CVE-2024-53861: Issuer field partial matches allowed in pyjwt

Summary The wrong string if check is run for iss checking, resulting in "acb" being accepted for "abc".

Details This is a bug introduced in version 2.10.0: checking the "iss" claim changed from isinstance(issuer, list) to isinstance(issuer, Sequence).

diff - if isinstance(issuer, list): + if isinstance(issuer, Sequence): if payload["iss"] not in issuer: raise InvalidIssuerError("Invalid issuer") else:

Since str is a Sequnce, but not a list, in is also used for string comparison. This results in if "abc" not in "abcd": being checked instead of if "abc" != "abc":. PoC Check out the unit tests added here: https://github.com/jpadilla/pyjwt-ghsa-75c5-xw7c-p5pm python issuer = "urn:expected"

payload = {"iss": "urn:"}

token = jwt.encode(payload, "secret")

# decode() succeeds, even though "urn:" != "urn:expected". No exception is raised. with pytest.raises(InvalidIssuerError): jwt.decode(token, "secret", issuer=issuer, algorithms=["HS256"])

Impact

I would say the real world impact is not that high, seeing as the signature still has to match. We should still fix it.