PyJWT CVE-2026-102265

No special product configuration is required beyond the application calling PyJWT 2.13.0's jwt.decode / PyJWS.decode on attacker-supplied token input at an endpoint reachable without authentication (AV:N/PR:N/UI:N). The only limiting factors are: the target must run PyJWT exactly in the 2.13.0-to-pre-2.14.0 range, and impact is bounded to a single failed request (HTTP 500) rather than a full service outage because Python's recursion limit turns the deep nesting into a discrete exception. Applications that already wrap PyJWT calls in broad exception handlers would absorb the RecursionError and avoid the 500, reducing real-world impact.

This is a genuine but low-severity availability issue, not a high-impact flaw. The provided CVSS 3.1 vector (5.3, AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L) is internally consistent with the description: a remote, unauthenticated attacker sends one crafted token with a deeply nested header; PyJWS._load catches ValueError but not RecursionError, so the RecursionError escapes PyJWT's documented exception hierarchy and surfaces as an unhandled exception, typically an HTTP 500 request-level failure. Impact is limited to availability, and only A:L - a single failed request rather than a sustained process crash or memory exhaustion, since Python's recursion limit converts the deep nesting into a caught-too-late exception rather than a hang. There is no confidentiality or integrity impact despite the 'Information Disclosure' tag, which is inaccurate. No EPSS score, no CISA KEV listing, and no public exploit code were supplied (the commit adds only unit tests, not a weaponized PoC), so exploitation status is 'no public exploit identified.' Net: real and trivially triggerable, but low real-world priority because it degrades one request, not the service, and the underlying library is ubiquitous only in the sense of the package - the actual attack surface is any endpoint that decodes attacker-supplied JWTs without wrapping PyJWT exceptions.

An attacker submits a JWT whose header segment base64url-decodes to a deeply nested JSON structure (e.g., ~1000 nested brackets) to an unauthenticated endpoint that calls PyJWT's decode on PyJWT 2.13.0. json.loads exceeds Python's recursion limit, raising a RecursionError that PyJWT's _load does not catch, so it propagates as an unhandled exception and the web framework returns HTTP 500. No public exploit is published, but the fix commit's test case demonstrates the exact trigger payload.