JWT Analyzer & Tester

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CRITICAL

alg:none bypass

Signature check skipped

CRITICAL

Weak HS256 secret

Common signing secret

CRITICAL

Sensitive payload

Plaintext private data

CRITICAL

kid injection

Unsafe key ID value

WARNING

Expired token

Expired with missing claims

GOOD

Valid RS256

Asymmetric signing example

Token Structure

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Decoded Claims

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Security Findings

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score, timestamps, and Python fix

Header Claims

Payload Claims

Sign With (HMAC)

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Generated Token

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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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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")

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