CVE-2026-48523: PyJWT: Algorithm allow-list bypass when decoding with `PyJWK` / `PyJWKClient` keys

PyJWT’s PyJWK verification path allows a verifier-side algorithm allow-list bypass.

In affected versions, when a JWT is decoded with a PyJWK object, PyJWT verifies that the header alg string is present in the caller’s algorithms=[...] list, but it does not actually use the header algorithm to verify the signature. Instead, it verifies with the algorithm already bound to the PyJWK object.

This lets an attacker who controls a registered JWK/JWKS private key sign with a disallowed algorithm and have the token accepted as long as the JWT header advertises an allowed algorithm. This affects the documented PyJWKClient usage flow and does not require any non-default flags or unsafe configuration.