Pierluigi Paganini
February 19, 2021
MassLogger Windows credential stealer is back and it has been upgraded to steal credentials from Outlook, Chrome, and instant messenger apps.
Cisco Talos experts uncovered attacks against users in Turkey, Latvia, and Italy, the infections have some similarities with attacks that targeted users in Bulgaria, Lithuania, Hungary, Estonia, Romania, and Spain in September, October, and November 2020.
The MassLogger infections were first spotted in the wild in April, since then the author of the malware are improving the malicious code.
Unlike other Masslogger trojan samples previously documented, the one employed in the new campaign uses the Microsoft Compiled HTML Help file format, which is a Microsoft proprietary online help format, to start the infection chain. The Microsoft file format can also contain active script components, in this case JavaScript, which is used to launch the malware in the attacks detected by Talos.
“Although operations of the Masslogger trojan have been previously documented, we found the new campaign notable for using the compiled HTML file format to start the infection chain.” reads the analysis published by Cisco Talos. “This file format is typically used for Windows Help files, but it can also contain active script components, in this case JavaScript, which launches the malware’s processes.”
The infection chain starts with an email message containing a legitimate-looking subject line and comes with a RAR attachment with a slightly unusual filename extension.
RAR archives can also be split into multi-volume archives, in the attacks observed by Talos, the filename creates files with the RAR extension named “r00” and later with the .chm file extension. This trick was implemented to bypass security programs that check for the file extension of the attachment.
Upon opening the attachments, the message “Customer service” is displayed while an obfuscated JavaScript code creates an HTML page, which in turn contains a PowerShell downloader that fetches from a legitimate server the loader used to launch the MassLogger payload.
The malware is able to exfiltrate stolen data via SMTP, FTP or HTTP. The latest version of MassLogger (version 3.0.7563.31381) implements features to steal credentials from Pidgin messenger client, Discord, NordVPN, Outlook, Thunderbird, Firefox, QQ Browser, and Chromium-based browsers such as Chrome, Edge, Opera, and Brave.
The Masslogger Trojan could also act as a keylogger, but the variant analyzed by the experts has disabled this functionality.
The malware is almost entirely executed in memory, for this reason in order to detect the threat it is important to conduct continuous background memory scans. The malware only leaves the attachment and the compiled HTML help file on the disk of the infected machine.
“Users are advised to configure their systems for logging PowerShell events such as module loading and executed script blocks as they will show executed code in its deobfuscated format. Talos will continue to track similar campaigns to make sure adequate protection is included in Cisco Secure products.” concludes the report.
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(SecurityAffairs – hacking, malware)
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