The past few months have seen an increase in new, versatile malware downloaders that gather a significant amount of data about users’ systems before deploying a malicious payload. That payload is determined on the users’ system.

Marap malware and Xbash are two notable recent examples. Marap malware fingerprints a system and is capable of downloading additional modules based on the findings of the initial reconnaissance. XBash also assesses the system, and determines whether it is best suited for cryptocurrency mining or a ransomware attack and deploys its payload accordingly.

Stealthy sLoad Downloader Used in Highly Targeted Attacks

A further versatile and stealthy malware variant, known as the sLoad downloader, can now be added to that list. SLoad first appeared in May 2018, so it predates both of the above malware variants, although its use has been growing.

The primary purpose of sLoad appears to be reconnaissance. Once downloaded onto a system, it will determine the location of the device based on the IP address and performs several checks to ascertain the type of system and the software that is running and will determine whether it is on a real device or in a sandbox environment. It checks the processes running on the system, compares against a hardcoded list, and will exit if certain security software is installed to avoid detection.

Provided the system is suitable, a full scan of all running processes will be performed. The sLoad downloader will search for Microsoft Outlook files, ICA files associated with Citrix, and other system information. sLoad is capable of taking screenshots and searches the browser history looking for specific banking domains. All of this information is then fed back to the attackers’ C2 server.

Once the system has been fingerprinted, further malware variants are downloaded, primarily banking Trojans. Geofencing is used extensively by the threat actors using sLoad which helps to ensure that banking Trojans are only downloaded onto systems where they are likely to be effective – If the victim uses one of the banks that the Trojan is targeting.

In most of the campaigns intercepted to date, the banking Trojan of choice has been Ramnit. The attacks have also been highly focused on specific countries including Canada, and latterly, Italy and the United Kingdom – Locations which are currently being targeted by Ramnit. Other malware variants associated with the sLoad downloader include the remote desktop tool DarkVNC, the Ursnif information stealer, DreamBot, and PsiBot.

The sLoad downloader is almost exclusively delivered via spam email, with the campaigns often containing personal information such as the target’s name and address. While there have been several email subjects used, most commonly the emails relate to purchase orders, shipping notifications, and missed packages.

The emails contain Word documents with malicious macros in ZIP files, or alternatively embedded hyperlinks which will download the ZIP file if clicked.

The sLoad downloader may be stealthy and versatile, but blocking the threat is possible with an advanced spam filter. End user training to condition employees never to click on hyperlinks from unknown senders nor open attachments or enable macros will also help to prevent infection.  Web filtering solutions provide an additional layer of protection to block attempts to download malicious files from the Internet.