Sending variable from server to the Game Instance after finishing an event

I am currently testing ways to make a auth system for the online game that I will try to make. Currently, I have VaRest plugin installed and the php file on my server with the GET request. My concern is that if I run GET request from the client, user can intercept or even change that request via software like fiddler or similar.
Now, I want to know if there is a way to send a GET request on the server, with variables taken from my Login menu, check my database for user credentials and then save bool as true or false, but on GAME INSTANCE.
For now, I only managed to get that variable saved if I play as Listener server and the variable is not printed on client.
Thank you.

For a start you should put things like password and username in the POST body and don’t use GET. Also use HTTPS. There’s a lot of information about the security of that online, as it’s not just an Unreal issue obviously. And there’s no reason that wouldn’t work through Game Instance, can you post your Blueprint, I’m not sure how you’re currently doing it.