The developers behind the Locky ransomware are tireless in their quest to evade security controls and gain a higher infection rate, and they have recently implemented a variety of changes to the ransomware’s code to support their mission.
Although it first emerged roughly two months ago, Locky has become one of the largest ransomware threats today, mainly hitting users in Germany and France, but already infecting computers in over 100 countries. The malware is said to be operated by the group behind Dridex, mainly because it has employed the same infection techniques and was distributed by the Dridex botnet.
Recently, Locky began showing code modifications that prevented security researchers from efficiently monitoring its activity, but some additional changes were observed over the past week, Check Point researchers say. According to the security firm, a new Locky variant is making the rounds that is more efficient in evading detection mechanisms than previous versions.
First and foremost, the researchers noticed that Locky is no longer distributed via malicious macros in Office documents attached to spam emails. The attackers are now using the Nuclear exploit kit (EK) to deliver the threat, making the infection process more efficient, as the ransomware will no longer blocked by email or document security inspections.
The researchers also noticed that Locky’s embedded configuration block, which contains info about the current sample and a list of static C&C server addresses, is now obfuscated and placed in a fake .reloc section at the file’s overlay. Locky de-obfuscates the configuration block only in run-time, making the data in it completely volatile, since it exists solely in memory.
Another change in the new Locky variant is the fact that, before the execution of any of its original logic, the ransomware attempts to place a shadow copy of itself in a newly allocated memory region, Check Point says. This technique helps the malware avoid common sandboxes, memory detections, or other dynamic analysis methods.
While the malware previously saved its configuration data in a fixed registry key HKCUSoftwareLocky, the new variant uses random registry keys and values (per-host) to save unique computer ID, public-key, and payment text. With this move, detection is no longer possible by simply looking for this registry key, making a previously released “vaccine” for the ransomware no longer effective.
The new Locky variant also employs changes in its communication patterns, which include new HTTP headers, as detailed last week, but shows new behavior as well. Now, the malware first attempts to use the default system Internet Explorer User-Agent and, if that fails, it uses a hardcoded user-agent instead, namely “Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.1; Trident/4.0; .NET4.0C; .NET4.0E).”
The DGA algorithm, the main pillar of Locky’s evasive communication methods, was changed too, with modifications being made to the algorithm flow and overall behavior. Researchers noticed a change in the possible top-level domains for the generated domains, as well as a new top-level-domain list that contains “ru, info, biz, click, su, work, pl, org, pw and xyz” extensions.
Furthermore, Check Point says the new ransomware variant contains a significantly different prolog routine within the domain generation algorithm (DGA).
Overall, the new Locky variant shows much lower detection rate compared to its predecessors, researchers say.
According to Check Point, a quick and effective reaction from the security industry prevented the original Locky variant from reaching its full infection potential, despite solid functionality and encryption algorithms. However, these latest upgrades and the distribution via an EK should help the ransomware achieve a higher infection rate.
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