In September 2017, we discovered a new targeted attack on financial institutions. Victims are mostly Russian banks but we also found infected organizations in Malaysia and Armenia.
The attackers were using a known but still very effective technique for cybercriminals looking to make money: gaining persistent access to an internal banking network for a long period of time, making video recordings of the day to day activity on bank employees’ PCs, learning how things works in their target banks, what software is being used, and then using that knowledge to steal as much money as possible when ready.
We saw that technique before in Carbanak, and other similar cases worldwide. The infection vector is a spear-phishing email with a malicious attachment. An interesting point in the Silence attack is that the cybercriminals had already compromised banking infrastructure in order to send their spear-phishing emails from the addresses of real bank employees and look as unsuspicious as possible to future victims.
The attacks are currently still ongoing.
Technical details
The cybercriminals using Silence send spear-phishing emails as initial infection vectors, often using the addresses of employees of an already infected financial institution, with a request to open an account in the attacked bank. The message looks like a routine request. Using this social engineering trick, it looks unsuspicious to the receiver:
The attachment we detected in this new wave is a “Microsoft Compiled HTML Help” file. This is a Microsoft proprietary online help format that consists of a collection of HTML pages, indexing and other navigation tools. These files are compressed and deployed in a binary format with the .CHM (compiled HTML) extension. These files are highly interactive and can run a series of technologies including JavaScript, which can redirect a victim towards an external URL after simply opening the CHM. Attackers began exploiting CHM files to automatically run malicious payloads once the file is accessed. Once the attachment is opened by the victim, the embedded .htm content file (“start.htm”) is executed. This file contains JavaScript, and its goal is to download and execute another stage from a hardcoded URL:
The goal of the script is to download and execute an obfuscated .VBS script which again downloads and executes the final droppe…
