Authlib is a Python library that implements OAuth and OpenID Connect servers. A cryptographic padding oracle vulnerability was found in the implementation of the JSON Web Encryption (JWE) RSA1_5 key management algorithm. The library registers RSA1_5 by default without explicit opt‑in and removes the constant‑time Bleichenbacher mitigation that the underlying cryptography library otherwise provides. As a result, an attacker who can influence the processing of a JWE token can, by exploiting the padding oracle, recover plaintext data or keys, leading to a confidentiality breach and potential further compromise of the system.

The vulnerability affects the Authlib Python library (authlib:authlib). All versions before 1.6.9 are impacted. The security advisory lists v1.6.9 and later as the fixed release.

The CVSS score is 8.3, indicating a high severity. The EPSS score is less than 1 %, suggesting a low probability of exploitation at the present time. It is not listed in the CISA Known Exploited Vulnerabilities catalog. The attack vector is likely remote: an adversary can craft a JWE token processed by the vulnerable library and repeatedly submit modified ciphertexts to observe padding error responses. Observation of the success of padding operations would allow reconstruction of the underlying plaintext or key material. This exploitation requires the attacker to have the ability to submit JWE requests to an application that uses Authlib.