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Vulnerability · Published Mar 16, 2026 · Assigned by GitHub_M

CVE-2026-28498Authlib: Fail-Open Cryptographic Verification in OIDC Hash Binding

Critical 9.1

In plain English

A critical-severity security flaw in authlib from authlib. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 15% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)9.1Critical
Exploit odds (30 days)0.2%top 84.8% of all CVEs
Exploited in the wildNo reportsCISA SSVC: poc
Attack toolingNoneno public tooling known

Official 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.

Affected packages & fixed versions

EcosystemPackageFixed inAdvisory
Debian:12python-authlib1.2.0-1+deb12u2DEBIAN-CVE-2026-28498
Debian:13python-authlibno fix listedDEBIAN-CVE-2026-28498
Debian:14python-authlib1.6.9-1DEBIAN-CVE-2026-28498
PyPIauthlib1.6.9PYSEC-2026-2117
PyPIauthlib1.6.9GHSA-m344-f55w-2m6j
Ubuntu:25.10python-authlibno fix listedUBUNTU-CVE-2026-28498
Ubuntu:26.10python-authlibno fix listedUBUNTU-CVE-2026-28498

Affected products (as reported)

VendorProductVersions
authlib authlib < 1.6.9

Weakness: CWE-354, CWE-573, CWE-325 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

References