Pierluigi Paganini July 04, 2015

Security researcher discovered a strain of the Kovter trojan that has been updating  Flash Player and Internet Explorer to prevent further infections.

The French security expert Kafeine have discovered a new strain of the Kovter malware noticing that the instance of the malicious code he was analyzing was attempting to download the latest version of the Flash Player.

The Kovter malware is used in Ad fraud campaigns, victims were infected by simply clicking on online advertisements and generate revenue for the websites that host the ads.

Flash Player was recently updated by Adobe in order to fix the critical vulnerability (CVE-2015-3113) that had been exploited by threat actors in targeted attacks. Kafeine reported that another exploit kit, the popular Magnitude, was integrated with the code to exploit the flaw in the Flash Player. Several exploit kits including Angler, Neutrino, RIG and Nuclear Pack have later integrated the exploit code for the same vulnerability.

Kafeine noticed a singular behavior for the new strain of the Kovter malware he tested, the malicious code, in fact, was trying to update Flash Player to the latest version 18.0.0.194. It is likely that threat actors patch the infected machine to prevent additional infections through other threat agents.

“Checking my systems I noticed multiple VM trying to grab last version of Flash and thought they were not properly setup allowing Flash Player to auto-update (which we do not want obviously – we want to keep them exploitable and also avoid behavioural/network noise).” wrote Kafeine in a blog post. “The goal is most probably to close the door of the system to additional infection via DriveBy.”

The Kovter malware does much more, it also updates the Internet Explorer browser to the latest version patching two flaws, the CVE-2013-2551 and CVE-2014-6332, exploited by a number of threat actors.

The behavior is not new, in the past security researchers observed other malicious code patching infected systems to close the door to further infection. Kafeine explained that the Betabot Trojan operates in a similar way to prevent future infections via exploit kits. The expert is surprised by the rapidity into patching the issue on the machine infected by the Kovter.

“An exploit get its way to almost all exploit kits in a matter of days, and owners of a big adfraud botnet decide to fix the issue on their ‘fleet’ almost as fast. I find this fast action/reaction interesting,” said Kafeine.

Kafeine reported that Kovter is also being served by the principal exploit kits in the wild, including the Angler EK, the Nuclear Pack, and Neutrino EK.