Conveyed through phishing emails, the Masslogger trojan's most recent variation is contained inside a multi-volume RAR archive using the .chm file format and .r00 extensions, said Switchzilla's security research arm. Cisco Talos added: “Masslogger is a credential stealer and keylogger with the ability to exfiltrate data through SMTP, FTP or HTTP protocols. For the first two, no additional server-side components are required, while the exfiltration over HTTP is done through the Masslogger control panel web application.”
CHM is an arranged HTML file that contains an embedded HTML file with JavaScript code to start the active infection process. Each phase of the infection is obfuscated to avoid detection using simple signatures. The subsequent stage is a PowerShell script that eventually deobfuscates into a downloader and downloads and loads the main PowerShell loader. The Masslogger loaders appear to be facilitated on undermined authentic hosts with a filename containing one letter and one number linked with the filename extension .jpg. For instance, "D9.jpg".
Masslogger is not an entirely new creation of the malware industry: Talos highlighted research by infosec chap Fred HK. He ascribed it to a malware underground persona who goes by the handle of NYANxCAT. Costs for Masslogger were apparently $30 for three months or $50 for a lifetime license. Cisco's analysis showed that Masslogger “is almost entirely executed and present only in memory” with just the email attachment and the HTML help file.