🔓 JWT Decoder

• Inspect & Debug JWT Tokens in Development

  Quickly decode tokens from your authentication flow (e.g., Auth0, Firebase) to verify their payload structure, check expiration times, and validate custom claims without writing temporary console log code.

• Validate Client-Side Permissions Securely

  Decode a JWT in a frontend application (like a React or Vue app) to conditionally render UI elements based on user roles or permissions stored within the token, ensuring a seamless user experience.

• Troubleshoot API Authentication Issues

  When an API request is rejected, paste the received JWT into the decoder to inspect if the token is expired, has incorrect audience (`aud`) claims, or lacks required scopes, speeding up backend integration.

• Educational Tool for Understanding JWT Structure

  Use the visual breakdown of the header, payload, and signature to learn how JWTs are constructed, making it easier for developers and students to grasp the concept of token-based authentication.

• Verify Token Integrity in a Trusted Environment

  After validating a token's signature on your secure server, use the decoder in a trusted admin panel to view detailed user session data for support or audit purposes without exposing the verification secret.

• Prepare & Test Custom Token Payloads

  When configuring a new authentication service, encode sample payloads with test data to ensure your backend will correctly parse and utilize the custom claims (like `userId` or `subscriptionTier`) you intend to send.

Validate Token Structure Before Decoding

Always perform a basic structural check on the JWT string before attempting to decode it. A valid JWT consists of three Base64Url-encoded segments separated by dots (header.payload.signature). A quick check can prevent unnecessary processing errors and improve user feedback.

• Split the token by '.' and verify it results in exactly three parts.
• Check that each part contains only valid Base64Url characters (no spaces, line breaks, or invalid symbols).
• This pre-validation is especially important when accepting token input directly from users.

Handle Decoding Errors Gracefully

Never assume the provided string is a valid or well-formed JWT. Implement robust error handling around the decode operation to catch and manage common issues like malformed JSON in the header or payload.

• Wrap your decode function call in a try-catch block.
• Provide clear, user-friendly error messages instead of exposing raw JavaScript errors (e.g., "Invalid token format" vs. "Unexpected token '}'").
• Log the technical details server-side for debugging, but keep client-side messages generic for security.

Verify Claims Post-Decoding

Decoding a JWT only makes the claims readable; it does not verify the token's authenticity or validity. Always treat the decoded payload as untrusted data and perform essential claim checks.

• Check the expiration time ('exp') against the current time.
• Validate the 'nbf' (not before) claim if present.
• Verify the intended audience ('aud') and issuer ('iss') match your application's expectations.
• Remember, these checks are necessary but insufficient without signature verification.

Never Trust Client-Side Decoding for Security

JWT decoding is a client-side operation that merely translates Base64Url to JSON. The signature is not verified. Any security decisions must be based on a cryptographically verified token.

• Use a JWT decoder for debugging, displaying user info from a verified token, or inspecting token structure.
• All access control and authentication decisions must rely on tokens verified by your backend server using the correct secret or public key.
• Treat any data from an unverified token as you would user input—validate and sanitize it.

Leverage Decoded Data for UI/UX

Once you have a verified token, the decoded payload is excellent for enhancing the user interface without additional API calls.

• Display the user's name or username directly from the 'sub' or a custom claim.
• Use 'exp' to show session timeouts or warn users before logout.
• Parse scopes or permissions from claims to conditionally render UI components (while still enforcing permissions server-side).

Sanitize Token Input

If your tool accepts JWT input from a user, treat it as untrusted data to prevent injection attacks or accidental system issues.

• Trim whitespace from the beginning and end of the input string.
• Consider setting a reasonable maximum length for the input field.
• Avoid storing or logging the raw token unnecessarily, especially in client-side analytics.