Microsoft reported that new distribution campaigns are using installer files from the Nullsoft Scriptable Install System (NSIS) to evade ransomware.
Not long ago, the NSIS installers were associated with different ransomware families, such as Locky, Cerber, Critroni (aka CTB-Locker), Crowti (aka CryptoWall), Wadhrama, and Teerac (aka Crypt0L0cker).
What the new NSIS installers do is trying to evade anti-virus detection by incorporating non-malicious components.
In addition to the installation engine system.dll, these components feature more non-malicious plugins, a .bmp file as the background image for the installer interface, and a non-malicious uninstaller component uninst.exe.
The new NSIS installers do not include the randomly named DLL file which was used for decrypting the encrypted malware. According to Microsoft, due to this significant change, the trace of malicious code in the NSIS installer package got highly reduced.
In February, Microsoft noticed an uptick in the adoption of the new installers that install ransomware. The new installers do not use a DLL file to decrypt the malicious payload, but pack a Nullsoft installation script instead, which loads the encrypted data file in the memory and executes its code area. In this way, the malicious payload is encrypted and the installation script is obfuscated.
The encrypted data file is loaded by the script into the memory and gets the offset to the code area (12137). After that, the script issues a call to the encrypted data file. The code area in the encrypted data file is the first decryption layer, however, the script further decrypts the code until it runs the final payload, Microsoft claims.
“By constantly updating the contents and function of the installer package, the cybercriminals are hoping to penetrate more computers and install malware by evading antivirus solutions. Given the pervasiveness of NSIS installers that distribute ransomware, they are likely part of a distribution network used by attackers to install their malware,” Andrea Lelli, Microsoft Malware Protection Center, says.
According to Microsoft, the campaigns which use the new NSIS installers follow a specific scheme: spam emails that mimic invoice delivery notifications are used to deliver a malicious attachment that could be a JavaScript downloader, a JavaScript downloader in a .zip file, a .LNK file that contains a PowerShell script, or a document with malicious macros. Once the victim opens the attachment, the NSIS installer is downloaded and the malware starts running.
“Cybercriminals will stop at nothing to attempt sidestepping security solutions in order to install malware on your computer. The fact that we’re seeing these innovations in cybercriminal operations that deliver ransomware reveals that they are highly motivated to achieve their ultimate goal: to siphon money off their victims. Unfortunately, for enterprises, the damage of successful malware infection can be so much more than just cash,” Andrea Lelli notes.
