CVE-2026-102266 PyJWK accepts empty HMAC keys, bypassing PyJWT's empty-key validation
Attackers can forge JWT tokens with arbitrary claims by exploiting empty HMAC key validation in PyJWK, bypassing authentication.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-102266?
CVE-2026-102266 is a high-severity authentication bypass vulnerability in jpadilla pyjwt, published on 28 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Weakness: CWE-347
Which versions are affected by CVE-2026-102266?
3 open source packages ship an affected version, including pyjwt and PyJWT. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
pyjwt PyPI | From 2.13.0 | 2.14.0 |
pyjwt PyPI | From 2.13.0 | 2.14.0 |
PyJWT PyPI | From 2.13.0 | 2.14.0 |
Products include Pyjwt Project Pyjwt.
How do I fix CVE-2026-102266?
Upgrade pyjwt to 2.14.0 or later.
Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.
Is CVE-2026-102266 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-102266 exists.
Other vulnerabilities in jpadilla pyjwt
- CVE-2026-102268 PyJWT: Asymmetric-PEM detection bypass: whitespace/line-ending-mutated public keys skip the HS/asymmetric confusion guard
- CVE-2026-32597 PyJWT accepts unknown `crit` header extensions (RFC 7515 §4.1.11 MUST violation)
- CVE-2026-102267 PyJWT: PyJWKClient follows redirects when fetching JWKS
- CVE-2026-102271 PyJWT: Public keys in DER form are accepted as HMAC secrets, bypassing the CVE-2022-29217 guard
- CVE-2026-102272 PyJWT BOM Bypass
Sources
Also known as AZL-105239, CLEANSTART-2026-OQ39308, CLEANSTART-2026-XF88803, DEBIAN-CVE-2026-102266, ECHO-4dcd-bf8f-9933, GHSA-9j54-fg26-wv3r and 17 more.