Newly observed ransomware campaigns are leveraging installer files from the Nullsoft Scriptable Install System (NSIS) to hide malicious code, Microsoft says.
The NSIS installers were recently associated with various well-known ransomware families, including Cerber, Locky, Teerac (also known as Crypt0L0cker), Crowti (aka CryptoWall), Wadhrama, and Critroni (aka CTB-Locker).
The new NSIS installers attempt to evade anti-virus detection by trying to look as normal as possible by incorporating non-malicious components. These include more non-malicious plugins, in addition to the installation engine system.dll; a .bmp file as the background image for the installer interface, and a non-malicious uninstaller component uninst.exe.
Unlike previously used NSIS installers, the new ones no longer feature the randomly named DLL file that was used to decrypt the encrypted malware. Because of this major change, the footprint of malicious code in the NSIS installer package is significantly reduced, Microsoft reveals.
Starting last month, Microsoft observed an uptick in the adoption of the new installers that install ransomware. Instead of using a DLL file to decrypt the malicious payload, the new installers pack a Nullsoft installation script that loads the encrypted data file in memory and executes its code area.
Not only is the malicious payload encrypted, but the installation script is also obfuscated. The script loads the encrypted data file into memory, then gets the offset to the code area (12137). Next, the script issues a call to the encrypted data file. According to Microsoft, the code area in the encrypted data file is the first decryption layer, but the script further decrypts the code until it runs the final payload.
“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, notes.
The distribution campaigns leveraging the new NSIS installers usually follow a specific scheme, Microsoft explains: 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. When the intended victim opens the attachment, the NSIS installer is downloaded, which is turn decrypts and runs the malware.
“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,” Lelli says.
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