PyJWT is a Python implementation of JSON Web Token standards.
CVE-2026-101918
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 jwt.decode(token, options={"verifysignature": False}) too. Notably, this project already fixed the identical bug class for the JWS header path — PyJWS.load catches (ValueError, RecursionError) and wraps it…
Assessment
- CVSS
- 7.5 (v3.1)
- Severity
- High
- EPSS
- 0.41% chance of exploitation in the next 30 days
- Known exploited
- Not listed in CISA KEV
- Exploit availability
- Nothing found in Exploit-DB, Metasploit, Nuclei or public PoC repositories
- Published
- 28 September 2026
- Last updated
- 7 October 2026
- Weakness
- CWE-248
- Assigner
- security-advisories@github.com
CVSS vectorCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
At the source
References
- https://github.com/jpadilla/pyjwt/commit/5fde08a6cf906aa7698de2d6391d88b73006b17b
- https://github.com/jpadilla/pyjwt/releases/tag/2.15.0
- https://github.com/jpadilla/pyjwt/security/advisories/GHSA-42vr-xj54-vc7v
- https://nvd.nist.gov/vuln/detail/CVE-2026-101918
- https://github.com/advisories/GHSA-42vr-xj54-vc7v
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