Of course it can, but not by brute forcing to get a private key with funds. The weakest point in any security system is the person using it. By clever social engineering you might hank out the password of an innocent user of an online wallet. Or maybe you got a hidden trojan on your PC that turned out to be a RAT and it has now your unencrypted bitcoin wallet. That's why you should ALWAYS be careful no matter what, always use two factor authentication and always encrypt your password. And never store your backup phrase in a document on your PC, that phrase should be on a physical paper hidden in your drawer.