Malware authors are using various techniques to evade detection, and those operating the Cerber ransomware are now employing a server-side “malware factory” , researchers at Invincea reveal.
The “hash factory” attack means that the server morphs the Cerber payload very often to generate unique hashes. In this particular case, a new hash is generated every 15 seconds, researchers say. Given that signature-based solutions often rely on the identification of hashes of known malware for detection, the technique allows the malware to avoid detection.
Cerber, which emerged on the threat landscape in early March, has become the third largest threat in the ransomware segment, trailing only CryptoWall and Locky, which have been on the top two positions for the past three months. Cerber now enjoys a 24 percent market share, though it is still far behind CryptoWall (41 percent) and Locky (34 percent), Fortinet’s Kenichi Terashita recently revealed.
Over the past few months, the ransomware emerged several times as the payload in different infection campaigns, such as the Magnitude and Neutrino campaigns that abused an exploit for a recently patched Flash Player vulnerability (CVE-2016-4117). A few weeks back, researchers also noticed that Cerber appeared to be flooding the subnet with UDP packets, supposedly being used in distributed denial of service (DDoS) attacks.
Cerber is most active in the United States (nearly 50 percent of infections), but also targets Taiwan, Japan, Australia, Brazil, Canada, Portugal, Spain, Malaysia, and Germany.
The new infection campaign observed by Invincea used weaponized Office documents to drop the ransomware onto victims’ machines. The malicious documents contain macros that leverage PowerShell, which in turn invoke commands encoded in Base64, the process resulting in a fileless infection with the Cerber ransomware.
While this attack method is not new, the fact that the server that delivered the payload in this campaign was generating a new hash for it every 15 seconds did stand out. What researchers couldn’t determine, however, was whether the payloads on the server were being programmatically generated locally, or were being generated remotely and uploaded by a script.
The binary being installed during the infection process was also found to be a match to a sample collected in September 2015, detected as a ransomware dropper delivered by Neutrino. Additionally, researchers tied the Cerber payload to the Dridex botnet, though as part of a run that didn’t use a server-side malware factory.