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                Excel Add-ins Deliver JSSLoader Malware

                The GOLD NIAGARA threat group has expanded its tactics for delivering the JSSLoader RAT, spoofing legitimate Microsoft Excel add-ins to infect systems. By: Counter Threat Unit Research Team

                Secureworks® Counter Threat Unit™ (CTU) researchers observed multiple malicious Microsoft Excel add-ins delivering JSSLoader malware. JSSLoader is a remote access trojan (RAT) that was first observed in 2019 and is used by the GOLD NIAGARA cybercrime group. An Excel add-in extends Excel functionality, typically uses the '.xll' file extension, and functions similar to a dynamic link library (DLL). These observations indicate a change to tactics, techniques, and procedures (TTPs), as the threat actors previously leveraged malicious executable files or Excel macros.

                The original delivery mechanism was unavailable for analysis, but the add-ins were reportedly delivered via invoice-themed emails. This approach is consistent with previous GOLD NIAGARA activity. The XLL files analyzed by CTU™ researchers use the ExcelDna.xll filename, possibly to mimic a legitimate Excel add-in project of the same name. Executing the XLL file launches Excel and displays a security warning (see Figure 1). If the user enables the add-in, its code executes within the context of the Excel process, attempts to download a JSSLoader binary to the %TEMP% directory, and then executes the binary.

                Excel security warning triggered by malicious XLL file.
                Figure 1. Excel security warning triggered by malicious XLL file. (Source: Secureworks)

                The JSSLoader RAT can harvest data about the compromised system and send it to a command and control (C2) server (see Figure 2), run commands, download additional malicious payloads, and execute files.

                C2 beacon sending harvested system information and decoded data.
                Figure 2. C2 beacon sending harvested system information and decoded data. (Source: Secureworks)

                The malware authors have made several changes over the years, including briefly shifting to C++ code in mid-2021. In late 2021, JSSLoader samples written in the original .NET language reemerged. Samples observed in 2022 continue to use .NET and include only minor differences from late 2021 samples. The .NET code lacks robust obfuscation, instead relying on variable .NET class and function names, simplistic encoding and decoding functions, a large number of unused strings, and string splitting/concatenation to avoid reliable detection (see Figure 3).

                JSSLoader code obfuscation.
                Figure 3. JSSLoader code obfuscation. (Source: Secureworks)

                Once executed, JSSLoader collects basic system information, sends the information to the C2 server, and then waits for commands. The malware uses Windows shortcut (.lnk) files for persistence.

                To mitigate exposure to this malware, CTU researchers recommend that organizations use available controls to review and restrict access using the indicators listed in Table 1. The URL and domains may contain malicious content, so consider the risks before opening them in a browser.

                Indicator Type Context
                a8da877ebc4bdefbbe1b5454c448880f36ffad46d6d50083d586eee2da5a31ab SHA256 hash Malicious Excel add-in used to download JSSLoader malware
                9f69f4c0791f2233d4777bcd54e54be063136a1c SHA1 hash Malicious Excel add-in used to download JSSLoader malware
                feca1b74d39cc8cf7219ecd8264c3fca MD5 hash Malicious Excel add-in used to download JSSLoader malware
                da480b19c68c2dee819f7b06dbfdba0637fea2c165f3190c2a4994570c3dae2a SHA256 hash JSSLoader executable
                8e44eb6f82441f84db1b4b5bf4b93a8f34005a93 SHA1 hash JSSLoader executable
                253cb5361e43bfb1931fa115336e7c16 MD5 hash JSSLoader executable
                910b6f3087b1d5342a2681376c367b53e30cf21dd9409fb1000ffb60893a7051 SHA256 hash JSSLoader executable
                15636fdd7bbab7e51b79b61ab7358cf7004ca97c SHA1 hash JSSLoader executable
                0cd9c62063026d4199c941b5f644c5ce MD5 hash JSSLoader executable
                http://physiciansofficenews.com/partners/visitor.exe URL JSSLoader executable
                divorceradio.com Domain name JSSLoader C2 server
                securmeawards.com Domain name JSSLoader C2 server
                weotophoto.com Domain name JSSLoader C2 server

                Table 1. Indicators for this threat.

                If you need urgent assistance with an incident, contact the Secureworks Incident Response team. For other questions on how we can help, use our general contact form.

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