JWT Analyzer & Tester

Sample Tokens

CRITICAL

alg:none Bypass

Algorithm "none" — skips signature check entirely

CRITICAL

Weak HS256 Secret ("secret")

Top-1 in every brute-force wordlist

CRITICAL

Sensitive Data in Payload

Password + credit card stored in plaintext

CRITICAL

kid Injection Attack

SQL injection in key ID header

WARNING

Expired + Minimal Claims

No iss/aud/jti, expired token

GOOD

Well-formed RS256

Asymmetric, all claims, 15-min expiry

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Token Structure

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Decoded Claims

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Header · Payload · Signature appear here

Verify Signature (HMAC)

Security Findings

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Analyze a token to see findings,
score, timestamps and Python fix

Header Claims

Payload Claims

Sign With (HMAC)

Quick Presets

Generated Token

Token appears here after generation…

Preview

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Known JWT Attack Vectors

CRITICAL alg:none Signature Bypass

Set alg:"none" — vulnerable libraries skip verification entirely. Attacker can forge any payload including privilege escalation. CVE pattern: jwt.decode(token, options={"verify_signature":False})

CRITICAL Algorithm Confusion (RS256→HS256)

If a server accepts both RS256 and HS256, attacker uses the public key (which is public!) as the HMAC secret. Server verifies with public key — it matches. Fix: never list both in one algorithms=[] allowlist.

CRITICAL kid Header Injection

kid: "../../dev/null" → empty key → trivial sign
kid: "x' UNION SELECT 'secret'--" → SQL injection
Validate kid strictly: re.match(r'^[a-zA-Z0-9\-_]{1,64}$', kid)

WARNING Weak Secret Brute-Force

Anyone with a JWT can run offline brute-force. hashcat tests billions/sec. Common secrets crack in milliseconds. The demo token uses "secret" — send to Analyzer to crack it instantly.

WARNING Replay Attack (no jti / no exp)

Without jti + server denylist, stolen tokens are valid until expiry. Even after logout. Fix: jti on every token + Redis denylist on logout.

WARNING Privilege Escalation via Payload Tampering

If signature verification is skipped or misconfigured, Base64-decode payload, change role:"user"→role:"admin", re-encode and submit. Never trust payload claims without verifying the signature.

Attack Demo Output

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Click "Demo →" to generate an attack token

Python Defence Template

🐍 Secure PyJWT Validation

import jwt, re from jwt.exceptions import DecodeError, ExpiredSignatureError # NEVER disable verification: # jwt.decode(token, options={"verify_signature": False}) ← BAD def safe_decode(token: str) -> dict: return jwt.decode( token, public_key_or_secret, algorithms=["RS256"], # exact list — never wildcard audience="myapp-api", issuer="https://auth.myapp.com", options={ "require": ["exp","iss","aud","jti","sub"], "verify_exp": True, } ) # Sanitize kid BEFORE using it to look up keys: def safe_kid(kid: str) -> str: if not re.match(r'^[a-zA-Z0-9\-_]{1,64}$', kid): raise ValueError("Invalid kid") return kid # Denylist check (run after decode): def check_revoked(payload: dict): if redis.exists(f"revoked:{payload['jti']}"): raise PermissionError("Token revoked")

Analyzed Token History

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