Patch Forecast
Amber warning
Patch this week.

CVE-2026-102266 · GHSA-9j54-fg26-wv3r

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

Risk score
71/100
CVSS
9.1 v3.1
Exploit chance (30 days)
0.2%
Published
28 Sep 2026

Open in the live forecast Today's forecast

What to know

What it is
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 attack…
Affected
PyJWT (pip) >= 2.13.0, < 2.14.0
Fix
Upgrade PyJWT to 2.14.0 or later.
Exploitation
Public proof of concept

Story map

Story map: PyJWTTHE FLAWSTHE FIXCVE-2026-102266PyJWK accepts empty HMACkeys, bypassing PyJWT'sempty-key validationCVSS 9.1 · EPSS 0%CVE-2026-102268Asymmetric-PEM detectionbypass:whitespace/line-ending-mutat…CVSS 9.1 · EPSS 0%CVE-2026-101917PyJWT: PyJWKClient stillamplifies unauthenticatedJWKS fetches on unknown kid…CVSS 7.5 · EPSS 0%CVE-2026-102274Malformed RSA JWK abortsparsing of an entire JWK SetCVSS 7.5 · EPSS 0%Upgrade PyJWT2.14.0or later on your release lineWho is exposedPyJWT (pip) <= 2.13.0AMBER · PATCH THIS WEEKPyJWTPyJWT is a Pythonimplementation of JSON WebToken standards. Prior to 2.14.0,is_pem_format in jwt/utils.pyis affected becauseis_pem_format does notrecognize every PEM…Where it standsPublic proof of conceptCONNECTS TOSame weaknessCWE-347 (CWE-347)69 other warnings in 30 days →Same productEarlier PyJWT warnings10 in the last 90 days →
Story map: the flaws, the fix, how it unfolded and what it connects to. Boxes link to the record or article.

At a glance

PyJWT: Amber, patch this week. PyJWT is a Python implementation of JSON Web Token standards. Prior to 2.14.0, is_pem_format in jwt/utils.py is affected because is_pem_format does not recognize every PEM representation accepted by the cryptography loader. This occurs when an application mixes HMAC and asymmetric algorithms and supplies a mutated pub…

The flaws

  • CVE-2026-102266: PyJWK accepts empty HMAC keys, bypassing PyJWT's empty-key validation (CVSS 9.1 · EPSS 0%)
  • CVE-2026-102268: Asymmetric-PEM detection bypass: whitespace/line-ending-mutated public keys skip the HS/asymmetric confusion guard (CVSS 9.1 · EPSS 0%)
  • CVE-2026-101917: PyJWT: PyJWKClient still amplifies unauthenticated JWKS fetches on unknown kid values (incomplete fix of CVE-2026-48524) (CVSS 7.5 · EPSS 0%)
  • CVE-2026-102274: Malformed RSA JWK aborts parsing of an entire JWK Set (CVSS 7.5 · EPSS 0%)

The fix

Upgrade PyJWT: 2.14.0 or later on your release line

Who is exposed: PyJWT (pip) <= 2.13.0

Where it stands

  • Public proof of concept

Connects to

Why it is Amber

  • CVSS 9.1 critical
  • Public exploit or proof of concept referenced
  • Remotely exploitable without logging in
  • On your watchlist: PyPI

Affected packages

PackageEcosystemVulnerableFixed in
PyJWTpip>= 2.13.0, < 2.14.02.14.0

References

Original 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.

Last activity 28 Sep 2026. Patch Forecast updates every hour.