CVE-2026-114615: Authlib incomplete algorithm-confusion fix permits HMAC forgery with prefixed public keys

Severity: critical (CVSS 9.1 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.1% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-10-22 · updated 2026-10-29
Weakness: CWE-184

Description

Authlib is a Python library which builds OAuth and OpenID Connect servers. From 1.3.1 until 1.9.0, OctKey.import_key() checks asymmetric-key markers only at byte zero while Authlib's PEM loaders accept leading whitespace, comments, or a byte-order mark. In deployments that use the documented mixed algorithm registry and already configure verification with PEM or certificate bytes containing such a prefix, an unauthenticated attacker who knows those same public-key bytes can use them as an HMAC secret and forge HS256 tokens with arbitrary identity or authorization claims. This is a bypass of the remediation for CVE-2024-37568. The module-level jwt object reaches OctKey.import_key() in authlib/jose/rfc7518/oct_key.py, where POSSIBLE_UNSAFE_KEYS fails to identify prefixed asymmetric key material. This issue is fixed in version 1.9.0.

Affected products

authlib: from 1.3.1 before 1.9.0 — fixed in 1.9.0
authlib: from 1.3.1, < 1.9.0

How to fix

Upgrade to a fixed version: authlib 1.9.0. Check the vendor advisory for mitigations.

References

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