Patchwork APT, Backdoor

Key takeaways

  • Cyble Research and Intelligence Labs (CRIL) recently encountered an ongoing campaign associated with the Patchwork APT group, which is likely aimed at Chinese entities.

  • This campaign continues the trend of the Patchwork APT group, which has previously targeted entities in China and Bhutan.

  • The threat actors (TAs) have utilized a malicious LNK file, likely originating from a phishing email, as the initial infection vector. This file executes a PowerShell script that downloads two files: a seemingly innocuous PDF intended to lure the user and a malicious Dynamic Link Library (DLL).

  • This campaign employs DLL sideloading techniques to execute the downloaded DLL using the legitimate system file “WerFaultSecure.exe,” thereby obfuscating malicious activity.

  • The loaded DLL decrypts and executes shellcode that modifies the AMSIscanBuffer and ETWEventWrite APIs. This manipulation aims to evade detection mechanisms, allowing the malware to operate stealthily within the compromised system.

  • The shellcode is subsequently used to decrypt and execute the final payload, stealing sensitive information from the victim’s machine.

Overview

Patchwork, also known as Dropping Elephant, is a highly active advanced persistent threat (APT) group that has been engaged in cyber espionage operations since 2009. Believed to be based in India, this group primarily targets high-profile organizations such as government, defense, and diplomatic entities across South and Southeast Asia.

Cyble Research and Intelligence Labs (CRIL) has been closely monitoring the activities of the Patchwork APT group since July 2024. On July 24, 2024, CRIL observed a campaign related to Patchwork APT. By pivoting through the pattern of files, CRIL observed several files associated with two major Patchwork APT campaigns: the first targeting Bhutan and the second targeting Chinese entities.

Campaign Targeting China

This campaign involves a malicious LNK file titled “COMAC_Technology_Innovation.pdf.lnk,” which references the Commercial Aircraft Corporation of China and specifically targets Chinese entities. This lure capitalizes on the 7th COMAC International Science and Technology Innovation Week, with TAs leveraging this event to focus on organizations in the aerospace, technology research, and government sectors, thereby increasing the success rate of their phishing campaign. Researchers from Aliyun have analyzed this campaign and published their findings in a blog post detailing the tactics used by Patchwork.

Campaign targeting Bhutan

Another notable campaign by this group observed in the same month targeted Bhutan with a file named ‘Large_Innovation_Project_for_Bhutan.pdf.lnk.’ This decoy document features a project proposal for Bhutan from the Adaptation Fund Board.

Ongoing Campaign

Among these, a newly identified LNK file, “186523-pdf.lnk”, appears to be linked to an ongoing campaign of the Patchwork group. This same sample was also shared by researcher Ginkgo and StrikeReady Labs on X (formerly Twitter).

When the malicious LNK file gets executed, this file downloads two components: a lure PDF and a malicious DLL containing encrypted shellcode. Additionally, it copies a system file from the victim’s machine, which is then leveraged to sideload the malicious DLL. This DLL then decrypts and executes the final payload directly in memory. The malware collects system information, such as the Process ID, public and private IP addresses, usernames, and more. Then, it transmits this data to the command and control (C&C) server, enabling further malicious activities, as shown in the image below.

This variant seems to be new compared to the payloads observed in previous campaigns. For tracking purposes, we are naming the malware “Nexe” Backdoor, as the string “Nexe” was found hardcoded in the binary used for C&C communication.

Figure 1 – Infection chain

Notably, this campaign needs specific targets, as the lure consists of a plain, empty PDF. However, the names of the payload servers used in this campaign, such as shianchi[.]scapematic.info and jihang[.]scapematic.info suggests that Chinese entities are likely being targeted. Typically, the Patchwork group’s payload server names are associated with the country they are focusing on.

Technical Details

The LNK file, disguised as a PDF, contains a PowerShell script that carries out several malicious actions. The image below shows its contents.

Figure 2 – LNK file content

The script first uses an “Invoke-WebRequest” command to download a file from the URL “hxxps://jihang[.]scapematic[.]info/eqhgrh/uybvjxosg” and saves it as a PDF in the “C:ProgramData” directory. This PDF file appears to be the lure document, but in this case, it contains no content and is simply a plain, empty PDF.

Next, the script downloads another file from a different URL on the same domain, “hxxps://shianchi[.]scapematic[.]info/jhgfd/jkhxvcf,” saving it initially as “hal” in the “C:ProgramData” directory. It then renames the file to “wer.dll” in the same location.

The script proceeds to copy the Windows system file “WerFaultSecure.exe” from “C:WindowsSystem32” to “C:ProgramData”, likely to facilitate DLL sideloading. The image below shows the downloaded files on the victim’s machine.

