PyJWT: Unauthenticated RecursionError DoS in pre-verification payload parse (PyJWKClient.get_signing_key_from_jwt / verify_signature=False)

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

5.3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

EPSS

• CVE-2026-101918: 0.41% estimated exploitation activity in the next 30 days; percentile 35.63 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary PyJWKClient.getsigningkeyfromjwt(token) — the first step of the JWKS verification flow documented in docs/usage.rst — must decode a token's payload before its signature can be checked, via jwt.apijwt.decodecomplete(token, options={"verifysignature": False}). That call parses the payload with json.loads in PyJWT.decodepayload (jwt/apijwt.py:297-300), whose except clause catches only ValueError. A payload that is valid JSON but nested ~20,000 levels deep makes json.loads raise RecursionError, which is not a ValueError and escapes as a raw, undocumented exception type — not DecodeError, InvalidTokenError, or PyJWTError, so every documented error-handling pattern in docs/usage.rst misses it. The token needs no valid signature and no network access — the crash happens during payload parsing, before the kid is even looked up. A single unauthenticated ~50KB request crashes the caller's auth handler (HTTP 500 / dead worker), repeatably. The same path is reachable through plain…

Published by the source

30 Sep 2026, 15:41 UTC

Last modified by the source

1 Oct 2026, 17:56 UTC

Source last checked

26 Oct 2026, 13:54 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-42vr-xj54-vc7v

2. Revision 2 · amended · 24 Oct 2026

   Amended from osv record GHSA-42vr-xj54-vc7v

3. Revision 1 · created · 23 Oct 2026

   Published from ghsa record GHSA-42vr-xj54-vc7v

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# Patch Next advisory brief: CVE-2026-101918

Advisory: PyJWT: Unauthenticated RecursionError DoS in pre-verification payload parse (PyJWKClient.get_signing_key_from_jwt / verify_signature=False)
IDs: CVE-2026-101918, GHSA-42vr-xj54-vc7v
Record: https://patchnext.com/advisories/cve-2026-101918 (revision 3, source checked 26 Oct 2026, 13:54 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: pyjwt 2.15.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI pyjwt: affected >= 2.0.0a1, <= 2.14.0; fixed in 2.15.0

## Suggested inspection (read-only)
- `python -m pip show pyjwt`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on pyjwt.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-42vr-xj54-vc7v (GitHub-reviewed advisory GHSA-42vr-xj54-vc7v lists affected ranges: PyPI pyjwt >=2.0.0a1 <=2.14.0)
- Verified: https://osv.dev/vulnerability/GHSA-42vr-xj54-vc7v (OSV record GHSA-42vr-xj54-vc7v lists affected ranges: PyPI pyjwt >=2.0.0a1 <2.15.0 (fixed))

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