Re: [OAUTH-WG] [EXT] Re: WGLC review of draft-ietf-oauth-security-topics-13
Hi Mike, > Wouldn't most RSs only trust access tokens from a single AS anyways? At the last OSW, there was broad agreement that this is typically the case. Otherwise, the mitigation that we suggested in the paper would not prevent the attack. > Would it be reasonable for the document to recommend that clients designate a specific AS for each RS that the client accesses (and not allow the user to select a different AS)? Would that help prevent the attack? In principle, this should prevent the attack. However, this would require the client to identify the correct AS for each RS, which might not always be possible (e.g., in more dynamic settings) and more complicated than just delegating this decision to the RS. Any opinions (also from other OAuth WG members) on this? Best regards, Pedram On 26.11.19 21:08, Peck, Michael A wrote: Hi Pedram, I understand why a client would need to allow use of multiple authorization servers if the client needs to access various resource servers each of which may trust different ASs (e.g. the client supports accessing resources at multiple cloud storage services). However, how common is the case that a client would need to allow selecting from multiple authorization servers for accessing a particular resource server? Would it be reasonable for the document to recommend that clients designate a specific AS for each RS that the client accesses (and not allow the user to select a different AS)? Would that help prevent the attack? Wouldn't most RSs only trust access tokens from a single AS anyways? Thanks, Mike On 11/26/19, 1:32 PM, "OAuth on behalf of Benjamin Kaduk" wrote: Hi Pedram, Thanks for confirming that the scenario is as I was trying to understand it. I don't think it's universal that all clients will give transitive access from the user to the accessed resource, though it's certainly common; the lack of exposition on that point is what I had been stumbling on. -Ben On Tue, Nov 26, 2019 at 06:33:04PM +0100, Pedram Hosseyni wrote: > Hi Ben, > > The attacker uses the (honest) client shown in Figure 4 as a regular > user. For example, the client might provide access to a cloud storage > via its website, i.e., by using the clients' website, a user can access > her files stored at the resource server. > > I'll try to clarify the attack with a simplified example. > > Let's assume that the client supports two authorization servers > AS_honest and AS_attacker. Intuitively, if the attacker phishes an > access token created by AS_honest for an honest user (Alice), one would > expect that sender-constraining the access token (e.g., via mTLS) > prevents the attacker from using this access token. > > The overall goal of the attacker is to use the sender-constrained access > token (which he cannot use directly at the resource server) to access > Alices cloud storage. > > The attack works as follows: > > First, the attacker visits the website of the client. Usually, the > attacker would now choose an AS, and after successful authentication, > access his files stored in the cloud. When selecting the AS, the > attacker chooses AS_attacker. In Step 5 of Figure 4, AS_attacker now > provides the phished access token. As this token is bound to this > client, the client can use it at the resource server for getting access > to the cloud storage of Alice. As the attacker is using the client > (through the clients' website), he now gets access to these files > (stored at the RS). > > Please let me know if you have any other questions. > > Best regards, > Pedram > > > On 26.11.19 16:51, Benjamin Kaduk wrote: > > Hi Pedram, > > > > On Thu, Nov 21, 2019 at 02:50:52PM +0100, Pedram Hosseyni wrote: > >> Also, for this or the next version of this document, the Cuckoo's Token > >> attack (see Section IV-A of http://arxiv.org/abs/1901.11520/ ), should > >> be addressed. We also discussed this issue extensively at the last OSW > >> in Stuttgart. > > I took a look at the paper, and I'm not sure I'm properly understanding the > > "Cuckoo's Token" attack. Looking at Figure 4 of the paper to have > > something concrete to refer to, I assume that the client, as a white box, > > is presumed to be honest. Since the access token is bound to the client, I > > assume that the attacker has to return the phished access token to the same > > client that originally (honestly) got it, as otherwise the token will not > > be usab
Re: [OAUTH-WG] WGLC review of draft-ietf-oauth-security-topics-13
