CVE-2026-102266: PyJWK accepts empty HMAC keys, bypassing PyJWT's empty-key validation
PyJWT is a Python implementation of JSON Web Token standards. From 2.13.0 until 2.14.0, HMACAlgorithm.fromjwk is affected because PyJWK verification path used the decoded key without applying preparekey validation. This occurs when a trusted JWK Set contains an oct entry with an empty k value. As a result, an attacker signs an HMAC token with the same zero-length key accepted by PyJWT. Consequently, forged token can carry arbitrary authenticated claims. This issue is fixed in version 2.14.0.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
PyJWTto a version that resolves this vulnerability.Fixed in 2.14.0
Event History
Frequently Asked Questions
Which deployments are exposed to this issue?
Deployments using PyJWT versions from 2.13.0 up to, but not including, 2.14.0 are affected when their trusted JWK Set contains an HMAC ("oct") key whose "k" value is empty. The affected path is PyJWK-based verification through HMACAlgorithm.from_jwk.
What does an attacker need to forge a token?
The attacker needs the application to trust a JWK Set containing an oct entry with an empty key value. They can then sign an HMAC token using the same zero-length key and supply arbitrary authenticated claims.
How can this be remediated?
Upgrade PyJWT to version 2.14.0, which fixes the issue. Also review trusted JWK Sets and remove or reject oct entries with an empty "k" value.