V51 - OAuth - DPoP proof replay attack protection

[randomstuff]

Should we add some requirement about DPoP proof replay attack protection?

Possible concrete mitigations:

• use a server-provided nonce
• limit the window of validity of the DPoP proof
• "jti" storage

FAPI 2.0 does not require the usage of DPoP server-side nonce:

[authorization servers] if using DPoP, may use the server provided nonce mechanism

if using DPoP, [clients] shall support the server provided nonce mechanism (as defined in Section 8 of [RFC9449]);

FAPI 2.0 has this requirement about "iat" validation (which is valid applicable to DPoP and other JWTs as well) but this actually does not help so much for our purpose:

to accommodate clock offsets, shall accept JWTs with an iat or nbf timestamp between 0 and 10 seconds in the future but shall reject JWTs with an iat or nbf timestamp greater than 60 seconds in the future.

[randomstuff]

See #2185

[randomstuff]

I don't believe, we should require the usage us DPoP nonce (FAPI 2.0 does not require it) or "jti" storage.

Possible options:

• do not mention this topic at all;
• only require that some mitigation is in place;
• require that "iat" validation uses a small window of validity.

[elarlang]

As we going to write requirements that must be followed by everyone (on a specific level), then I recommend ignoring the topic till we are not 100% sure or it is too specific an edge case with a small impact.

[TobiasAhnoff]

I think this does not need to be a requirement, FAPI also recognizes this as an "edge case", given confidential clients and alignment with other FAPI requirements (which we also have for L3), and in https://openid.bitbucket.io/fapi/fapi-2_0-security-profile.html#section-6.2 states that

These mitigations may have potential complexity, performance or scalability trade-offs. Attacker type A5 represents a powerful attacker and mitigations may not be necessary for many ecosystems.

So, I vote for "do not mention this topic at all" (even if e g nonce is a good mitigation to implement for some scenarios)