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In TRU Positives, eSentire’s Threat Response Unit (TRU) provides a summary of a recent threat investigation. We outline how we responded to the confirmed threat and what recommendations we have going forward.
Here’s the latest from our TRU Team…
What did we find?
In late September 2023, the eSentire Threat Response Unit (TRU) observed and prevented an infection by the NetWire RAT. NetWire was a publicly available remote administration tool with password-stealing and keylogging capabilities that first emerged in the wild in 2012.
In March 2023, the FBI shut down NetWire and arrested Mario Zanko, who resided in Croatia, and was responsible for distributing the RAT. Zanko was also responsible for operating the NetWire website where sales took place (Figure 1).
Figure 1: Website that used to sell NetWire and now seized by FBI
We assess with high confidence that the primary source of infection was a drive-by download. The user visited a website offering the Microsoft Office 2019 Professional Plus installer, along with an activator program named "Office 2013-2019 C2R Install".
The downloaded RAR archive, labeled “x64 v16.0.12130.20272_Multi.rar”, contained a binary file named OInstall.exe (MD5: 38be94769e4f59d9a90e551e505c2e07). Unfortunately, we could not identify the exact website or source from which the user obtained the archive.
The initial malicious file contained an AutoIT script that, upon execution and decryption, drops the OInstall.exe (Office 2013-2019 C2R Install) (MD5: b326fc82fb91811c223965d0d63c7a42) and install.exe (MD5: 6037361243f8c390326debbea5b85ac2) file under the %TEMP% folder, which is a .NET binary that we will be analyzing in this article.
Upon decompiling/detokenizing the AutoIt script, we got an encrypted file (Figure 2). The detokenized AutoIt script is decrypted using XOR operations, as illustrated in Figure 3. Notably, the key for each decryption is derived from the last byte of the preceding encrypted string.
Figure 2: Detokenized AutoIt script with decrypted dataFigure 3: XOR decryption function
The decrypted script also contains two embedded base64-encoded binaries that are mentioned previously in this article – OInstall.exe and install.exe.
Core Payload – install.exe
We will look at the core payload, install.exe. Immediately after loading the binary in dnSpy, we can see the resource file that might contain some encrypted data based on the size (121278 bytes). Next, we will try to look for the function that might be responsible for locating and loading the resource section.
The highlighted function in Figure 4 looks quite interesting. It extracts every 5th character from the obfuscated string (“aL0mTs;1lUmnUrgzBFN7:NS^B/R\\pm/qfg^(:Vrj??O<g<E8X@ggHKVujPsXi=VUnd]>7j>90\\N[D=JH0]7Wl-:`lE9uMU:a@J2Q-l>Q^f04<0AHL^E-V9vZFw3sb]r7NV{d?r@3cwYh2w:[5}efGX)uVTg/2Vik(Pt<<?PJYe<jusjs;vGKiloHKzWpoAefJgL>\\`S>[P6ih0jMH\\-[pDi9lVLy]?9u=+=NCY)5tP6(\\<9:/ReJ5(<XZO?Weia<[6O?fwh<RleE01am@cogvA0Nsf:F6><Gm`[YU`Ra^[wy-U@8nz0`O<AeHAu-ndREZeKuQ0iH2`-uVs?9_XLA]BdJe*9;N6)e9Km)5qkn?2KfC”)
to recreate the original string.
Figure 4: Function that contains the obfuscated string
We can recreate the deobfuscation function as shown in Figure 5 and apply our obfuscated string.
Figure 5: Function responsible for deobfuscating the string and the reproduced code
As a result, we receive the string “asmz://(?<guid>[0-9a-fA-F]{32})/(?<size>[0-9]+)(/(?<flags>[a-zA-Z0-9]*))?”. This string pattern is a regular expression that describes a specific string format and can extract parts of it. Let's break down what this pattern is looking for:
Starts with "asmz://".
Followed by a 32-character hexadecimal GUID capture group.
Followed by a forward slash and a number (size).
Optionally, followed by another forward slash and alphanumeric flags.
