system
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You do not have to change anything on your web site. Hackers can do that for you.
That is why I asked you if you had checked the login process of your web site. Have you checked it yet?
Of course, it could be a false positive by avast. Someone (you or one of your users) will have to send in a report to avast for it to be checked for a false positive. This can be done through the avast GUI.