JWT Decoder

JSON Web Tokens (JWT) are a compact, self-contained standard for securely transmitting information between parties as a JSON object. The format is defined in RFC 7519 and consists of three Base64url-encoded parts separated by dots: header.payload.signature. The header contains the token type (JWT) and the signing algorithm used (typically HS256 for HMAC-SHA256 or RS256 for RSA-SHA256). The payload contains claims — statements about the user or token properties. Standard claims include iss (issuer), sub (subject/user ID), aud (audience), exp (expiration time as Unix timestamp), iat (issued at time), and jti (JWT ID for uniqueness). Custom claims add application-specific data like user roles, permissions, or feature flags. The signature cryptographically verifies that the token hasn't been tampered with — only the server holding the secret key can create valid signatures. The AllTools JWT Decoder decodes the header and payload sections in the browser using atob() — no server needed since decoding doesn't require the secret key. It also displays the expiration time in human-readable format and indicates whether the token has expired based on the current time.

JWT debugging is a daily task for developers building authenticated APIs and single-page applications. When authentication fails or permissions behave unexpectedly, inspecting the JWT payload reveals exactly what the server believes about the current user — their ID, roles, and expiration time. The AllTools JWT Decoder instantly reveals all claims without transmitting your token to external services. This matters because JWTs often contain sensitive user information — user IDs, email addresses, role assignments — that shouldn't be shared with third-party debugging tools. Never paste production JWTs into external decode tools. Common JWT debugging scenarios include: verifying the exp claim matches the expected session duration, confirming the sub claim contains the correct user identifier, checking that custom role or permission claims are present and correct, and verifying the iss claim matches the expected issuer. Security best practices for JWTs include setting short expiration times (15 minutes to 1 hour for access tokens), using refresh tokens for long-lived sessions, storing JWTs in httpOnly cookies rather than localStorage to prevent XSS access, and validating all claims server-side on every request rather than trusting client-side decoding.