# CVE-2026-101918: PyJWT recursion error denial of service

Source: https://junglewise.ai/threats/cve-2026-101918-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-2026-101918 · Severity: medium · CVSS 5.3 · Published 2026-09-28

PyJWT, a Python library that validates digital tokens used for secure authentication and authorization, fails to properly handle malicious tokens with deeply nested data structures. An attacker can send a specially crafted token that causes the application to crash with an HTTP 500 error, disrupting service for legitimate users without requiring authentication.

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

## Quick answers

**Is CVE-2026-101918 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-101918 being exploited in the wild.

**How severe is CVE-2026-101918?** Medium severity, with a CVSS score of 5.3, published 28 September 2026.

**Which products and versions are affected?** PyJWT 2.0.0a1 to 2.15.0; pyjwt (PyPI) >= 2.0.0a1, <= 2.14.0, ECOSYSTEM:introduced=2.0.0a1,fixed=2.15.0.

**Is there a fix for CVE-2026-101918?** Yes. pyjwt (PyPI): upgrade to 2.15.0 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-101918 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-101918?** Check whether you run 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 2.15.0 or later

  ```sh
  pip install "pyjwt>=2.15.0"
  ```


## Technical details

PyJWT versions 2.0.0a1 through 2.15.0 catch ValueError exceptions during JSON payload parsing but do not catch RecursionError exceptions. An attacker can supply a JWT with a recursively nested payload that triggers RecursionError when passed to json.loads(), bypassing the library's documented exception handling and causing an unhandled exception. The vulnerability affects PyJWKClient.get_signing_key_from_jwt and jwt/api_jwt.py with verify_signature=False, allowing unauthenticated denial of service via network-supplied malicious tokens.

## Affected products

- PyJWT PyJWT 2.0.0a1 to 2.15.0

## Identifiers

- CVE-2026-101918
- GHSA-42vr-xj54-vc7v
- PYSEC-2026-4141

## Timeline

- 2026-09-28: disclosed
- 2026-09-23: patched: Version 2.15.0 released with fix

## References

- [https://github.com/jpadilla/pyjwt/commit/5fde08a6cf906aa7698de2d6391d88b73006b17b](https://github.com/jpadilla/pyjwt/commit/5fde08a6cf906aa7698de2d6391d88b73006b17b)
- [https://github.com/jpadilla/pyjwt/releases/tag/2.15.0](https://github.com/jpadilla/pyjwt/releases/tag/2.15.0)

## 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)