# CVE-2026-102266: PyJWT HMAC key validation bypass in JWK parsing

Source: https://junglewise.ai/threats/cve-2026-102266-pyjwt-is-a-python-implementation-of-json-web-token-standards  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-09-28. Updated: 2026-09-28  
License: CC BY 4.0 with a link back

CVE-2026-102266 · Severity: high · CVSS 7.4 · Published 2026-09-28

PyJWT is a Python library for creating and verifying JSON Web Tokens, which are used for secure authentication and authorization in modern applications. Versions 2.13.0 through 2.14.0 accept zero-length HMAC keys from trusted JWK Sets without proper validation, allowing an attacker to forge authentication tokens with arbitrary claims that will be accepted as legitimate.

Technologies: [Jpadilla PyJWT](https://junglewise.ai/threats/technologies/pyjwt), [pyjwt (PyPI)](https://junglewise.ai/threats/technologies/pypi-pyjwt). Vendors: [Jpadilla](https://junglewise.ai/threats/vendors/jpadilla), [PyPI](https://junglewise.ai/threats/vendors/pypi).

## Quick answers

**Is CVE-2026-102266 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-102266 being exploited in the wild.

**How severe is CVE-2026-102266?** High severity, with a CVSS score of 7.4, published 28 September 2026.

**Which products and versions are affected?** Jpadilla PyJWT 2.13.0 to 2.13.x; pyjwt (PyPI) >= 2.13.0, < 2.14.0, ECOSYSTEM:introduced=2.13.0,fixed=2.14.0.

**Is there a fix for CVE-2026-102266?** Yes. pyjwt (PyPI): upgrade to 2.14.0 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-102266 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-102266?** Check whether you run Jpadilla PyJWT or pyjwt (PyPI) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- pyjwt (PyPI): upgrade to 2.14.0 or later

  ```sh
  pip install "pyjwt>=2.14.0"
  ```


## Technical details

The HMACAlgorithm.from_jwk method fails to apply prepare_key validation to the decoded key material when processing oct (symmetric key) entries from a JWK Set. An attacker can exploit this by crafting a JWK Set entry with an empty k value; PyJWT will accept this zero-length key for token verification, enabling the attacker to sign forged tokens with the same weak key. The vulnerability requires network access to the token verification endpoint and trust in a compromised or attacker-controlled JWK Set.

## Affected products

- jpadilla PyJWT 2.13.0 to 2.13.x

## Identifiers

- CVE-2026-102266
- GHSA-9j54-fg26-wv3r
- PYSEC-2026-4143

## Timeline

- 2026-09-28: disclosed
- 2026-09: patched: Fixed in version 2.14.0

## References

- [https://github.com/jpadilla/pyjwt/commit/f91ed44dd65baaf457f4b3353ed35e98a753934c](https://github.com/jpadilla/pyjwt/commit/f91ed44dd65baaf457f4b3353ed35e98a753934c)
- [https://github.com/jpadilla/pyjwt/releases/tag/2.14.0](https://github.com/jpadilla/pyjwt/releases/tag/2.14.0)

## Related threats

- [PyJWT HS256/384/512 token forgery via empty HMAC key in oct JWK](https://junglewise.ai/threats/pyjwt-hs256-384-512-verify-accepts-empty-hmac-key-via-oct-jwk-pyjwk-a3ba63d2)
- [PyJWT decode() options-dict mutation enabling claim-verification bypass](https://junglewise.ai/threats/cve-2026-103001-pyjwt-decode-reintroduces-options-dict-mutation-enabling-silent)
- [PyJWT OKP algorithm key validation bypass](https://junglewise.ai/threats/cve-2026-102275-pyjwt-is-a-python-implementation-of-json-web-token-standards)
- [PyJWT unhandled ValueError in JWK Set parsing](https://junglewise.ai/threats/cve-2026-102274-pyjwt-is-a-python-implementation-of-json-web-token-standards)
- [PyJWT HMAC key validation bypass in HMACAlgorithm.prepare_key](https://junglewise.ai/threats/cve-2026-102273-pyjwt-is-a-python-implementation-of-json-web-token-standards)