pyjwt is a JSON Web Token implementation in Python. An incorrect string comparison is run for iss checking, resulting in "acb" being accepted for "_abc_". This is a bug introduced in version 2.10.0: checking the "iss" claim changed from isinstance(issuer, list) to isinstance(issuer, Sequence). 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":. Signature checks are still present so real world impact is likely limited to denial of service scenarios. This issue has been patched in version 2.10.1. All users are advised to upgrade. There are no known workarounds for this vulnerability.