The requirements match with the name of our resource file within the install.exe binary “asmz://87aec78c4c6931fe7cdf575766b54ea2/138240/z”, where “87aec78c4c6931fe7cdf575766b54ea2” is the GUID capture group, “138240” is the size and “z’ is the flags.
The resource file is then decompressed with DeflateStream. This is a stream class provided by .NET to handle the deflate compression algorithm. Upon decompressing the resource file, we get a DLL file (MD5: 797b3318a7323b73df2bdc910f6ce92f). The method “ResourceDecrypt” is responsible for retrieving the resource file within the DLL binary and then decrypting the file AES.
Figure 6: Method responsible for retrieving the resource file and decrypting it with AES
We receive another executable upon the AES decryption (MD5: a60238d3d7d3d6d90bae440cc4ff25ea). We will proceed with renaming the method names to simplify the analysis.
AntiVM
The “antivm” method checks the system's registry for indications that it's running within a virtual machine (VM). It accesses the BIOS description in the registry to retrieve the system's product name and manufacturer, accesses another system-related registry key to obtain the BIOS version, then checks if any of the retrieved values contain "VM", "VMware", or "VBOX".
If the values contain the mentioned strings, the binary terminates the current running process (Figure 7).
Figure 7: AntiVM function
Persistence
The method “persistence” is responsible for setting up the persistence mechanism on the infected machine by creating a folder named “apppatch” under %APPDATA%, the binary copies itself into the created folder under “mtstocom.exe” and adds itself to the “Load” registry value to load the malicious binary when the user logs on under the registry path “HKEY_CURRENT_USER\Software\Microsoft\Windows NT\CurrentVersion\Windows” as demonstrated in Figure 8.
Figure 8: Persistence method
UAC Bypass
The binary determines the version of the operating system. If the version is 7, it performs the UAC bypass using Eventvwr.exe; if it's 10, it uses Fodhelper.exe
for the bypass, as shown in Figure 9.
Figure 9: UAC Bypass function
Frenchy Shellcode
Finally, it decrypts one of the resource files using AES and then decompresses it, which leads us to the final payload – NetWire RAT (MD5: 2da7b1cf405f791165e42ce5a3efad97), the second decrypted file is a Frenchy Shellcode (frenchy_shellcode_002 – based on the mutex, MD5: abdb5f121849f3f3718768d37abe0173) that is responsible for process hollowing into RegAsm.exe process.
The shellcode function in Figure 10 extracts a list of functions from the Process Environment Block (PEB). It then dynamically resolves the addresses of the NtOpenSection and NtMapViewOfSection functions using the list and a function labeled as “mw_resolve_fnc”. “mw_resolve_fnc” is an export address table (EAT) lookup function.
Given the base address of a module (DLL) and the name of a function, it will locate and return the address of that function from the module's export table.
Figure 10: The code responsible for resolving the functions dynamically
NetWire
NetWire encrypts its configuration with RC4 as shown in Figure 11.
Figure 11: Encrypted data and RC4 decryption function
The initial binary contained an obfuscated AutoIt script. This script subsequently dropped the primary payload responsible for activating the NetWire RAT.
This core payload leveraged the Frenchy Shellcode technique to execute process hollowing, resulting in the deployment of NetWire RAT within RegAsm.exe.
Apart from the core functionalities, the payload comes equipped with features for AntiVM detection, UAC (User Account Control) bypass, and ensuring persistence within the compromised system.
For stealth, NetWire employs RC4 encryption to secure its configuration data.
Recommendations from our Threat Response Unit (TRU) Team:
The eSentire Threat Response Unit (TRU) is an industry-leading threat research team committed to helping your organization become more resilient. TRU is an elite team of threat hunters and researchers that supports our 24/7 Security Operations Centers (SOCs), builds threat detection models across the eSentire XDR Cloud Platform, and works as an extension of your security team to continuously improve our Managed Detection and Response service. By providing complete visibility across your attack surface and performing global threat sweeps and proactive hypothesis-driven threat hunts augmented by original threat research, we are laser-focused on defending your organization against known and unknown threats.
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