PyJWT: Malformed RSA JWK aborts parsing of an entire JWK Set

PyJWKSet.from_dict/from_json and PyJWK.from_dict/from_json are exported public API (jwt/__init__.py). PyJWKClient.get_jwk_set (jwt/jwks_client.py:158) feeds a fetched JWKS HTTP response directly into PyJWKSet.from_dict with no per-key pre-validation, so this is reachable through the documented PyJWKClient flow whenever the fetched JWKS contains a malformed key alongside valid ones.

Proof of concept

import jwt

good_jwk = {
    "kty": "RSA",
    "n": "<a valid base64url-encoded RSA modulus, e.g. from a real 2048-bit public key>",
    "e": "AQAB",
}

bad_jwk = {
    "kty": "RSA",
    "n": good_jwk["n"],
    "e": "AQAB",
    "d": "AAAAAA",  # not the true private exponent for n/e, no CRT params present
}

jwks_doc = {"keys": [bad_jwk, good_jwk]}

jwt.PyJWKSet.from_dict(jwks_doc)
# raises: ValueError: Unable to compute factors p and q from exponent d.
# (uncaught -- PyJWKSet.__init__ never returns, `good_jwk` is never added)

Impact

PyJWKSet.__init__ raises before completing, so no key in the JWK Set is added to the resulting set, including keys unrelated to the malformed entry. This requires the malformed key to already be present in a JWK Set the application parses (e.g. one entry in an aggregated/federated key set, or a key affected by transit corruption before signature verification of the JWKS transport itself). Applications that vet each key individually before adding it to a trusted set are not affected. The failure is an uncaught ValueError, not one of PyJWT's documented jwt.exceptions.* types, so exception handling written against PyJWT's documented contract (except jwt.exceptions.PyJWTError) will not catch it either.

Suggested remediation

Wrap the key-construction call in PyJWK.__init__ (api_jwk.py:82) so a ValueError is converted into InvalidKeyError (a PyJWTError subclass):

try:
    self.key = self.Algorithm.from_jwk(self._jwk_data)
except ValueError as e:
    raise InvalidKeyError(f"Unable to construct key from JWK: {e}") from e

This lets PyJWKSet.__init__'s existing except PyJWTError: continue skip the one malformed key as its own comment already states is intended.

Responsible Disclosure Timeline

Per the OWASP Vulnerability Disclosure Cheat Sheet and Google Project Zero's 2020 disclosure policy:

• Day 0 (date of submission): to be set to the actual API response's created_at timestamp when this report is submitted. If submitted on the date of this draft (2026-09-05), Day 0 = 2026-09-05.
• Day 90 (full-public-disclosure deadline, regardless of fix status): Day 0 + 90 days -- 2026-12-04 if Day 0 is 2026-09-05.
• A brief mutually-agreed extension (standard 14-day grace period) is available if a fix is scheduled but not yet shipped by Day 90 -- extending to 2026-12-18 in that case.
• This window may shorten instead of extend if the issue is confirmed under active exploitation.

Try It Yourself (Sandbox)

Reproduce the PoC above, and attempt your own fix, in an isolated sandbox with no access to production systems, secrets, or real data.

• Sandbox: jwt.py
• Course: Tenant Zero: AI & SaaS Authz Failures

Credit

Discovered and reported by SecDim Security Research: secdim.com, @secdim, [email protected].

Maintainer update — 2026-09-08

We reproduced the reported behavior on PyJWT 2.13.0 with cryptography installed: a malformed RSA private JWK containing an invalid d value and no CRT parameters raised a plain ValueError while parsing a JWK set. Because PyJWKSet skips PyJWTError instances only, that exception aborted parsing before subsequent valid keys were loaded. The impact is a conditional availability failure for applications that parse a key set containing a malformed entry; it is not a signature-forgery or claims-verification bypass.

The independently verified affected range is >= 2.9.0, <= 2.13.0; earlier releases were not checked. A narrow fix has been prepared in commit 8915570 based on master commit 5fa7594: PyJWK converts key-construction ValueError exceptions to InvalidKeyError, allowing the existing PyJWKSet skip path to continue. Regression coverage verifies that a malformed RSA key is skipped while a valid key in the same set remains usable. The full suite passes with 370 tests and 4 intentional cryptography-environment skips; formatting, lint, and targeted type checks pass. The advisory remains in triage while the fix goes through release planning.