Experience with Spring Security. JWT token: will the server intercept a request with a foreign token? What if a hacker changes the token's payload?

In Spring Security, when using JWT tokens, the server verifies the token's signature to ensure it was issued by a trusted authentication center and has not been altered.

If a hacker replaces the token with someone else's, but the signature does not match, the server will reject the request — it is impossible to intercept and use someone else's token without the private key.

If the hacker changes the token's payload (for example, changing the role or user ID), the signature becomes invalid, and the server will also reject such a token.

Thus, the security of JWT is based on cryptographic signatures. Without the correct signature, the server will not accept the token, even if the payload has been changed.

It is important that the server always verifies the signature and expiration date of the token, and uses reliable secrets or keys for signing.