Web users and advertisers often disagree about online ads, but malvertising — display ads injected with malicious code that tries to execute when an ad loads in a Web browser — is hard to love. Now it seems that malvertising campaigns are spreading from Web browsers to other Web-using applications, such as Skype.
Ads normally show up in a Skype user's home screen, and during audio conversations. Finnish security firm F-Secure found that some of those ads were a part of a recent malvertising campaign.
If a user were to click one of those Skype ads, the user's browser would be taken to malicious website that harbored the notorious Angler browser exploit kit. (The same malicious ads appeared on MSN.com, Wikia.com, the Italian version of eBay and the Daily Mail website.)
Angler is one of several browser exploit kits, constantly changing bundles of malware that attack visiting Web browsers with exploits of multiple security flaws, then install various kinds of malware once they get through. In this case, Angler tried to infect Windows PCs with the Teslacrypt strain of encrypting ransomware, which locks up a user's files until the user pays a ransom of several hundred dollars.
Since Angler is designed to attack Web browsers, not Skype, the chat service's users were safe as long as they didn't click on the malicious ads. To protect yourself from future malvertising that appears on Skype or other non-browser Internet-connected desktop applications, simply never click on those ads.