CVE-2026-28498

Authlib: Fail-Open Cryptographic Verification in OIDC Hash Binding

Published Mar 16, 2026 CVSS 4.0 · 8.2 High EPSS < 1% KEV · Not listed
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.

Remediation

No solution or workaround provided in the CVE record

Tracking

Sign in to view the affected projects.

Advisories

Source ID Title
Debian DLA Debian DLA DLA-4579-1 python-authlib security update
Debian DLA Debian DLA DLA-4708-1 python-authlib security update
Github GHSA Github GHSA GHSA-m344-f55w-2m6j Authlib: Fail-Open Cryptographic Verification in OIDC Hash Binding
Ubuntu USN Ubuntu USN USN-8557-1 Authlib vulnerabilities
History
Type Values Removed Values Added
CPEs cpe:2.3:a:authlib:authlib:*:*:*:*:*:*:*:*
Metrics cvssV3_1

{'score': 9.1, 'vector': 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N'}

cvssV3_1

{'score': 7.5, 'vector': 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N'}

Type Values Removed Values Added
First Time appeared Authlib
Authlib authlib
Vendors & Products Authlib
Authlib authlib
Type Values Removed Values Added
References
Metrics threat_severity

None

cvssV3_1

{'score': 9.1, 'vector': 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N'}

threat_severity

Important

Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'no', 'Exploitation': 'poc', 'Technical Impact': 'partial'}, 'version': '2.0.3'}

Type Values Removed Values Added
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.
Title Authlib: Fail-Open Cryptographic Verification in OIDC Hash Binding
Weaknesses CWE-354
CWE-573
References
Metrics cvssV4_0

{'score': 8.2, 'vector': 'CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N'}

Weaknesses

Sources

Status
PUBLISHED
Assigner
GitHub_M
Published
Updated
Sep 10, 2026, 12:04 PM
Reserved
Feb 27, 2026, 8:57 PM
Status
Modified
Published
Mar 16, 2026, 6:16 PM
Modified
Sep 10, 2026, 1:18 PM
Severity
Important
Public date
Mar 16, 2026, 6:03 PM