What is a JSON Web Token (JWT)?
A JSON Web Token is a compact, URL-safe token format defined by RFC 7519. It consists of three Base64Url-encoded parts separated by dots: a header specifying the signing algorithm, a payload containing claims like user identity and expiration, and a signature that verifies the token hasn't been tampered with. JWTs are widely used for authentication in REST APIs, single sign-on (SSO) systems, and microservice architectures.
Why Decode JWTs Locally?
Most popular JWT debugging tools — including jwt.io — send your token to a remote server for processing. For development tokens this may be acceptable, but production JWTs often contain sensitive claims like user IDs, email addresses, roles, and organization identifiers. Pasting these into a third-party website creates an unnecessary data exposure risk.
This local JWT debugger uses your browser's native atob() API
to decode the token entirely on your machine. No network requests are made, no tokens are logged,
and no data leaves your browser tab. It's the safest way to inspect JWTs containing production credentials.
Security Analysis Features
Beyond basic decoding, this debugger performs real-time security analysis on every token you inspect.
It detects the dangerous alg: "none" vulnerability,
which allows unsigned tokens to bypass signature verification — a common attack vector in misconfigured OAuth implementations.
The tool also checks exp (Expiration) and
nbf (Not Before) claims to flag expired or not-yet-valid tokens,
helping you debug authentication failures in your API integrations without guessing at timestamps.