Author: Vlad Pasca
A Hybrid Analysis perspective and deep technical dive into the new Turla APT backdoor
Turla starts its attack by using shortcut files to infect victims
Evasion techniques employed by the group involve unhooking and disabling ETW and AMSI for stealth
Backdoor implements custom commands for execution of malicious PowerShell scripts and file creation
In a recent campaign, the Russian APT group Turla (also known as Venomous Bear), used shortcut files (.lnk) to infect systems with a fileless backdoor. The malware employs multiple evasion techniques such as disabling ETW and AMSI, and unhooking. Our contribution to existing research consists of analyzing the backdoor from a Hybrid Analysis perspective and presenting the implementation of the malicious routines. We will present the deobfuscation process and perform a complete technical analysis of the malware that reveals its functionalities.
A Hybrid Analysis Perspective
Turla’s backdoor was obfuscated using the “SmartAssembly” obfuscator to complicate the analysis. It implements evasion techniques to extend malicious activity and influence the logging process. The backdoor commands can be used to create new files and run malicious PowerShell scripts using PowerShell runspaces.
We’ve analyzed an attack that started with a shortcut file called “Advisory23-UCDMS04-11-01.pdf.lnk” detonated via Hybrid Analysis. As shown in Figure 1, the file’s icon is set to PDF in order to trick the user.
Figure 1 - PDF icon set
However, a legitimate PDF called “Advisory23-UCDMS04-11-01.pdf” is displayed during the execution, as seen in the runtime screenshots from the Hybrid Analysis detonation report:
Figure 2 - Legitimate PDF displayed
The detonation report also displays the process tree (Figure 3 ) showing that the initial process creates a file called “ChromeConnection” in the temporary folder. This is executed using the MSBuild tool:
Figure 3 - Process tree
The final payload is a fileless backdoor. The sample executes the Main function of the backdoor with six custom parameters. The presence of these six custom parameters is highly suspicious and should be a first red flag for taking a closer look at this sample. We could identify this operation in the “Extracted Strings” section of the Hybrid Analysis report (see Figure 4).
Figure 4 - Final backdoor is executed with 6 parameters
As shown below, the PDF and the created file can be downloaded from the “Extracted Files” section of the report.
Figure 5 - Extracted Files section on Hybrid Analysis
A Deep Dive into The Dropper
Based on these observations, we are able to determine that the fileless backdoor is worth a deeper investigation. After finding the final dropped file (SHA256: 7091ce97fb5906680c1b09558bafdf9681a81f5f524677b90fd0f7fc0a05bc00) we can download it locally and analyze it using PEStudio. We determine that the sample is a .NET executable obfuscated using the “SmartAssembly” obfuscator (Figure 6).
Figure 6 - PEStudio detects the SmartAssembly obfuscator
Dumbassembly performs initial deobfuscation operations on the malware (Figure 7).
Figure 7 - Dumbassembly tool deobfuscates the backdoor
Simple Assembly Explorer is used to further deobfuscate the resulting executable, as highlighted in Figure 8.
Figure 8 - Simple Assembly Explorer options
It’s very important that we use the right options for deobfuscating the code.
Finally, de4dot is used to restore the remaining obfuscated code, as displayed in Figure 9 and Figure 10 below, which shows the difference between the decompiled codes.
Figure 9 - Before deobfuscation
Figure 10 - After deobfuscation
The process creates a mutex called “{C916E9A6-EEDF-4648-9A29-9E5713F4E79A}” to ensure that only one copy of the malware is running at a single time.
The first three parameters passed to the program are Base64-decoded and then decrypted using the XOR operator, with the first byte representing the key.
Figure 11 - Parameters are passed to the decryption function
Figure 12 - Implementation of the operations
We’ve developed a custom Python script that decrypts the required parameters. The C2 server https[:]//files.philbendeck[.]com is revealed after the decryption.
The last three parameters are used to compute the receive timeout, sleep time, and reconnect timeout, respectively. The default values are 30 seconds for the first two and 30 minutes for the third.
Figure 13 - Last parameters are used to compute the timeouts
The malicious process obtains the network interfaces on the local computer via a function call to GetAllNetworkInterfaces, and then extracts the MAC address. The address is modified to delete the “-“ character and the result is concatenated with the first parameter previously decrypted (Figure 14).
Figure 14 - MAC address extraction
The value computed above is XOR-ed with a randomly generated byte and stored in a variable called “strEncodedID”. This is used to compute a unique identifier of the infected machine.
Figure 15 - Unique identifier stored in strEncodedID variable
The following DLLs are mapped to memory: ntdll.dll, KernelBase.dll, and kernel32.dll. The purpose of this operation is to bypass hooks that might have been installed, by mapping “fresh” (not hooked) DLLs to replace the .text hooked sections with the clean ones . To accomplish that, the memory protections of .text sections of the loaded DLLs need to be changed to 0x40 (PAGE_EXECUTE_READWRITE) using VirtualProtect (Figure 16).
