APT-Style Evasion Techniques Spotted in “Kofer” Ransomware Campaign

Researchers have come across a massive ransomware campaign in which attackers continuously generate new variants of the same malware in an effort to evade detection.

Over the past two weeks, Israel-based security firm Cybereason has identified a large number of different ransomware samples that appear to be created by the same group using an automated algorithm to “mix and match” different components of the threat.

According to Cybereason, this campaign, dubbed “Operation Kofer,” is the first drive-by ransomware attack to incorporate a level of complexity usually seen in advanced persistent threat (APT) operations.

Experts say the attackers are mainly targeting users in Europe, including Spanish, Polish, Swiss and Turkish speakers. New ransomware variants are generated every few days — in some cases every few hours — and delivered to potential victims through spam campaigns that carry ransomware such as CryptoWall 3.0 and Crypt0L0cker disguised as PDF documents.

While there are many similarities between the ransomware variants used in the Kofer campaign, each sample has a unique hash and unique characteristics, which helps the malware evade signature- and hash-based detection.

Several anti-detection techniques have been observed by researchers, including the storage of the encrypted ransomware payload inside a PE file, the inclusion of junk resources to make the malware look benign, and execution of the malware as a child process in an effort to evade detection by sandboxes and dynamic detection solutions.

Some of the malware variants analyzed by Cybereason are designed to check for the presence of virtual environments before conducting any malicious activities.

As for persistence, the ransomware copies itself to random or preset paths that are added to autorun locations in the registry.

Some of the ransomware variants use the Tor anonymity network for command and control (C&C) communications. Experts have pointed out that network-only tools lacking an endpoint element will likely fail to detect the malicious Tor activity since they cannot distinguish legitimate Tor processes from the malware.

“If the Kofer variants are in fact coming from a single source, then this can indicate the commoditization of ransomware at a whole new scale,” said Uri Sternfeld, Senior Security Researcher at Cybereason. “Our best suggestion to minimize the impact of ransomware is to run frequent backups using an external drive and use endpoint monitoring and detection technologies to limit the scope of such attacks.”