What’s Vulnerable › Vulnerabilities

CVE-2026-102266: PyJWK accepts empty HMAC keys, bypassing PyJWT's empty-key validation

• Severity: critical (CVSS 9.1 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.2% probability of exploitation in the next 30 days
• Public exploit or detection template: yes
• Published: 2026-09-28 · updated 2026-10-07
• Weakness: CWE-347

Description

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 products

• jpadilla pyjwt: 2.13.0

References

• https://github.com/jpadilla/pyjwt/commit/f91ed44dd65baaf457f4b3353ed35e98a753934c
• https://github.com/jpadilla/pyjwt/releases/tag/2.14.0
• https://github.com/jpadilla/pyjwt/security/advisories/GHSA-9j54-fg26-wv3r
• https://github.com/jpadilla/pyjwt/security/advisories/GHSA-9j54-fg26-wv3r

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