293. That's How APT-Q-37 Abuses C# Compiler

Hello everyone!

As you know, adversaries may deliver malicious files to victims as uncompiled code. So today we'll look at an example of Obfuscated Files or Information: Compile After Delivery (T1027.004).

According to the report, APT-Q-37 leveraged malicious XLAM files. The macro in the file decodes a copy of the backdoor source code data written in C# and saves it as C:\programdata\cayote.log. Then csc.exe is used to compile it as C:\Programdata\USOShared\vlcplayer.dll:

"C:\Windows\Microsoft.NET\Framework\v4.0.30319\csc.exe" /r:"C:\Windows\Microsoft.NET\Framework\v4.0.30319\System.Net.Http.dll" /target:library /out:C:\Programdata\USOShared\vlcplayer.dll "C:\Programdata\cayote.log"

For example, we can look for suspicious csc.exe executions for compiling a DLL under ProgramData folder:

event_type: "processcreatewin"

AND

proc_file_path: "csc.exe"

AND

cmdline: ("target:library" AND "programdata")

See you tomorrow!

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