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.from_jwk is affected because PyJWK verification path used the decoded key without applying prepare_key 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

jpadilla · pyjwt

CVSS v3.1 NVD

9.1 CRITICALCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

CVSS v3.1 GITHUB

7.4 HIGHCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N

EPSS

0.18% chance of exploitation in the next 30 days, 7th percentile

Published

2026-09-28

Updated

2026-09-29

Proof-of-concept exploits (1)

• advisories/GHSA-9j54-fg26-wv3r

Security advisories

• jpadilla/pyjwt/security/advisories/GHSA-9j54-fg26-wv3r
• jpadilla/pyjwt/commit/f91ed44dd65baaf457f4b3353ed35e98a753934c

References

• CVE Record
• NVD entry