Re: [OAUTH-WG] Downgrade attacks on PKCE
Ryan Kelly <rfkelly@mozilla.com> Mon, 01 June 2020 00:03 UTC
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From: Ryan Kelly <rfkelly@mozilla.com>
Date: Mon, 01 Jun 2020 10:03:33 +1000
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To: Daniel Fett <fett@danielfett.de>
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Subject: Re: [OAUTH-WG] Downgrade attacks on PKCE
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On Sat, 30 May 2020 at 17:59, Daniel Fett <fett@danielfett.de> wrote: > Aaron, Dick, Torsten and I today discussed the downgrade attacks on PKCE > [1] and how to mitigate them in OAuth 2.1 and 2.0. We came to the > conclusion that we have two options: > [..snip..] > *2. "Dynamic" Solution* > > Each AS that supports PKCE MUST check whether a code challenge is > contained in the authorization request. This information MUST be bound to > the code that is issued. > > When a code arrives at the token endpoint, the AS MUST do the following > check: > > 1. If there was a code_challenge in the authorization request for > which this code was issued, there must be a code_verifier in the token > request and it must be verified according to RFC7636. (This is no change > from the current behavior in RFC7636.) > 2. If there was no code_challenge in the authorization request, any > request to the token endpoint containing a code_verifier MUST be rejected. > > Properties: > > - No change in behavior needed for properly implemented clients. > Backwards compatible for all existing deployments. > - Implementation is mainly some logic for the authorization endpoint, > token endpoint, and a new data field in the authorization session > maintained by the AS. > - Slightly more complex to implement for the AS, maybe. > > We would like to hear the feedback from the working group on these two > solutions before proceeding to propose wording for the affected documents. > When we added PKCE support in Firefox Accounts, we implemented the suggested check (2) above purely as a defensive-coding measure, expecting that it might help us catch buggy clients. I think it makes sense to add as a required behaviour since it seems like a pretty clear signal of mismatched expectations between client and server. Cheers, Ryan
- [OAUTH-WG] Downgrade attacks on PKCE Daniel Fett
- Re: [OAUTH-WG] Downgrade attacks on PKCE Neil Madden
- Re: [OAUTH-WG] Downgrade attacks on PKCE Ryan Kelly
- Re: [OAUTH-WG] Downgrade attacks on PKCE Filip Skokan
- Re: [OAUTH-WG] Downgrade attacks on PKCE Dick Hardt
- Re: [OAUTH-WG] Downgrade attacks on PKCE Mike Jones
- Re: [OAUTH-WG] Downgrade attacks on PKCE John Bradley
- Re: [OAUTH-WG] Downgrade attacks on PKCE Vladimir Dzhuvinov
- Re: [OAUTH-WG] Downgrade attacks on PKCE Gavin Henry
- Re: [OAUTH-WG] Downgrade attacks on PKCE Kazuki Tsuzuku