hello, A recent thread on the JOSE mailing list reminded me of how parsing JSON strings when not required can lead to scenarios where an attacker can exploit a vulnerable json parser, possibly before verification occurs... as tokens can sometimes be the first untrusted user input processed by systems, this can be very bad.
The word "parse" does not occur once in https://datatracker.ietf.org/doc/html/draft-ietf-oauth-selective-disclosure-jwt-05 And yet, there are instances of "JSON-encoded", which presumes "JSON decoding", and through implementation experience, I am aware that it is possible to decode and parse SD-JWTs and their disclosures without performing any verification of hashes or digital signatures. I think it would be wise to add a comment to security considerations stating something to the effect of: In general UTF-8 json strings should not be *parsed* until after a signature verification or a hash match has been confirmed, in the case that implementations parse strings that have not yet been authenticated either by public key signature verification or comparison of a hashed value to strings that have been verified by a public key, it's possible that a malicious issuer, or holder might exploit a verifier that relies on a vulnerable parser. There is this section of discovering issuer keys: https://datatracker.ietf.org/doc/html/draft-ietf-oauth-selective-disclosure-jwt-05#name-issuer-signature-key-distri My experience tells me that if the issuer has multiple keys, the verifier is often likely to employ one of these 2 strategies to find the correct key: 1. try all the keys they know for the issuer 2. try a specific key with a specific key id, possibly after parsing the protected header to obtain the key id. In the case that the parsing library is vulnerable, the attacker can craft a protected header that exploits the verifier prior to signature verification. Apologies if this has already been discussed. Regards, OS -- ORIE STEELE Chief Technology Officer www.transmute.industries <https://transmute.industries>
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