It took me some time to warm to the ECDH based idea but I do think it has a lot of merit. For whatever it's worth, I put the idea forth as one potential path forward during the general PoP interim meeting back in March ( https://datatracker.ietf.org/meeting/interim-2020-oauth-02/materials/slides-interim-2020-oauth-02-sessa-oauth-20-proof-of-possession-00.pdf slide 8) but the WG consensus was to pursue a signature style PoP for the current efforts, which was followed not long after by WG adoption of DPoP. I could potentially see an ECDH-style-POP effort making a resurgence in the future. But it's fundamentally pretty different.
On Tue, Nov 24, 2020 at 4:03 AM Neil Madden <neil.mad...@forgerock.com> wrote: > I agree with Daniel that I’d be a bit wary of assuming that this could > never be exploited. For example, a server-side web app that signs DPoP > proofs on behalf of client-side Javascript (to keep the key safe on the > server) and reuses the key for different users could be a risk. > > IMO this is another symptom of the general issue of using signatures for > authentication - they are too strong for the job. The fact that a signature > is equally valid to all parties and at all times means you have to be very > careful to include enough context in the signature calculation to ensure > all these kinds of attacks are eliminated. And you have to ensure that all > RSes check the context. > > Contrast that to my suggestion to use ECDH [1], which was already immune > to such attacks by including the access token in the key derivation step. > (And in such a way that requires no additional data on the wire and is > almost impossible for the RS not to verify). Even without including the > access token in the KDF, the attack could only happen if the client reused > its key and the RS reused a challenge key pair. > > Macaroon access tokens [2] are also immune to this attack, as in that case > the constraints that go in the DPoP proof are directly attached to the > access token itself so there is no way to reuse them separately. > (Interestingly, macaroons have a more direct analogue of DPoP in the form > of discharge macaroons. Such discharge macaroons are required to be > “prepared” before use, which cryptographically binds them to the equivalent > of the access token - so this kind of attack was also considered and > addressed there). > > [1]: > https://mailarchive.ietf.org/arch/msg/oauth/1Zltt75p5taPw0DRmhoKLbavu9s/ > [2]: > https://neilmadden.blog/2020/07/29/least-privilege-with-less-effort-macaroon-access-tokens-in-am-7-0/ > > — Neil > > On 24 Nov 2020, at 09:38, Daniel Fett <f...@danielfett.de> wrote: > > > Thanks Justin for bringing this to our attention. > > Right now, I don't think that this is a big problem and I wasn't able to > come up with a way to improve the attack. I hope that it doesn't come back > to haunt us when somebody does a more in-depth analysis... > > That said, the lack of binding to the access token makes it easier to > precompute proofs if somebody has a limited code execution opportunity in > the client. We have this paragraph in the spec: > > Malicious XSS code executed in the context of the browser-based > client application is also in a position to create DPoP proofs with > timestamp values in the future and exfiltrate them in conjunction > with a token. These stolen artifacts can later be used together > independent of the client application to access protected resources. > The impact of such precomputed DPoP proofs can be limited somewhat by > a browser-based client generating and using a new DPoP key for each > new authorization code grant. > > The recommendation could be to use a fresh key pair for each token request. > > -Daniel > > > Am 20.11.20 um 20:26 schrieb Justin Richer: > > While working on an implementation of DPoP recently, I realized that the > value of the access token itself is not covered by the DPoP signature at all. > What I’m wondering is whether or not this constitutes an attack surface that > we care about here. Here’s how it works: > > > Let’s say that a client creates a DPoP key and uses that key to request two > tokens, T1 and T2, for different users, Alice and Bob, respectively. Alice is > malicious and wants to get Bob’s stuff. Alice manages to get a hold of Bob’s > token value, T2, through some means. Normally DPoP wouldn’t let Alice create > a new request using T2 since Alice doesn’t have the client’s key. However, if > Alice gets the client to create a request for her using T1, she can copy the > signature from that request onto a new request using T2. Since the signature > doesn’t cover the token value and the key is the same, the RS should accept > both requests, right? > > An important aspect is that the parts needed to make this attack work are a > little weird: you’d need access to a valid signed request from the client > with T1 as well as access to a valid T2 attached to the same key in order to > make this substitution. However, this is effectively the same attack area > that bearer tokens have in a lot of ways, since it doesn’t require the > attacker gaining access to the singing key to generate or modify a signature, > nor does it require the attacker to generate or modify an access token > (merely obtain one). > > > I’d like to understand if this is an actual attack against DPoP. If it isn’t, > how is it countered by DPoP today? If it is, do we discuss in the DPoP draft? > I didn’t see a mention of it there. If it’s not, should we discuss it there? > > > The old OAuth PoP draft mitigates this attack by putting the access token > itself inside the signature body instead of a second header. Another option > would be to include a hash of the token value (such as OIDC’s “at_hash” > method used in ID Tokens) in the DPoP payload. With either of these > approaches, Alice having access to T1, T2, and a signed message for T1 does > not allow her to use T2 effectively. > > — Justin > _______________________________________________ > OAuth mailing listOAuth@ietf.orghttps://www.ietf.org/mailman/listinfo/oauth > > > -- https://danielfett.de > > _______________________________________________ > OAuth mailing list > OAuth@ietf.org > https://www.ietf.org/mailman/listinfo/oauth > > > ForgeRock values your Privacy <https://www.forgerock.com/your-privacy> > _______________________________________________ > OAuth mailing list > OAuth@ietf.org > https://www.ietf.org/mailman/listinfo/oauth > -- _CONFIDENTIALITY NOTICE: This email may contain confidential and privileged material for the sole use of the intended recipient(s). Any review, use, distribution or disclosure by others is strictly prohibited. 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