Your uncle’s frozen Mac says it’s infected after viewing a Google ad. Now what?

Researchers say they recently found Google ads delivering a sophisticated tech support scam that freezes the screens of both Windows and Mac devices and displays messages urgently instructing them to phone a bogus call center.

The ads were displayed all over the web, including on high-traffic maps, weather, real-estate, document-hosting, and sports sites. Users who called the number were then urged to pay hefty fees, grant remote access to their devices, or divulge personal information. From August 31 to September 14, security firm Netskope observed users from 619 customer organizations click on the malicious ads, although none of them were actually scammed because Netskope blocked the content.

Roughly 62 percent of the organizations were based in the US, with Japan and Australia accounting for the Nos. 2 and 3 spots. Since the firm has visibility into only a tiny sliver of Internet activity, the number of people exposed to the ads—including those who fell victim to it—is likely much higher. Netskope tracked more than 250 Google Ads campaign IDs across at least 284 legitimate publisher sites.