JWT Security Audit & Payload Inspector Studio

JSON Web Tokens are everywhere in modern auth, yet many developers treat them as opaque strings. Understanding their structure makes debugging login flows far easier. A JWT has three parts separated by dots: a header, a payload, and a signature. Each part except the signature is base64url-encoded JSON, which means it is readable by design — encoding is not encryption.

The header typically declares the algorithm and token type. The payload carries claims such as the subject, issued-at time iat, and expiration exp. Because the payload is plaintext to anyone who decodes it, never place passwords or secret data inside. The signature is what provides integrity: it is computed over the first two parts using a secret (for HS256) or a private key (for RS256), so tampering is detectable.

Decoding and verifying are different operations. Decoding simply reveals the JSON so you can inspect claims. Verifying recomputes the signature with the correct secret or public key and confirms the token is authentic and unaltered. A token can decode perfectly yet fail verification if its signature is invalid or it has expired.

An inspector accelerates debugging. When a user reports random logouts, paste their token and check the exp claim — if it is in the past, the session expired as designed. When roles seem wrong, inspect the payload's role claim. Always verify server-side before trusting any claim; client-side decoding is only for visibility.

Security hygiene matters. Decode tokens locally rather than pasting them into unknown web tools, since tokens grant access. Our inspector runs entirely in your browser, keeping the token on your device while you inspect headers, claims, and expiry with confidence.