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If you see the source code, you will find some script under script tag. When I erased that on the source code, it still worked suggesting it's a config setting. Therefore we need to change the config and register it on console.

By the way, I found that there are ls, id, flag help command avaiable, but flag is only possible with admin authority.

By changing the username, we can use flag so change it like so.

Then you can see the flag!