Description
Authlib is a Python library which builds OAuth and OpenID Connect servers. Prior to 1.9.1, Authlib's OctKey.import_key() and HMAC signing and verification path accept zero-length key material. If an application's secret lookup can return an empty string or empty bytes, a remote attacker can compute the deterministic HS256, HS384, or HS512 signature for arbitrary token claims and have Authlib treat the token as authentic. This issue is fixed in version 1.9.1.
Affected products
authlib: authlib
References3
- github.com/authlib/authlib/security/advisories/GHSA-8m4g-j4wp-w… ExploitMitigationVendor Advisory
- github.com/authlib/authlib/commit/4ff30db442c70e29486e1982cb0ad… Patch
- github.com/authlib/authlib/releases/tag/v1.9.1 Release Notes
Metrics
- CVSS 3.1
-
9.1
CRITICAL
Score source: NVD
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N - EPSS — exploit probability
- 0% percentile: 23 · as of 8 November 2026
- CWE
- CWE-287CWE-326CWE-1391
Could this be exploited in your environment?
A patch closes the known hole. A penetration test shows what an attacker can still reach — with proof.