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

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

— Microsoft

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 fetchdata() 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, getsigningkey(kid) (jwt/jwksclient.py:185-211), on any unknown kid, calls getsigningkeys(refresh=True), and refresh=True bypasses jwksetcache unconditionally and forces a fresh fetchdata(). The kid is read from the unverified token header (getsigningkeyfromjwt decodes with verifysignature=False), so no valid token and no authentication is required. lrucache 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 doGET(self): hits['n'] += 1 self.sendresponse(200); self.sendheader('Content-Type','application/json'); self.endheaders() self.wfile.write(JWKS) def logmessage(self,a): pass srv = socketserver.TCPServer(('127.0.0.1',0), H); port = srv.serveraddress[1] threading.Thread(target=srv.serveforever, daemon=True).start() c = PyJWKClient(f'http://127.0.0.1/:{port}/jwks.json', cachekeys=True, lifespan=3600) for i in range(8): try: c.getsigningkey(f'attacker-unknown-kid-{i}') except Exception: pass before = hits['n'] for in range(5): try: c.getsigningkey('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).

Maintainer update — 2026-09-10

The maintainer confirmed the reported behavior against PyJWT 2.13.0. With JWKS caching enabled, an unknown kid previously forced an unconditional JWKS refresh, including when the same unknown value was repeated while the cached key set was still valid. This allowed unauthenticated token headers to cause unnecessary outbound JWKS requests and repeated parsing work.

The fix is now on master in commit ba4853a. PyJWKClient now applies a 30-second cooldown after successful JWKS fetches before permitting another unknown-kid refresh, serializes concurrent refresh decisions per client, and allows callers to configure or disable the cooldown. Cache-disabled behavior and immediate retry after failed fetches remain unchanged.

Regression tests cover repeated unknown kids, cooldown expiry, concurrent misses, cache-disabled operation, and invalid cooldown values. The available full tox matrix, Ruff, and mypy checks pass. The fix will be included in the next released 2.x version.

Maintainer update — 2026-09-11

The verified fix for this advisory is included in PyJWT 2.14.0, released on 2026-09-11 and available on PyPI. PyJWT 2.14.0 is the first release containing the fix. This advisory is now published with 2.14.0 recorded as the patched version.

— GitHub