Welcome back :D, This post is the continuation of SteamHide malware analysis, here we’ll go through the second stage FinalMalware.exe dropped by Hide binary inside image.exe downloader by decrypting the ICC profile after pulling out image from stream profile images. Let’s get started!!

Initial triage for ‘FinalMalware.exe’

File identification

Again DIE and PortexAnalyzer outputs indicates that it’s a PE 32bit .NET managed assembly file, we’ll use dnSpy for further examination

Strings to attach

When a .NET binary is not obfuscated then the strings are good giveaways to its capabilities prior analysis.

Some interesting strings are follows

• system executable file cmstp.exe
• some registry hives and registry manipulating functions
• functions related to file discovery
• some random but interesting strings like BroMal, broVPN, uNoFGmsEX and so on
• string mentioning Temp\ directory
• some pastebin urls
• string mentioning a virtual machine VMware

How does it behave

Starts with ‘Form1_Load()’

‘Evasion.IsInsideVM()’ : Anti-VM Check

• performs a typical anti-vm technique MITRE ATT&CK T1497.001. This method checks the presence of virtual machine by making a query via SelectQuery() on Win32_DiskDrive WMI class which represents a physical disk drive of computer running the Windows,
• when the ManagementObjectSearcher.Get() method on this object is invoked, the ManagementObjectSearcher executes the given query and returns a collection of management objects(ManagementObjectCollection) that matches the query, then loop through collection to get the value of Win32_DiskDrive.Caption property. But checks for only two VM entries ie. VBOX and VMware, childish enough ‘cause when i debugged this sample in my QEMU based Windows VM, the caption value was QEMU HARDDISK, resulting in no VM dectection, as you can see below:

TIP#2: In powershell, run Get-WmiObject -Class Win32_DiskDrive to check your VM’s caption value

‘Admin.Check()’: are you the admin

On a successful Anti-VM check, it gets the current users WindowsPrincipal(WindowsIdentity.GetCurrent()) windows Identity to check if the user has admin role or not by calling WindowsPrincipal.IsInRole method with WindowsBuiltInRole.Administrator enum which returns a boolean respectively.

‘Evasion.CheckFirstRun()’: am i new here

if user is not admin, checks if the file named ie. uNoFGmsEX\uNoFGmsEX..txt exist under Local Application Data folder of user ie. %LOCALAPPDATA%, if not, then creates one, as shown in Procmon output

‘Form1.TryPrivilegeEscalation()’: want some power

if the sample is running for the first time, then it will try to escalate privileges by utilising the technique MITRE ATT&CK T1218.003 which is performed by exploiting system binary cmstp.exe using the command  C:\Windows\system32\cmstp.exe" /au C:\Windows\Temp\[randomFileName].inf which in turn runs the malware with elevated privilege, here’s more on the topic cmstp.exe as lolbin.

See the process in action below

‘Form1.AddPersistence()’: lemme settle for later

• When this sample runs itself with admin privilege via cmstp.exe, it persist by making auto run key by setting the registry key Software\\Microsoft\\Windows\\CurrentVersion\\Run: BroMal with value set to the malware path itself

what’s with the shortcuts

• PopulateSearchPaths(): stores the paths to user Desktop and Documents folder to ShortcutUtils.searchPaths array
• PopulatedShortcuts(): which calls ShortcutUtils.EnumerateShortcuts() to traverse through every directory and sub-directories within searchPaths looking for the *.lnk files, if found then add to the ShortcutUtils.shortcutList list variable
• then nothing is done with that at least as of now

‘App.EnumerateVulnerable()’ : your weakness

The malware seems to look for the existence of a SquirrelTemp\SquirrelSetup.log, if found then saves the path, but nothing is done with that info. Maybe to further exploit them

Then the sample abruptly ends.

what does future holds for SteamHide

This version of sample lacks functionality and there’s are many code segments that are either incomplete or not used by now, like:

• there’s a method CreateTwitterRequest which allows making twitter requests, which can be used as twitter bot for C2 and stuff
• CheckMutexRunning : create a mutex which can be used to avoid re-infecting the system
• CodePieceManager class : contains method regarding compiling source code to MSIL assembly, maybe to add functionality on the fly
• Updater class: which can be used to update to a new version in the same way done by hide binary inside image.exe downloader

IOCs

I had fun analyzing this sample and learned a lot in the meantime. Here’s the first hand report by Karsten Hahn Picture this: Malware Hides in Steam Profile Images

Thanks for tuning in, I’ll see you around :)