre DeMarre
>>
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