GHSA-gg9x-qcx2-xmrh

Описание

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 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:

src/joserfc/_rfc7518/oct_key.py:52-63:

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:

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) calls _decode_jws(...) → deserialize_compact(value, key, algorithms, registry).
3. deserialize_compact (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):

Server harness:

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

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

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

Interpretation:

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(...):

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.