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New Formbook Campaign Delivered Through Phishing Emails

Mar 11 2022

Summary

Since the beginning of 2022, the unfolding geopolitical conflict between Russia and Ukraine has resulted in the discovery of new malware families and related cyberattacks. In January 2022, a new malware named WhisperGate was found corrupting disks and wiping files in Ukrainian organizations. In February 2022, another destructive malware was found in hundreds of computers in Ukraine, named HermeticWiper, along with IsaacWiper and HermeticWizard.

Aside from new malware families and novel attacks, previously known malware families continue to be used against organizations in Ukraine and throughout the world. Recently, Netskope Threat Labs came across an interesting phishing email addressed to high-ranking government officials in Ukraine containing Formbook (a.k.a. XLoader), which is a well-known malware operating in the MaaS (Malware-as-a-Service) model. This malware provides full control over infected machines, offering many functionalities such as stealing passwords, grabbing screenshots, downloading, and executing additional malware, among others.

The email seems to be part of a new spam campaign, since there were multiple emails with the same subject and body addressed to other recipients. Most of them contain an infected spreadsheet encrypted with the “VelvetSweatshop” password, which is a known Formbook behavior. The infected spreadsheet delivers the threat through vulnerability described under CVE-2017-11882 and CVE-2018-0798. However, the email addressed to government officials in Ukraine contains a .NET executable, responsible for loading Formbook in a multi-stage chain:

Formbook infection chain

In this blog post, we will analyze all the layers from the email attachment to the last Formbook payload.

Phishing Email 

The infection flow starts with a generic phishing email that uses a common technique, tricking the victim into downloading the payload by pretending to be a shipping invoice.

Screenshot of phishing email containing malicious attachment.
Phishing email containing a malicious attachment.

The attachment is a compressed file containing the first Formbook stage.

Screenshot of Email attachment carrying Formbook.
Email attachment carrying Formbook.

Also, as we mentioned previously, we found similar emails delivering malicious spreadsheets, so we believe that this is part of a new spam campaign delivering multiple threats.

Screenshot of similar phishing email with a malicious attachment.
Similar phishing email with a malicious attachment.

Analysis – Summary

Before executing the last file (Formbook), the malware is divided into multiple stages, which we have summarized below.

  1. Stage 01 is a loader, responsible for decoding and executing the next stage;
  2. Stage 02 is another loader, responsible for obtaining the encrypted bytes of Stage 03 from the resources of Stage 01, decrypting and executing it;
  3. Stage 03 is a known packer/loader named CyaX-Sharp, responsible for decrypting and executing the last stage;
  4. Stage 04 is the Formbook payload, which injects itself into other processes, as described later in this analysis.
Diagram showing a summary of Formbook loading process
Summary of Formbook loading process

Analysis – Stage 01


The first stage is a .NET executable likely compiled on February 21, 2022. This file is a loader, responsible for decoding and executing the next stage.

Screenshot of binary details of the first stage.
Binary details of the first stage.

Once we decompile the file, we can see that the real executable name is “VarArgMet.exe”. This stage doesn’t contain any code obfuscation but does contain an obfuscated string and an encrypted resource which we will discuss later.

Screenshot of first stage decompiled.
First stage decompiled.

Also, this file seems to be an infected version of a public .NET project named PlaylistPanda, created in 2009. Looking at the entry point, we can see the same code that is published in the PlaylistPanda public repository, where the MainForm function is called, followed by InitializeComponent.

Example of entry point of the first stage.
Entry point of the first stage.

In this malicious version, the InitializeComponent function contains the main code of the first stage. Once running, the code reads an obfuscated and base64 encoded string stored in a variable named x121312x121312, which contains the next stage. Once it’s deobfuscated and decoded, the file is passed as an argument to the function Springfield.  

Furthermore, this loader contains a lot of junk code that will never be executed, possibly to confuse analysts and slow down analysis.

Example of loader’s main code, decoding and executing the next stage.
Loader’s main code, decoding and executing the next stage.

The Springfield function then loads the second stage as a .NET assembly, which is saved in a variable named DebuggerVisualizer.

Example of second stage being loaded as a .NET assembly.
Second stage being loaded as a .NET assembly.

The DebuggerVisualizer variable is then passed as an argument to the EraInfo function, which executes the second stage by calling the CreateInstance function with the payload and three strings as arguments:

  • 5A6F6E654964656E746974795065726D697373696F6E417474726962 (ZoneIdentityPermissionAttrib)
  • 6F513037 (oQ07)
  • PlaylistPanda
Example of second stage being executed.
Second stage being executed.

Analysis – Stage 02


The second stage is a .NET DLL, likely compiled on February 16, 2022. This file is another loader responsible for executing the third stage, which is stored in the resources of the first stage.

Screenshot of binary details of the second stage.
Binary details of the second stage.

Once we decompile the file, we can see that the real name is “SpaceChemSolver.dll”. This file doesn’t have any sort of code obfuscation or protection. The entry point of this stage is the RunCore function, which is called within SharpStructures.Main.

Screenshot of second stage’s name.
Second stage’s name.

This code is responsible for loading and executing the third stage, which is encrypted and stored as a resource named ZoneIdentityPermissionAttrib in the first stage (PlaylistPanda), masqueraded as a bitmap image.

Example of third stage execution flow.
Third stage execution flow.

After loading the fake image from the first stage resources, the function ConstructionResponse is responsible for decrypting the binary using XOR operations with the string “oQ07”.

Screenshot of the function that decrypts the third stage.
Function that decrypts the third stage.

Once decrypted, the second stage loads the third stage as a .NET assembly, like we saw previously, executing a function named yjO9HynvmD.