Figure 16 - DLLs that will be mapped
Figure 17 - Windows APIs used for mapping
Figure 18 - Memory protection changed using VirtualProtect
The MAC address concatenated with the first parameter described before is set to be an AES-128 key that will be used in upcoming C2 communication activities (Figure 19).
Figure 19 - AES algorithm initialization
The hostname and username are retrieved and encrypted using the AES algorithm. The result is Base64-encoded and exfiltrated to the C2 server using a POST request (see Figure 20 and Figure 21).
Figure 20 - Create a POST request to the C2 server
Figure 21 - AES encryption and Base64 encoding
The process reads the server’s response by calling the GetResponse method. It expects 38 bytes in the response (Figure 22).
Figure 22 - Server’s response is read and verified
If any exception occurs, the binary Base64-encodes the hostname concatenated with the username, and downloads a resource from the C2 server based on the “search=” parameter, as shown below.
Figure 23 - Download a resource if any exception occurs
The value “strEncodedID” is encrypted using the AES algorithm and Base64-encoded. The encrypted data is sent to the C2 server (Figure 24).
Figure 24 - Exfiltration of the unique strEncodedID identifier
The server’s response is decrypted and Base64-decoded. The structure of the result is “value1|value2|…”, where the first value is the command to be executed and the remaining values represent the required parameters.
Figure 25 - Server’s response contains the command to be executed
Turla’s process of encrypting communication between the victim and C2 is a sophisticated attempt to avoid detection by automated cybersecurity solutions and SecOps teams. This method of hiding communications and the exchange between victim and C2 makes the attack very difficult to discover and analyze, even by defenders employing networking tools.
Custom Backdoor Commands
“uf” command
This command is used to create a new file and populates it with content received from the C2 server. The first parameter is the file path and the second parameter represents the file’s content that is Base64-decoded before being written (Figure 26).
Figure 26 - A new file is created and written on it
“op” command
The command has three subcommands: “rct”, “st” and “rt”. It’s used to modify the reconnect timeout, sleep time, and receive timeout, respectively.
Figure 27 - Reconnect timeout is changed
“cps” command
The process closes a PowerShell Runspace using the Runspace.Close function:
Figure 28 - Close a PowerShell Runspace
“ps” command
The command can be used to run PowerShell scripts. The process disables ETW and turns off AMSI during the malicious activity.
Figure 29 - Command’s result is sent to the C2 server
The malicious process creates a PowerShell Runspace via a function call to CreateRunspace and an empty PowerShell instance (Figure 30).
Figure 30 - Create a PowerShell Runspace
The following functions will be patched: EventWrite, EtwEventWrite, ReportEventW, AmsiOpenSession, and AmsiScanBuffer. These functions are targeted because they’re used by ETW and AMSI, providing Telemetry to Security Products.
Figure 31 - Functions used by ETW and AMSI are patched
The patching operation is done by modifying the first instruction of the functions. The backdoor first changes the protection of the region using VirtualProtect, then copies the new instructions and changes the protection back to original(Figure 33).
Figure 32 - Make the code of the functions modifiable
For example, the code of the EventWrite method is modified to always return a value of 0, avoiding to create the ETW events to consume. The bypass of the AmsiScanBuffer function consists of returning the E_INVALIDARG value, as highlighted in the figure below, to avoid sending those buffers to the AMSI Provider.
Figure 33 - New instructions of EventWrite
Figure 34 - New instructions of AmsiScanBuffer
Moreover, the process loads the “System.Management.Automation” assembly and disables ETW of the PowerShell session by setting the value of the “m_enabled” field from the “Tracing.PSEtwLogProvider” class to 0, by leveraging Reflection.
Figure 35 - Disable ETW of the PowerShell session
If the “ldscr” subcommand is specified, the process can run PowerShell scripts specified in the C2 server’s response. The AddScript and AddCommand functions are utilized to run scripts and collect the output. Finally, the output is exfiltrated to the C2 server.
Figure 36 - Scripts’ output is exfiltrated to the C2 server
Figure 37 - Scripts are passed to the AddScript function
Final Thoughts
Hybrid Analysis is a great platform for identifying and analyzing APT samples. It provides the context and data that can be investigated further during the dynamic analysis of the malware. If you want to perform a more in-depth analysis of the sample, you can download the sample by registering with a Hybrid Analysis account and becoming a vetted user.
This example highlighting Turla shows the value of the platform. After deobfuscating the backdoor, we were able to analyze its commands that turned out to be intuitive and very effective.
Indicators of Compromise
C2 server
https[:]//files.philbendeck[.]com
SHA256
cac4d4364d20fa343bf681f6544b31995a57d8f69ee606c4675db60be5ae8775
b6abbeab6e000036c6cdffc57c096d796397263e280ea264eba73ac5bab39441
8d6fe8e336e020410753ff15ece5f36bae992f7f234385a23590a11ed734792d
7091ce97fb5906680c1b09558bafdf9681a81f5f524677b90fd0f7fc0a05bc00
Mutex
{C916E9A6-EEDF-4648-9A29-9E5713F4E79A}
