JWT Token Analyzer & Decoder

Decode and analyze JWT tokens free online. See header, payload, expiry status, and security warnings instantly. Does not verify signatures -- decode only.

Paste a JWT token

How to Use This Tool

Paste your JWTAdd the full token including all three dot-separated parts

Read the decoded claimsSee header and payload in formatted JSON

Check the warningsReview flagged issues like expiry or insecure algorithm

JWT Token Analyzer & Decoder is a free online tool that works entirely in your browser — no signup, no data uploaded, no limits.

JWTs Are Readable by Design -- Decoding Is Not a Security Flaw

A JSON Web Token has three parts separated by dots: a header, a payload, and a signature. The header and payload are Base64Url-encoded, not encrypted, which means anyone holding the token can read its contents without any key. The signature is what actually protects the token from being forged or tampered with -- and verifying that signature is deliberately not something this decode-only tool attempts.

This separation matters: decoding a JWT to inspect its claims is normal, expected behavior used constantly in debugging and development. What would be insecure is treating an unverified decoded payload as trustworthy without checking the signature server-side, which is a different step entirely.

The alg:none Vulnerability Worth Knowing About

Several early JWT library implementations had a flaw where a token with its algorithm header set to "none" would be accepted without any signature check at all. An attacker who discovered a vulnerable verifier could forge tokens claiming to be any user. Modern, well-maintained libraries explicitly reject "none" by default, but this tool flags the pattern specifically because misconfigured or outdated systems can still be exposed.

Reading Standard Claims

exp (expiration time) and iat (issued at) are Unix timestamps that well-formed tokens typically include. A token missing exp never expires by its own claim, which is often unintentional and worth flagging during a security review of how your application issues tokens.

Frequently Asked Questions

Decode only. JWT signatures require the secret key (HMAC) or public key (RSA/ECDSA) the token was signed with, which this tool does not have access to and should not be given -- pasting a secret key into a third-party tool would itself be a security risk. This shows you what the token claims, not whether those claims are cryptographically verified.

It checks for an expired token (exp claim in the past), checks if the algorithm is set to 'none' (a known JWT vulnerability that bypasses signature verification entirely if a server doesn't explicitly reject it), and flags missing standard claims like exp or iat that well-formed tokens typically include.

Some early JWT libraries had a bug where setting the algorithm header to 'none' would cause the library to accept the token without verifying any signature at all, effectively letting an attacker forge any token. Modern libraries patch this, but it's worth flagging since misconfigured systems can still be vulnerable.

No. JWT payloads are Base64Url-encoded, not encrypted, by design -- they are meant to be readable by anyone, with security coming from the signature verification step that confirms the claims haven't been tampered with. Decoding a token is expected behavior, not a security flaw in itself.

The decoding happens entirely in your browser with nothing sent to a server, but as a general security practice, avoid pasting tokens containing sensitive claims into any tool you don't fully control, even client-side ones, especially if the token is still within its valid expiry window.