Smoke Loader Backdoor Gets Anti-Analysis Improvements

The infamous Smoke Loader backdoor now has more complex anti-analysis techniques that allow it to remain a potent malware delivery mechanism, PhishLabs security researchers warn.

Also known as Dofoil, Smoke Loader has been advertised on dark web forums since at least mid-2011. Packing a modular design, the malware can receive secondary execution instructions and/or download additional functional modules. Lately, the loader has been used in the distribution of malware such as the TrickBot banking Trojan and GlobeImposter ransomware.

The Smoke Loader installer, the security researchers explain, spawns an EnumTools thread to detect and evade analysis tools, and uses an API to enumerate running analysis utilities. The malware checks for twelve analysis processes via a hash-based method, and terminates itself if one is found running. As part of an anti-VM check, it also queries the name and the volume information of the infected machine, along with a registry key.

“There are two main paths of execution in Smoke Loader, the installer and the loader. The installer path runs prior to spawning and injects into a new instance of a Windows Explorer process. Post injection, the loader runs and executes the core functionality of the module. Before injection occurs, Smoke Loader performs several checks to determine information about the system on which it is running,” PhishLabs says.

Smoke Loader was observed leveraging the VirtualProtect API call to change the protection of the allocated memory region, the security researchers reveal. Toward the end of the loader execution path, the malware also checks whether injection should occur, and execution continues if injection has not yet been performed.

The malware was observed performing networking checks to ensure the loader has Internet access (it can generate fake traffic for that). The security researchers also noticed that, unlike previous versions, the latest Smoke Loader variant uses a custom XOR-based algorithm to decode strings within the sample. Previously, the strings weren’t encoded.

“While Smoke Loader’s distribution is not as wide spread as other malware families, it is under continued development and very effective at what it does. The loader’s longevity indicates that the developers are committed to persistence and protection of their loader from the latest analysis techniques. Even though it dates back to 2011, the loader has undergone several transformations that allow it to continue to be a potent malware delivery mechanism in 2017,” PhishLabs concludes.

Related: RIG Exploit Kit Exposes Millions to SmokeLoader Backdoor