JWT Parser
Decode, encode, and analyze JSON Web Tokens with signature verification
JWT Token
Decode and inspect JSON Web Tokens instantly. Parse JWT headers, payloads, and signature algorithms to debug authentication flows, verify token claims, and troubleshoot authorization issues — with complete client-side processing that never transmits your tokens to any server.
Decoding JSON Web Tokens for Authentication Debugging and API Security Analysis
JSON Web Tokens (JWTs) have become the dominant authentication mechanism for REST APIs, single-page applications, and microservice architectures. A JWT is a compact, URL-safe token composed of three Base64URL-encoded sections separated by periods: a header describing the token type and signing algorithm, a payload containing the claims (user identity, roles, expiration), and a cryptographic signature for verification.
While JWTs look like opaque strings to the naked eye, decoding them is essential for debugging authentication problems. Expired tokens, incorrect audience claims, missing role scopes, and malformed issuer URLs are all invisible until you decode the payload and inspect its contents. Security engineers auditing external APIs regularly need to inspect JWTs to verify they're signed with appropriate algorithms (RS256 or ES256 rather than the insecure HS256 with a weak secret) and that they don't expose excessive user data.
This JWT parser decodes all three sections of a JWT in your browser, presenting the header algorithm, all payload claims with human-readable names, the expiration timestamp in your local timezone, and the signing algorithm in use. Because decoding only requires Base64URL decoding (not signature verification), the entire operation happens locally with no external calls. Your tokens — which may carry session state, user roles, or tenant identifiers — remain completely private.
JWT Security Best Practices Every Developer Should Follow
JWTs appear simple but harbor several common security pitfalls. The 'alg: none' vulnerability allows attackers to remove the signature entirely if the server accepts tokens with no algorithm — always reject tokens with alg set to 'none'. The algorithm confusion attack exploits servers that accept both symmetric (HS256) and asymmetric (RS256) algorithms: an attacker signs a token with HS256 using the server's public key as the HMAC secret, and a naive implementation might accept it. Always explicitly specify which algorithm your server accepts.
Regarding claims: always validate the `exp` (expiration), `nbf` (not before), `iss` (issuer), and `aud` (audience) claims on your server — these fields exist in the payload but serve no purpose if your server doesn't check them. Keep JWTs short-lived (15–60 minutes max for access tokens) and implement proper refresh token rotation. Never put sensitive data like raw passwords, credit card numbers, or Social Security numbers in JWT payloads — the payload is encoded, not encrypted, and any party that intercepts the token can read every claim.
Debugging Authentication and Authorization Failures
When a user gets a 401 or 403 error, decode their JWT to immediately see if the token is expired, missing required role claims, targeting the wrong audience, or issued by the wrong identity provider.
Auditing Third-Party Service Tokens
Inspect JWTs from OAuth providers, cloud services, or external APIs to verify the signing algorithm, token lifetime, claim structure, and whether the token contains more user data than necessary.
Understanding Authentication Library Behavior
When integrating nextauth, passport.js, Auth0, Cognito, or Keycloak, decode the tokens they issue to understand exactly what claims are included, how expiration is handled, and how token refresh cycles work.
Learning JWT Structure and Claims
For developers learning about JWT-based authentication, this decoder makes the abstract concrete — paste any JWT and immediately see its three-part structure, which claims are standard (RFC 7519) vs. custom.
- 1
Paste Your JWT Token into the Input Field
Copy your JWT from a browser's localStorage, a cookie, an Auth header in DevTools, or your API testing tool and paste the full dot-separated string. The parser handles both compact JWTs and those with extra whitespace or newlines.
- 2
Inspect the Decoded Header
The header panel shows the token type (typically 'JWT'), the signing algorithm (RS256, HS256, ES256, etc.), and any Key ID (kid) used to look up the signing key. The algorithm tells you the security strength of the token's signature.
- 3
Review All Payload Claims
The payload panel displays every claim with human-readable labels: sub (subject/user ID), iat (issued at), exp (expiration — shown in your local timezone), aud (intended audience), iss (issuer URL), and any custom claims.
- 4
Check Expiration Status and Copy Individual Sections
A clear valid/expired indicator shows whether the token's `exp` claim is in the future or past. Use the copy buttons next to each section to grab individual parts — the raw payload JSON, a specific claim, or the full decoded header.
Full JWT Decoding — Header, Payload, and Signature
Decodes all three JWT sections simultaneously: the header (algorithm, key ID), the payload (all standard and custom claims), and the signature section metadata. Unlike simple decoders that only show the payload, this gives you the full picture.
Human-Readable Expiration and Timestamp Display
Unix timestamps in JWT claims (`iat`, `exp`, `nbf`) are automatically converted to your local timezone with date and time, alongside a real-time countdown or elapsed time indicator showing whether the token is currently valid.
Algorithm Detection and Security Flagging
The header parser identifies the signing algorithm and flags potentially dangerous configurations — specifically the 'alg: none' attack vector and weak HS256 configurations — with visual warnings to alert security-conscious developers.
Copy Per-Section Output
Independent copy buttons for the header JSON, payload JSON, and individual claim values let you quickly extract just the user ID, role list, or expiration time without selecting the entire decoded output.
Zero Server Transmission — Complete Token Privacy
JWT parsing uses only Base64URL decoding, which runs natively in the browser. Your tokens never leave your machine, making this safe for production tokens that carry real user sessions, tenant data, or permission scopes.
Found this tool useful?
Share your experience and help others discover it.