Hey Scotty,  new one on me.  Went to youtube official page and now a popup saying I need to send my name and password to 'cumbby.in:80.  force quit does not help.
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Sounds like you've run into an ad. This is a very convincing ad. Usually poses as a different site and force quitting and restarting your browser only takes you right back to the same place. There are bad guys out there that buy domain names very similar to popular ones. For example, if you add an extra y to nytimes.com you will go to a completely bogus site.

Should be a simple fix. Force Quit Safari and before restarting, hold down the shift key. That should get you back on the safe internet again. You will also want to open safari preferences and check extensions. Uninstall all extensiions.