JWT Decoder & Inspector — Free Online Tool

Decode any JSON Web Token instantly. All processing is 100% client-side — your token never leaves your browser.

Privacy guarantee: This tool runs entirely in your browser. No network requests are made. Your token data stays on your device.

What is a JSON Web Token (JWT)?

A JSON Web Token (JWT) is an open standard (RFC 7519) for securely transmitting information between parties as a JSON object. JWTs are commonly used for authentication and information exchange in web APIs. The token is digitally signed, so you can verify its integrity — but by default the payload is only Base64-encoded, not encrypted.

A JWT consists of three parts separated by dots: Header (algorithm and token type), Payload (claims/data), and Signature (used to verify the token hasn't been tampered with).

JWT Standard Claims

  • iss (Issuer) — Who created and signed the token
  • sub (Subject) — Who the token is about (usually user ID)
  • aud (Audience) — Who the token is intended for
  • exp (Expiration Time) — When the token expires (Unix timestamp)
  • iat (Issued At) — When the token was issued
  • nbf (Not Before) — The token isn't valid before this time
  • jti (JWT ID) — Unique identifier for the token

JWT Security Tips

  • Never store sensitive data in JWT payloads — the payload is only Base64-encoded, not encrypted. Anyone can decode it.
  • Always verify the signature server-side — a decoded JWT doesn't mean it's valid. Always check the signature against your secret key.
  • Keep expiry times short — use refresh tokens for long sessions. Short-lived access tokens (15-60 min) limit exposure.
  • Avoid the "none" algorithm — some vulnerable implementations accept unsigned tokens. Always enforce a specific algorithm.
  • Use HTTPS only — transmit JWTs over encrypted connections to prevent interception.

JWT Security Best Practices for Production

JSON Web Tokens are widely used for authentication, but improper implementation creates serious security vulnerabilities. Follow these battle-tested practices when deploying JWTs in production systems.

Token Storage: Where to Keep JWTs

  • HttpOnly Cookies (Recommended): Store JWTs in HttpOnly, Secure, SameSite=Strict cookies. This prevents JavaScript access, eliminating XSS-based token theft. The browser automatically sends the cookie with each request.
  • localStorage (Avoid): Any XSS vulnerability in your app or third-party scripts can read localStorage and steal the token. Never store sensitive tokens in localStorage for production applications.
  • sessionStorage: Slightly better than localStorage since it clears when the tab closes, but still vulnerable to XSS attacks during the session.
  • In-memory (Single Page Apps): Store the token in a JavaScript variable. Most secure against XSS, but the token is lost on page refresh, requiring silent re-authentication via refresh tokens.

Common JWT Vulnerabilities and Mitigations

  • Algorithm Confusion Attack: An attacker changes the alg header from RS256 to HS256, then signs the token with the public key. Always validate the algorithm server-side and reject unexpected algorithm values.
  • Token Expiry Too Long: Access tokens should expire in 15–30 minutes. Use short-lived access tokens paired with longer-lived refresh tokens (7–30 days) stored securely server-side.
  • Missing Audience/Issuer Validation: Always verify the aud (audience) and iss (issuer) claims to prevent tokens from one service being accepted by another.
  • No Token Revocation: JWTs are stateless — once issued, they're valid until expiry. Implement a token blocklist (using Redis) for logout, password changes, and compromised accounts.

JWT vs Session-Based Authentication

JWTs are ideal for stateless APIs, microservice architectures, and mobile apps where server-side session storage is impractical. Session-based auth is better for traditional server-rendered web apps where you need instant revocation and don't want to manage token refresh logic. Many production systems use a hybrid approach: JWTs between microservices internally, with session cookies for the user-facing frontend.

Frequently Asked Questions about JWT

Is it safe to decode a JWT token in the browser?

Yes, decoding a JWT is always safe because the header and payload are only Base64URL-encoded, not encrypted. Anyone who has the token can read its contents — that is by design. This tool decodes your JWT entirely in your browser; your token is never sent to any server. This is also why you should never store passwords, private keys, or sensitive secrets in a JWT payload.

Can I verify a JWT signature without the secret key?

No. JWT signature verification requires the secret key (for HMAC algorithms like HS256) or the public key (for RS256, ES256). You can always decode the header and payload without a key, but you cannot verify the token has not been tampered with. Signature verification must happen server-side. This tool shows you the decoded token contents for inspection, not signature validity.

Why is my JWT token expiring unexpectedly?

JWT tokens contain an exp claim — a Unix timestamp for when the token expires. If the current time exceeds this value, the token is rejected. This is by design for security. Check the exp value here to understand when your token expires. If tokens expire too quickly, check for clock skew between your server and client — even a few minutes difference causes issues. Use NTP to keep server clocks synchronized.

What algorithm should I use for signing JWT tokens?

For most applications, RS256 (RSA + SHA-256) is recommended because it uses asymmetric keys — you sign with a private key and verify with a public key, so you can share the public key safely. HS256 (HMAC + SHA-256) is simpler but requires sharing the secret key with every service that verifies tokens. Never use the "none" algorithm — some vulnerable implementations accept unsigned tokens, which is a critical security flaw.

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