For a brief period this week, cybercriminals managed to infect Google's and Microsoft's online ad networks with malicious advertisements that attacked users' PCs, according to security consultancy Armorize.
The attacks started around 5 December and lasted a few days, sending victims who clicked on the ads to malicious web pages. Those pages took advantage of known software bugs to install backdoor programs that gave the attackers control of the victims' PCs, or to install software that made it appear as though the PCs were filled with malicious software.
Google acknowledged Friday that it had experienced some issues on its DoubleClick network but said it had put a stop to them quickly.
"[T]he DoubleClick Ad Exchange, which has automatic malware filters, independently detected several [ads] containing malware, and blocked them instantly - within seconds," Google spokesman Jay Nancarrow said via email. "Our security team is in touch with Armorize to help investigate and help remove any affected creatives from any other ad platforms."
Nancarrow wouldn't say how the malicious ads got onto Google's ad network, but Armorize Chief Technology Officer Wayne Huang said cybercriminals may have tricked Google by serving the ads from a domain similar to that used by a legitimate ad-serving company, AdShuffle, based in Irving, Texas. AdShuffle couldn't be reached for comment Friday.
Armorize and others spotted similar ads on Microsoft's Hotmail service, according to Huang. Microsoft said via email Friday that it was was looking into the matter and could not comment in time for this report.
The ads exploit bugs in Adobe Reader, Java and other PC software, Huang said. The bugs have been previously identified, which means people with up-to-date software and antivirus products should not be at risk.
Criminals have slid malicious ads into circulation before. Last year, the New York Times was tricked into running a fake ad for the Vonage VoIP service. It generated fake antivirus warnings that encouraged readers to buy bogus security software.
The Doubleclick and Hotmail ads appear to have been more dangerous, however, in that they attacked computers and installed malicious software, such as the HDD Plus fake system optimization tool.
"This time it's different. It's using drive-by downloads," Huang said. "You visit a site and then you see a fake antivirus pop up." It looks like a genuine antivirus message, he said, and is already installed on the victim's computer. "Even if you reboot, it's already there."
If the cybercriminals were able to get their bad ads on Google's and Microsoft's networks, they'll probably try to do the same thing on other networks too, he said.
Amorize planned to publish a blog post about the infection here on Friday evening.