For a long time, Apple shied away from discussing any types of malware: viruses, trojan horses, even adware. This latter form of malware is also called ad-injection software, and, as Apple says, “come from third-party download sites.” This can result in annoying ads popping up on your Mac, or being inserted into web pages. Again, quoting Apple:
“If your Mac has ad-injection software installed, you might see pop-up windows, ads, and graphics while surfing the web, even if “Block pop-up windows” is selected in Safari preferences. Ad-injection software might also change your homepage and preferred search engine.”
Apple has created a technical document, Remove unwanted adware that displays pop-up ads and graphics on your Mac, explaining how to get rid of these annoyances. But it’s not that simple. You need to check a lot of system folders for obscure files, such as com.genieo.completer.update.plist, com.VSearch.bulk.installer, or com.genieoinnovation.macextension.client.plist. And some of the file names may vary, so Apple explains how to look for files that may contain any of a number of different words.
This is all quite disturbing, and highlights the risks of installing software from many third-party websites. But it’s not just these sites that install crap like this; Oracle recently added adware to its OS X Java installer.
So what should you do? Most commercial antivirus software will remove adware, but it’s best to have a look in the folders that Apple mentions in its document. I actually check those folders from time to time, because software that I’ve tried out can leave files behind which may launch processes that I don’t need to have running. It’ll take a few minutes, but if you are seeing unwanted ads on your Mac, you should definitely do it.