Googlesyndication

Mac users have recently reported some 'suspicious' pop-up messages named 'Googlesyndication' displayed to them while visiting particular websites. That issue occurs on all popular browsers, including Safari, Chrome, Firefox, and others. The message asks users to download some files from URLs like 'tpc.googlesyndication.com' and 'safeframe.googlesyndication.com.' In other cases, users report having been redirected through these URLs to other unknown websites.

Though such behavior may look suspicious, Googlesyndication is not a virus. It is a legitimate Google platform through which advertising content for Google's AdSense program is stored and delivered. Therefore, this activity is unlikely to be related to an adware threat or any other malware infection. Instead, the Googlesyndication pop-ups are an internal issue of the AdSense network and a legitimate part of Google's ad delivery service.

Yet, it does not mean that all ads delivered through Google's advertising network are safe since rogue ads that redirect users to potentially malicious websites have managed to bypass implemented security defenses on many occasions. Therefore, users who come across pop-ups that ask them to download weird files should ignore such messages and/or exit pages that do not seem legit.
Though it is impossible to prevent Googlesyndication prompts as they come from Google's advertising network, and it is the company's responsibility to deal with malicious ads, users still have the option of blocking particular websites that seem untrusty through the web browser's settings.