CVE-2026-28498: authlib

Vulnerability summary

CVE-2026-28498 is a critical (CVSS 9.1) vulnerability in authlib. EPSS estimates a 0.27% chance of exploitation in the next 30 days, higher than 17.6% of scored vulnerabilities. 1 public exploit is recorded.

Timeline

1. 16-03-2026
   PublishedVulnerability first disclosed.
2. 10-09-2026
   Last modifiedVulnerability information updated.

Description

Authlib is a Python library which builds OAuth and OpenID Connect servers. Prior to version 1.6.9, a library-level vulnerability was identified in the Authlib Python library concerning the validation of OpenID Connect (OIDC) ID Tokens. Specifically, the internal hash verification logic (_verify_hash) responsible for validating the at_hash (Access Token Hash) and c_hash (Authorization Code Hash) claims exhibits a fail-open behavior when encountering an unsupported or unknown cryptographic algorithm. This flaw allows an attacker to bypass mandatory integrity protections by supplying a forged ID Token with a deliberately unrecognized alg header parameter. The library intercepts the unsupported state and silently returns True (validation passed), inherently violating fundamental cryptographic design principles and direct OIDC specifications. This issue has been patched in version 1.6.9.

• CWE-354Improper Validation of Integrity Check ValueThe product does not validate or incorrectly validates the integrity check values or "checksums" of a message. This may prevent it from detecting if the data has been modified or corrupted in transmission.
• CWE-573Improper Following of Specification by CallerThe product does not follow or incorrectly follows the specifications as required by the implementation language, environment, framework, protocol, or platform.
• CWE-325Missing Cryptographic StepThe product does not implement a required step in a cryptographic algorithm, resulting in weaker encryption than advertised by the algorithm.