---

name: jwt-attack-surface description: Audit JWT implementation for algorithm confusion, secret weakness, claim validation issues, and token handling vulnerabilities. Use when reviewing authentication systems using JWT.

JWT Attack Surface Audit

Purpose

Identify JWT implementation vulnerabilities including algorithm confusion, weak secrets, missing validation, and token handling issues.

Focus Areas

• Algorithm Confusion: alg=none, RS256→HS256 confusion
• Weak Secrets: Short/guessable HMAC secrets
• Missing Validation: Signature bypass, claim verification
• Token Handling: Storage, transmission, revocation
• Claim Issues: Expiration bypass, privilege escalation

Critical Vulnerabilities

Algorithm None Attack

Copy// VULNERABLE - accepts "alg": "none"
jwt.verify(token, secret, { algorithms: ['HS256', 'none'] });

// SECURE - explicit algorithm whitelist
jwt.verify(token, secret, { algorithms: ['HS256'] });

RS256 to HS256 Confusion

Copy# If server uses RS256 (asymmetric):
# - Attacker gets public key
# - Signs token with public key using HS256
# - Server verifies with public key as HMAC secret

# VULNERABLE - accepts multiple algorithms
jwt.decode(token, public_key, algorithms=['RS256', 'HS256'])

# SECURE - single algorithm only
jwt.decode(token, public_key, algorithms=['RS256'])

Weak HMAC Secret

Copy// VULNERABLE - short/guessable secret
const secret = "secret123";
jwt.sign(payload, secret);

// SECURE - strong random secret (>= 256 bits)
const secret = crypto.randomBytes(64).toString('hex');

Missing Expiration Check

Copy// VULNERABLE - ignoring exp claim
claims, _ := jwt.Parse(token, keyFunc)
// No expiration validation!

// SECURE - validate expiration
claims, err := jwt.ParseWithClaims(token, &Claims{}, keyFunc)
if err != nil || claims.ExpiresAt.Before(time.Now()) {
    return ErrExpired
}

Output Format

Copyfindings:
  - title: "JWT accepts 'none' algorithm"
    severity: critical
    attack_scenario: "Attacker crafts unsigned JWT with alg=none, bypasses authentication"
    preconditions: "None"
    reachability: public
    impact: "Complete authentication bypass"
    confidence: high
    cwe_id: "CWE-327"
    affected_assets:
      - "src/auth/jwt.rs:45"
      - "/api/auth/verify"

Audit Checklist

Token Generation

Copy[ ] Secret is cryptographically random (>= 256 bits)
[ ] Algorithm is explicitly set (not from token)
[ ] Claims include: iat, exp, iss, aud
[ ] Expiration is reasonable (< 24h for access tokens)
[ ] Sensitive data not in payload (use references)

Token Verification

Copy[ ] Algorithm whitelist (single algorithm preferred)
[ ] Signature verification BEFORE parsing claims
[ ] Expiration (exp) validated
[ ] Issuer (iss) validated
[ ] Audience (aud) validated
[ ] Not-before (nbf) validated if present

Token Handling

Copy[ ] Stored in httpOnly cookie (not localStorage)
[ ] Transmitted over HTTPS only
[ ] Revocation mechanism exists
[ ] Refresh token rotation implemented
[ ] Token not leaked in URLs/logs

Attack Scenarios

1. None Algorithm

CopyHeader: {"alg": "none", "typ": "JWT"}
Payload: {"sub": "admin", "role": "admin"}
Signature: (empty)

2. Key Confusion (RS256→HS256)

Copy# Get public key from /api/.well-known/jwks.json
# Sign with public key using HS256
# Server uses public key as HMAC secret

3. Weak Secret Brute Force

Copy# Tools: hashcat, jwt_tool
hashcat -m 16500 jwt.txt wordlist.txt

4. Claim Tampering

Copy# Modify claims if signature not verified
{"sub": "user", "role": "user"} → {"sub": "admin", "role": "admin"}

Severity Guidelines

KYCo Integration

Register JWT implementation vulnerabilities:

1. Check Active Project

Copykyco project list

2. Register Finding

Copykyco finding create \
  --title "JWT accepts 'none' algorithm" \
  --project PROJECT_ID \
  --severity critical \
  --cwe CWE-327 \
  --attack-scenario "Attacker crafts unsigned JWT with alg=none, bypasses authentication" \
  --impact "Complete authentication bypass" \
  --assets "src/auth/jwt.rs:45,/api/auth/verify"

3. Import Scanner Results

Copy# Import semgrep JWT rules
semgrep --config p/jwt --sarif -o jwt-audit.sarif .
kyco finding import jwt-audit.sarif --project PROJECT_ID

Common CWE IDs for JWT Issues

• CWE-327: Use of Broken or Risky Cryptographic Algorithm
• CWE-287: Improper Authentication
• CWE-347: Improper Verification of Cryptographic Signature
• CWE-798: Use of Hard-coded Credentials (weak secret)
• CWE-613: Insufficient Session Expiration