Authlib Authlib
CVE-2026-28498
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.
What this means for your business
- It affects Authlib. It matters if your company, or a supplier that handles your data, runs it.
- Code showing how to exploit it has been published, but real attacks have not been confirmed.
- An attacker can use it remotely, over a network, without a login, and without anyone at your company clicking anything.
- FIRST's prediction model gives it a 0.2% chance of attack attempts being seen in the next 30 days, ranking above 13% of all known flaws.
What to do
- 1Check whether your company or your suppliers run Authlib, and which version. The affected versions are listed further down this page.
- 2If you do, apply the vendor's fix. A patch or vendor advisory has been published.
Not sure if your company is exposed?
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Scoring
- CVSS
- 7.5 (v3.1)
- Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N- CVSS v4
- 8.2
- Weakness
- CWE-354
- Assigned by
- security-advisories@github.com
Dates
- Published
- 2026-03-16
- Last modified
- 2026-09-10
- Sources
- NVD
Affected products
- Authlib Authlib- 1.6.9
As listed in the NVD configuration data. Not a statement about your estate.
References
- https://github.com/authlib/authlib/commit/b9bb2b25bf8b7e01512d847a95c1749646eaa72b
- https://github.com/authlib/authlib/releases/tag/v1.6.9
- https://github.com/authlib/authlib/security/advisories/GHSA-m344-f55w-2m6j
- 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