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Vulnerability record · CVE-2026-28498 · published 16 March 2026

CVE-2026-28498: Authlib vulnerability

Authlib · Authlib

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.

8.2 CVSS 4.0 High EPSS 0.26% · top 84.4% CWE-354 · CWE-354CWE-573 · CWE-573
8.2CVSS 4.0 base score
0.26%EPSS exploitation probability, 30 days
NoNot in CISA KEV
1Affected product versions listed by NVD
12References, 1 tagged exploit
10 Sep 2026Last modified by NVD

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.

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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:

Affected products

1 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

References

LinkTags
https://github.com/authlib/authlib/commit/b9bb2b25bf8b7e01512d847a95c1749646eaa72b Patch
https://github.com/authlib/authlib/releases/tag/v1.6.9 ProductRelease Notes
https://github.com/authlib/authlib/security/advisories/GHSA-m344-f55w-2m6j ExploitMitigationVendor Advisory
https://access.redhat.com/errata/RHSA-2026:6309
https://access.redhat.com/errata/RHSA-2026:6497
https://access.redhat.com/errata/RHSA-2026:6567
https://access.redhat.com/errata/RHSA-2026:6568
https://access.redhat.com/errata/RHSA-2026:6720
https://access.redhat.com/errata/RHSA-2026:6912
https://access.redhat.com/security/cve/CVE-2026-28498
https://bugzilla.redhat.com/show_bug.cgi?id=2448182
https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-28498.json

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Related vulnerabilities

Same products first, then exploited flaws of the same weakness class.

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Source: NIST National Vulnerability Database (record CVE-2026-28498), CISA KEV, FIRST EPSS (scores of 2026-09-27). This page is refreshed as NVD updates the record.