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JWT Security Checker

Paste a JWT and get a quick security-focused scan. This tool decodes the header/payload and flags common implementation risks (it does not verify signatures).

Category: Developer · URL: /tools/jwt-security-checker.html
Use this to reproduce scans consistently.
Warn if exp is far in the future.
Need signature verification? Try JWT Online Checker (decode + claim checks + HS verification).
(output will appear here)
JWT Online Checker Hash Tool (offline) Regex Tool Online XML Tool JSON Generator from Schema Spring Cron Formatter
Privacy: runs locally in your browser. No uploads, no tracking scripts.

How to use

This is a static analysis tool (no signature verification, no network calls).

  1. Paste the JWT into the input box.
  2. (Optional) Set Now to a specific epoch time to reproduce results.
  3. Click Run Security Check to get a compact report and findings.
Keywords this page targets (natural cluster): jwt security checker, jwt vulnerability checker, jwt alg none check, jwt header security audit, jwt claim validation exp nbf iat, jwt expired token checker, jwt long lived token risk, jwt kid injection check, jwt jku x5u security, jwt embedded jwk risk, jwt crit header checker, base64url jwt validator, jws token security scan, jwe token header check, jwt insecure configuration detection, jwt best practices checker, jwt issuer audience missing check, jwt signature missing check, jwt token format validator
Secondary intents covered: Decode a JWT and review header/payload safely offline, Detect alg=none and other dangerous algorithm settings, Check if exp/nbf/iat are missing, invalid, or risky, Spot suspicious kid values (path traversal/URL-like), Flag remote key lookup risks (jku/x5u) and embedded keys (jwk), Identify malformed tokens (segment count, Base64URL, JSON), Quickly copy a compact JSON report for bug reports

FAQ

Does this tool verify the JWT signature?

No. It only decodes and runs security checks on the header/claims without validating the signature.

Why is alg=none considered dangerous?

If your backend accepts alg=none, attackers can forge tokens without a signature. Most systems should reject it.

What are jku and x5u risks?

They can point to remote key material; if your server fetches keys from attacker-controlled URLs, it can be tricked into accepting forged tokens.

Why is a suspicious kid flagged?

kid is untrusted input; path traversal or URL-like values can trigger file reads or SSRF in buggy key resolvers.

What does “long-lived token” mean here?

If exp is far in the future (default > 30 days), compromise impact is higher. Prefer short-lived access tokens with refresh flows.

Can this tool check JWE (encrypted JWT)?

It can decode and analyze the JWE header, but it can’t read encrypted claims without decryption keys.

Is any data sent to a server?

No. The checker runs in your browser and does not make network requests.