CVE-2026-102265: PyJWT: Uncaught RecursionError in jwt.decode() on deeply nested token header

PyJWS._looad() (jwt/api_jws.py:337) splits the compact token, base64url decodes the header segment and hands it to json.loads() before _verify_signature() runs. The parse is wrapped in except ValueError as e: raise DecodeError(...). That is what makes the documented contract sufficient for untrusted input: except jwt.PyJWTError (or InvalidTokenError) around jwt.decode(token, key, algorithms=[...]).

CPython’s JSON decoder raises RecursionError (a RuntimeError subclass, not a ValueError) once nesting crosses the native stack guard. That exception leaves _load(), then jwt.decode(), and matches no PyJWTError handlerr at all. No key and no valid signature are needed: the header is parsed first, so a single unauthenticated request with an unsigned token suffices.

Scope: the recursion threshold is ~65,000 nesting levels; below it the same input yields a clean DecodeError (a 346,800-byte flat header is rejected normally, so depth, not size, is the trigger). A token this large cannot ride in an Authorization header (rejected at HTTP field limits,431 under gunicorn), but body transport (RFC 7662 introspection style) is a standard pattern. Under Flask 3.1.3 / gunicorn 26.2.0 each crafted request returns HTTP 500 instead of 401 and writes a full traceback to the error log; the sync worker survives.