Example of third stage being loaded.
Third stage being loaded.

Analysis – Stage 03 (CyaX-Sharp)

The third stage is yet another .NET file, but this time it’s protected with .NET Reactor. The compilation date is also near the other files, on February 21, 2022. This file is a known loader/packer named CyaX-Sharp, which is commonly used to deliver malware like AgentTesla and Warzone RAT.

Screenshot of binary details of the third stage.
Binary details of the third stage.

Before executing the payload, this packer offers many functionalities such as Virtual Machine and Sandbox detection. These features can be enabled or disabled through configuration, which is stored in a string within the binary.

Screenshot of CyaX-Sharp configuration string.
CyaX-Sharp configuration string.

Once it’s running, it starts by parsing the configuration string and then calling the functions related to the features for which the option is enabled.

Screenshot of CyaX-Sharp main function.
CyaX-Sharp main function.

The malware checks if there’s another instance running through a Mutex object named “WuhpBQuQigdPUFFvzgV”.

Screenshot of Mutex created by the third stage.
Mutex created by the third stage.

Then, the malware checks if the process is running with administrative privileges, and it adds the path of the executable to the exclusion list of Microsoft Defender.

Screenshot of simple Windows Defender bypass.
Simple Windows Defender bypass.

In this specific file, the Virtual Machine and Sandbox verification are disabled. However, just to demonstrate how it works, this malware is able to detect virtualized environments by checking the presence of specific values in the Windows Registry, used by software like VirtualBox and VMware.

Screenshot of functionality to detect virtualized environments.
Functionality to detect virtualized environments.

For sandbox detection, the malware searches for common file names, loaded modules, and windows titles.

Screenshot of functionality to detect sandboxes.
Functionality to detect sandboxes.

CyaX-Sharp also offers a feature to download and execute additional payloads, which is also disabled in this sample.

Screenshot of functionality to download and execute additional payloads.
Functionality to download and execute additional payloads.

It then copies itself to AppData, as “YtGUemuxgzC.exe”.

Screenshot of malware copying itself to AppData.
Malware copying itself to AppData.

The permission of this file is then changed to avoid anyone from deleting it.

Example of changing recently copied AppData permission.
Changing recently copied AppData permission.

To execute this copy, a very simple persistence technique is implemented via Windows scheduled tasks.

Example of malware’s persistence.
Malware’s persistence.

The final stage is then loaded from a resource named “fVkXSK7E”, which contains the encrypted bytes of Formbook.

Example of CyaX-Sharp loading the final stage.
CyaX-Sharp loading the final stage.

Before decrypting the payload, CyaX-Sharp builds the path string of the executable that will be used to inject Formbook. In this case, the malware is configured to use “vbc.exe”.

Formbook is then decrypted through bitwise operations using the bytes of the string “SUASbkTWociWWQ”.

Screenshot of CyaX-Sharp decrypting Formbook.
CyaX-Sharp decrypting Formbook.

Formbook is injected into “vbc.exe” via Process Hollowing, which we have already explained in more detail in this analysis. All the APIs are loaded dynamically via GetProcAddress and LoadLibraryA APIs.

Screenshot of APIs related to Process Hollowing.
APIs related to Process Hollowing.

We can find Formbook fully decrypted by inspecting the “vbc.exe” process memory, or by dumping the bytes once it’s decrypted in the third stage.

Screenshot of Formbook injected into “vbc.exe”
Formbook injected into “vbc.exe”

Analysis – Stage 04 (Formbook)

The last stage is Formbook, which is an infostealer sold as a service (MaaS) on hacking-related forums since 2016. This malware provides many functionalities, such as:

  1. Grabbing keystrokes (Keylogger);
  2. Grabbing screenshots;
  3. Grabbing HTTP(s) forms from network requests;
  4. Stealing data from the clipboard;
  5. Stealing data from common software, such as browsers, email, and ftp clients;
  6. Shutdown/Reboot the OS;
  7. Download and execute additional files;
  8. Remotely execute commands;
  9. Encrypted C2 communication;

The malware is written in ASM/C, and the compilation timestamp seems to be altered, as it indicates it was created in 2003.

Screenshot of binary details of Formbook payload.
Binary details of Formbook payload.

The primary entry point of Formbook is straightforward. Once running, it calls the main function which is named “InjectMaliciousPayload” in this IDA database. Most of the strings are obfuscated using the “Stack Strings” technique, which can be defeated with FLOSS. A list of decoded strings for this sample can be found in our GitHub repository.

Screenshot of Formbook’s primary entry point.
Formbook’s primary entry point.

It then executes a sequence of functions to assess the environment and determine whether it’s going to run, by verifying the presence of blacklisted processes and usernames, for example.

Screenshot of Formbook anti-analysis mechanisms.
Formbook anti-analysis mechanisms.

After the anti-analysis mechanisms, Formbook proceeds by creating and injecting itself into a randomly chosen process from Windows directory. In this case, it is injected into “svchost.exe”.

Screenshot of Formbook injecting itself into another process.
Formbook injecting itself into another process.

Also, another instance is injected into “explorer.exe”, responsible for the C2 communication. We found 65 different domains in this sample, where 64 are only used as decoys.

Screenshot of Formbook trying to connect to domains.
Formbook trying to connect to domains.

The real C2 of this sample is “www.biohackingz[.]one”.

Screenshot of Formbook C2 communication.
Formbook C2 communication.

This domain was first seen on February 21, 2022 on VirusTotal.

Screenshot of analysis of the C2 domain.
Analysis of the C2 domain.

Once the communication is established, Formbook parses the data to determine the action that needs to be taken.

Screenshot of part of the function that parses the C2 response.