JWT Misconfiguration

What Is Broken and Why

JWTs that accept alg: none, trust the algorithm declared in the token header, or use guessable HMAC secrets allow attackers to forge arbitrary tokens — including admin-level claims — without knowing any signing key. Algorithm confusion attacks exploit servers that accept both RS256 (asymmetric) and HS256 (symmetric): the attacker signs with the public key (which is public) using HS256 and the server verifies it as a valid HMAC.

Key Signals

• Authorization: Bearer <base64url>.<base64url>.<base64url> in requests
• JWTs stored in cookies (check httpOnly, Secure, SameSite flags)
• JWKS endpoint at /.well-known/jwks.json or /oauth/certs (exposes public key)
• alg field in JWT header — watch for HS256, RS256, none
• kid (Key ID) field in JWT header — check for path traversal or SQLi
• Short or dictionary-based HMAC secret (crack with hashcat)

Methodology

1. Decode the JWT header and payload (base64url decode each part).
2. Note the alg value; attempt to change it to none and remove signature.
3. If alg is RS256, fetch the public key from JWKS endpoint; re-sign with HS256 using the public key as the HMAC secret.
4. If alg is HS256, attempt to crack the secret with hashcat or jwt_tool.
5. Check kid value — test for path traversal (../../dev/null), SQLi, or SSRF.
6. Modify exp claim to far-future timestamp; attempt to use expired tokens.
7. Escalate claims: change role, admin, sub, userId in payload after forging.

Payloads & Tools

# jwt_tool — Swiss army knife for JWT attacks
pip install jwt_tool
jwt_tool TOKEN -X a           # alg:none attack
jwt_tool TOKEN -X s           # algorithm confusion (RS256→HS256)
jwt_tool TOKEN -C -d wordlist.txt  # crack HMAC secret

# hashcat JWT cracking
hashcat -a 0 -m 16500 TOKEN wordlist.txt

# Manual alg:none
# 1. Decode header: {"alg":"RS256","typ":"JWT"}
# 2. Change to:     {"alg":"none","typ":"JWT"}
# 3. Re-encode and append empty signature: header.payload.
# (trailing dot is required)

# kid SQLi
{"kid": "' UNION SELECT 'attacker_secret' --", "alg":"HS256"}
# Sign token with 'attacker_secret'

# kid path traversal (sign with empty string)
{"kid": "../../dev/null", "alg":"HS256"}
# /dev/null reads as empty → sign with empty string ""

Bypass Techniques

Attack	Technique
alg: none	Change header alg to none, drop signature, keep trailing dot
Algorithm confusion	Fetch RS256 public key; use it as HS256 HMAC secret
Weak secret	Crack short/dictionary HMAC with hashcat -m 16500
kid path traversal	Point kid to /dev/null or known empty file; sign with ""
kid SQLi	Inject SQL into kid to return attacker-controlled key from DB
Missing exp	If no expiry check, reuse old tokens indefinitely
jku/x5u injection	Point to attacker-hosted JWKS to supply own public key
Embedded JWK	Inject jwk into header containing attacker's own public key

Exploitation Scenarios

Algorithm confusion to admin: Setup → API uses RS256; JWKS endpoint public at /.well-known/jwks.json. Trigger → Fetch public key → re-sign token with HS256 using public key as secret → set "role":"admin" in payload. Impact → Full admin access without any private key.

alg:none on misconfigured library: Setup → Old version of JWT library doesn't reject alg: none. Trigger → Set alg: none, modify sub to another user's ID, remove signature. Impact → Arbitrary account takeover.

kid path traversal to RCE: Setup → kid is used to load key from filesystem without sanitization. Trigger → kid: "../../proc/self/fd/0" with socket input; or ../../tmp/evil. Impact → Attacker controls signing key; full token forgery.

False Positives

• alg: none rejected with 401 — library properly validates algorithm.
• RS256 server that only accepts RS256 (not HS256) — algorithm confusion not applicable.
• JWKS endpoint present but server uses pinned key in code — jku/x5u injection blocked.

Fix Patterns

// Always whitelist algorithm — never derive from token header
jwt.verify(token, secret, { algorithms: ['HS256'] });

// For RS256: pin the public key in code, don't trust jku/jwk headers
jwt.verify(token, publicKeyPem, { algorithms: ['RS256'] });

// Always validate exp
const decoded = jwt.verify(token, secret, {
  algorithms: ['HS256'],
  ignoreExpiration: false  // default false — make it explicit
});

Related Skills

JWT attacks are a class of [[auth-bypass]] specific to token-based systems. kid SQLi chains into [[sql-injection]]; kid path traversal chains into [[path-traversal]]. Weak session management after JWT compromise connects to [[cookie-attacks]] and [[session-fixation]].