Is it safe to store sensitive data in JWT Payload?

Question

I'm currently learning JWT implementation with PHP and want to use JWT tokens instead of sessions for my RESTful application.

During signature creation, I'm doing something like this

token = base64Header + '.' + base64Payload + '.' + signature  

Here we are just using base64 the Payload. If I paste in sites like https://jwt.io/#debugger, the Payload gets decrypted (even if the signature is wrong).

My questions,

1. Is JWT only for verifying the signature with server when sending data?
2. Is insecure to keep sensitive data in Payload?
3. If insecure, any way to secure the Payload?

Below is the sample code I wrote

<?php
    $headers = base64_encode(json_encode([
        "typ" => "JWT",
        "alg" => "HS256"
    ]));
    $claims = base64_encode(json_encode([
        "sub" => "1234567890",
        "name" => "John Doe",
        "admin" => true,
        "jti" => "870a3de5-ea7b-4062-abef-11180e530f5a",
        "iat" => 1492603378,
        "exp" => 1492606978
    ]));
    $payload = $headers.".".$claims;
    $signature = base64_encode(hash_hmac("sha256", $payload, 'secret', true));
    $encodedJWT = $payload.".".$signature;
    // eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiYWRtaW4iOnRydWUsImp0aSI6Ijg3MGEzZGU1LWVhN2ItNDA2Mi1hYmVmLTExMTgwZTUzMGY1YSIsImlhdCI6MTQ5MjYwMzM3OCwiZXhwIjoxNDkyNjA2OTc4fQ.nvw-bAUgr7H_xr3q8_Yz8rCNtMohtn2YlCmcLoLBWlc

Answer 1

If I paste in sites like https://jwt.io/#debugger, the Payload gets decrypted (even if the signature is wrong).

Third parties cannot verify the signature because they don't have the secret key. The payload doesn't get decrypted - it gets decoded.

Ideally you should not store sensitive data in the payload since the payload is only base64 encoded and not encrypted. This means anyone who gets a hold of the token can view the contents of the payload by simply base64 decoding it.

If you have a token in local storage of a web browser and your site has an XSS vulnerability it makes it trivial to steal the token. It's bad enough that the attacker has a valid JWT (which will hopefully expire soon anyway) but if it contains sensitive data then you're in real trouble. Imagine having to notify all users on your site that they must now change various bits of sensitive data about themselves because of a potential mass compromise.

Keep the JWT lightweight. Store the users ID, their roles/grants withing the system. If you feel like you have to add sensitive data to the payload try and rethink your solution.