The vulnerability in python-jose allows an attacker who has the service’s public key to construct a valid HS256 JSON Web Token (JWT) that will verify successfully when the token validation routine does not explicitly restrict the algorithm. Because the library fails to treat the DER-encoded public key as an asymmetric key, the key is interpreted as an HMAC secret. The forged token can carry arbitrary claims such as elevated privileges or altered timestamps, giving the attacker unauthorized access or the ability to subvert application logic. This represents an injection‑type flaw (CWE‑347) that compromises integrity and authentication of the application.

The affected product is the python‑jose library published by mpdavis, up to and including version 3.5.0. All users of that library that rely on JWT verification without algorithm restriction are potentially affected. No other vendors or products are listed as impacted.

The CVSS score of 9.3 places this issue in the critical severity range. No EPSS data is available, and it is not listed in CISA’s KEV catalog. The vulnerability is exploitable in environments where the public key of the service is publicly known or can be obtained by an attacker. The lack of explicit algorithm checks means that an attacker can forge a token and have it accepted by the application’s authentication layer, leading to unauthorized actions or privilege escalation. The attack vector is therefore application‑level, potentially remote, depending on the exposure of the public key.