I have received a "WARNING! on my Pad.
It says MAC OS is infected with Spyware and other malicious applications.
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Best reply
Just ignore it. It’s a malicious ad trying to trick you into giving up personal data or money
If it comes back you may need to
Force quit safari
go into your icloud settings and turn off iCloud syncing for safari
throw away your safari history
and reboot your ipad
You may also wish to not revisit the site you picked it up from. Either they know they are dishing up nasty stuff or the ads they serve have been hacked to dish this stuff out.