Exploit kits are essentially malicious Web applications that check if visitors run outdated software on their computers and then exploit vulnerabilities in that software to install malware. They usually target popular applications that are accessible through browser plug-ins, such as Java, Flash Player and Adobe Reader.
The attacks launched by exploit kits are called drive-by download attacks and have become one of the main methods of distributing malware.
According to an independent malware researcher who uses the pseudonym Kafeine, aside from Java and Flash Player, Angler EK is now also targeting Silverlight, a runtime environment for rich Internet applications developed by Microsoft.
Angler EK appeared last month, shortly after the creator of the popular Blackhole exploit kit was arrested in Russia, and is being used by the cybercriminal gang behind the Reveton ransomware that impersonates law enforcement agencies and asks victims to pay non-existent fines.
Before switching to Angler, the Reveton gang used Cool Exploit Kit, a more high-end version of Blackhole, Kafeine said in a blog post.
Starting Thursday, Angler includes an exploit for a remote code execution vulnerability in Silverlight 5 that’s known as CVE-2013-0074 and was patched by Microsoft in March, Kafeine said.
According to Timo Hirvonen, a senior researcher at antivirus company F-Secure, it’s unusual for authors of exploit kits to target Silverlight. “I do not remember seeing exploit kits using Silverlight exploits before,” he said via email.
It’s not clear how many users have Silverlight installed on their computers, but their number is likely to be in the tens of millions.
Most Netflix users need Silverlight, and there are more than 40 million Netflix subscribers, so there are enough potential targets to make a Silverlight exploit attractive, Hirvonen said.
Angler EK loads the Silverlight exploit only if the Java or Flash Player versions installed on the computer are not vulnerable, according to Hirvonen. He believes the Silverlight exploit was added to Angler now because exploit code for CVE-2013-0074 was recently made public on the Packet Storm security website and cybercriminals are frequently reusing publicly available exploits.
Silverlight users should make sure they have all the patches available for the software installed. Silverlight security patches are normally distributed through the Windows Update mechanism.