I got the same alert yesterday afternoon..wasnt happy after a long hard day and feelig mad so rang the number .
I got "helped" by a lady that seemed just like telstra support I have used when having other computer problems...so tired and vulnerable I let her into "Look" at my Mac laptop...then she directed me to a site technintion called Square Technology Inc with a number of 1800 235 946 and a very legitimate looking website with Norton security and asked me for my credit card details to pay $ 199 USD..about $ 230 Aus..I said what a bout paypal and was told pay pal was 'down" at the moment..!!
Thats when I got suspicious!!
I was dreading having to drive 45 mins to nearest computer fixer on Monday morning so thought hat as my laptop is new I would ring apple store and got support...
They walked me through over the phone exactly how to delete the safari files and empty trash to get rid of any files the hackers may have left..
I worry i gave them my address and email...lucky not my bank details and money...
Had I not been so tired in the first place I may have been more alert to it...