CVE-2026-101917: PyJWT: PyJWKClient still amplifies unauthenticated JWKS fetches on unknown kid values (incomplete fix of CVE-2026-48524)

Summary CVE-2026-48524 (GHSA-fhv5-28vv-h8m8, “PyJWKClient unbounded JWKS endpoint requests via attacker-controlled kid values (DoS)”) was fixed in 2.13.0 by stopping fetch_data() from clearing the cache on a fetch error. That closed one amplification path but did not add the mitigation the advisory’s title implies: there is still no rate-limit, negative-cache, or minimum-refresh-interval for unknown kids.

At HEAD, get_signing_key(kid) (jwt/jwks_client.py:185-211), on any unknown kid, calls get_signing_keys(refresh=True), and refresh=True bypasses jwk_set_cache unconditionally and forces a fresh fetch_data(). The kid is read from the unverified token header (get_signing_key_from_jwt decodes with verify_signature=False), so no valid token and no authentication is required. lru_cache does not cache the raised exception, so even the same unknown kid repeated re-fetches on every call.

Affected pyjwt <= 2.13.0 (the latest release; the patched release for CVE-2026-48524). No fixed version yet.

Proof of concept (verified on 2.13.0, cache enabled = realistic prod config) import threading, http.server, socketserver, json from jwt import PyJWKClient hits = {’n’: 0} JWKS = json.dumps({“keys”:[{“kty”:“oct”,“kid”:“real”,“k”:“AAAA”}]}).encode() class H(http.server.BaseHTTPRequestHandler): def do_GET(self): hits[’n’] += 1 self.send_response(200); self.send_header(‘Content-Type’,‘application/json’); self.end_headers() self.wfile.write(JWKS) def log_message(self,*a): pass srv = socketserver.TCPServer((’127.0.0.1’,0), H); port = srv.server_address[1] threading.Thread(target=srv.serve_forever, daemon=True).start() c = PyJWKClient(f’http://127.0.0.1/:{port}/jwks.json’, cache_keys=True, lifespan=3600) for i in range(8): try: c.get_signing_key(f’attacker-unknown-kid-{i}’) except Exception: pass before = hits[’n’] for _ in range(5): try: c.get_signing_key(‘same-unknown’) except Exception: pass print(‘distinct unknown kids: 8 -> fetches:’, hits[’n’]) print(‘same unknown kid x5 -> extra fetches:’, hits[’n’] - before)

Output: distinct unknown kids: 8 -> fetches: 9 same unknown kid x5 -> extra fetches: 5 Each unknown kid forces a fresh JWKS fetch; a repeated identical unknown kid still re-fetches every time against an unexpired cache. No rate-limit or negative-cache.

Impact One unauthenticated request -> one outbound JWKS HTTP fetch + full JSON parse on the victim server. An attacker floods tokens carrying junk kids, so the victim hammers its own JWKS/IdP endpoint (amplification: attacker -> victim -> IdP), exhausting victim CPU/sockets and potentially tripping the JWKS provider’s rate-limit, causing an application-wide auth outage. This is the unauthenticated DoS the parent advisory is named for, still reachable after the 2.13.0 fix.

Suggested fix Guard the forced refresh on unknown kids: negative-cache unknown kids for a short TTL, or enforce a minimum interval between forced JWKS refreshes, so a repeated or unknown kid cannot force unbounded fetches.

Note: the same-kid-repeated result (5 identical unknown kids producing 5 fetches against an unexpired cache) shows this is request amplification, not legitimate key-rotation handling, since that refresh can never succeed.

Reported by Babakizo (Securva).