Hi Ben, The attacker uses the (honest) client shown in Figure 4 as a regular user. For example, the client might provide access to a cloud storage via its website, i.e., by using the clients' website, a user can access her files stored at the resource server. I'll try to clarify the attack with a simplified example. Let's assume that the client supports two authorization servers AS_honest and AS_attacker. Intuitively, if the attacker phishes an access token created by AS_honest for an honest user (Alice), one would expect that sender-constraining the access token (e.g., via mTLS) prevents the attacker from using this access token. The overall goal of the attacker is to use the sender-constrained access token (which he cannot use directly at the resource server) to access Alices cloud storage. The attack works as follows: First, the attacker visits the website of the client. Usually, the attacker would now choose an AS, and after successful authentication, access his files stored in the cloud. When selecting the AS, the attacker chooses AS_attacker. In Step 5 of Figure 4, AS_attacker now provides the phished access token. As this token is bound to this client, the client can use it at the resource server for getting access to the cloud storage of Alice. As the attacker is using the client (through the clients' website), he now gets access to these files (stored at the RS). Please let me know if you have any other questions. Best regards, Pedram On 26.11.19 16:51, Benjamin Kaduk wrote: Hi Pedram, On Thu, Nov 21, 2019 at 02:50:52PM +0100, Pedram Hosseyni wrote: Also, for this or the next version of this document, the Cuckoo's Token attack (see Section IV-A of http://arxiv.org/abs/1901.11520/ ), should be addressed. We also discussed this issue extensively at the last OSW in Stuttgart. I took a look at the paper, and I'm not sure I'm properly understanding the "Cuckoo's Token" attack. Looking at Figure 4 of the paper to have something concrete to refer to, I assume that the client, as a white box, is presumed to be honest. Since the access token is bound to the client, I assume that the attacker has to return the phished access token to the same client that originally (honestly) got it, as otherwise the token will not be usable at the RS. The paper concludes that in step 6, the client gets access to the honest resource owner's resources, and furthermore that the attacker has access to those resources through the client. It's that last part that I'm not sure I understand -- if the client is honest, why would it return resource information to the attacker? The best I can come up with is that there's some sense of a "session" between the user and client, such that the client links its resource accesses with the "session" on behalf of which the access occurs, and is willing to return such information back to the user only on the "linked session". (The countermeasure makes sense and is a good practice, of course.) Thanks, Ben -- Pedram Hosseyni, M.Sc. Room V38 2.438 Institute of Information Security - SEC Universität Stuttgart Universitätsstraße 38 D-70569 Stuttgart Germany Phone: +49 711 685 88454 https://sec.uni-stuttgart.de ___ OAuth mailing list OAuth@ietf.org https://www.ietf.org/mailman/listinfo/oauth
[OAUTH-WG] WGLC review of draft-ietf-oauth-security-topics-13
Dear all, I have a few comments about the leakage of access tokens and the underlying assumptions: Section 2, A5 should be clarified: "a resource server can be compromised by an attacker": Is the assumption that the attacker cannot get access to the resources stored at the compromised RS (or only parts of it)? Otherwise, the attacker does not need to get an AT anymore. A more differentiated view on RS compromisation is already given in Section 4.8.2, but this is not reflected in A5. Also, A5 states that "an access token may be sent to an attacker-controlled resource server due to a misconfiguration," which does not require the compromisation of an honest RS. Perhaps one could be more generic here and just say that the AT leaks to the attacker. However, a misconfigured RS endpoint not only gets the request to this endpoint (containing an AT) but can also respond and provide the RO with wrong resources. At the same time, if, say, the RO thinks that she is connected to her cloud storage, the attacker would get access to all uploaded data. Is this really what A5 should express, or is the primary focus on the leakage of the AT? Also, for this or the next version of this document, the Cuckoo's Token attack (see Section IV-A of http://arxiv.org/abs/1901.11520/ ), should be addressed. We also discussed this issue extensively at the last OSW in Stuttgart. Typo: Section 3.5: MTLS -> mTLS Best regards Pedram Hosseyni -- Pedram Hosseyni, M.Sc. Room V38 2.438 Institute of Information Security - SEC Universität Stuttgart Universitätsstraße 38 D-70569 Stuttgart Germany Phone: +49 711 685 88454 https://sec.uni-stuttgart.de ___ OAuth mailing list OAuth@ietf.org https://www.ietf.org/mailman/listinfo/oauth