Figure 3 – Downloaded files

Finally, it creates a scheduled task named “EdgeUpdate” to run “WerFaultSecure.exe” at regular intervals, ensuring persistence on the compromised system. The image below shows the scheduled task created on the system.

Figure 4 – Task Scheduler to execute the WerFaultSecure.exe

Figure 4 – Task Scheduler to execute the WerFaultSecure.exe

DLL Sideloading

Threat actors leveraged the DLL sideloading technique to load the malicious DLL file using the legitimate WerFaultSecure.exe, as shown in the image below.

Figure 5 – DLL sideloading

After the DLL is successfully loaded, it decrypts the encrypted shellcode within it and writes the decrypted content into the memory of the WerFaultSecure process, as shown in the image below.

Figure 6 – Writing decrypted content in WerFaultSecure address space

Bypassing Security Mechanisms via Memory Patching

The injected shellcode is crafted to circumvent AMSI and Microsoft’s event tracking systems by patching specific bytes in the EtwEventWrite, AmsiScanString, and AmsiScanBuffer APIs, as shown in the images below.

Figure 7 – Patching Security Tool APIs

Once the shellcode overwrites these APIs, it creates a section object from the previously decrypted content and maps it into the address space of WerFaultSecure. This allows the final VC++ compiled payload to execute without triggering any security alerts.

Final Payload

Once the payload is successfully loaded into memory, it utilizes the LoadLibraryW() API to load the necessary modules for execution, as shown in the image below.

Figure 8 – Loading required modules for execution

After loading the necessary modules, the malware creates a mutex named “dsds” to ensure that only one instance of the malware runs on the victim’s system at a time, as shown in the figure below.

Figure 9 – Mutex Creation

After creating the mutex, the malware retrieves a handle to the console window associated with the calling process. It then hides the console window and continues running in the background.

Figure 10 - Hides the console window
Figure 10 – Hiding the console window

The malware then utilizes the GetAdaptersInfo() and GetHostName() functions to collect information about the network adapters and the device name on the compromised machine, as shown in the image below.

Figure 11 – Fetching system network adapter details

The malware queries https://myexternalip.com/raw using a specific user agent to obtain the victim’s public IP address, as demonstrated in the image below.

Figure 12 – Malware retrieving public IP

After gathering key system details, including the MAC address, username, and IP address, the malware computes the SHA256 hash for these values before further encryption, as shown in the image below.

Figure 13 – Generating SHA256

After generating the hash, the malware encodes it into Base64 format. The resulting data then enters another encryption loop using the Salsa20 algorithm, which represents a change from the previous encryption method used in prior campaigns. This is followed by an additional round of Base64 encoding. The figure below shows the encryption code with key and nonce.

Figure 14 – Salsa20 Encryption

In addition to the previously mentioned details, including the MAC address, username, and IP address, the malware also retrieves and encrypts the following information using the same sequence: it first converts the data into Base64 format, then applies the Salsa20 encryption algorithm and finally encodes it again in Base64:

  • Process ID

  • Local IP address

  • Windows version

  • Username

  • Hardcoded user-agent string

Each piece of encrypted system information is concatenated and separated by the “$” symbol. The image below displays the encrypted system information.

Figure 15 – Encrypted System information

The encrypted data corresponds to the following fields:

  • MAC address $ username  $ public IP address $ private IP address $ Windows version $ username $ Process ID $ Nexe (hardcoded string) $ User-agent string

Using the final generated string, the malware initiates an HTTP request to a hardcoded domain, “iceandfire[.]xyz,” which is embedded in the code, as illustrated in the image below.

Figure 16 – Sending request

After constructing the HTTP request, the malware transmits encrypted data to its C&C server. However, since the C&C was not active during the analysis, we couldn’t fully assess its behavior. Despite this, following the POST request, the malware creates two threads capable of performing various tasks, as shown in the image below.

Figure 17 – Creation of Threads

The thread extracts partial content from the initially generated string, which includes the encrypted MAC address, username, and public IP address of the victim’s machine, and attempts to send this data to the same domain.

Figure 18 – Send request

The threads read the server’s response following the request and then compare the response with the following values:

  • upload

  • uplexe

  • download

  • filelist

  • screenshot

This comparison helps the thread to determine the actions or commands that it should execute in the system.

Conclusion

The ongoing evolution and enhancement of the Patchwork APT group’s malware capabilities highlight their commitment to remaining at the forefront of espionage and cyber operations. The latest attack exemplifies their ability to evade security alerts and execute malicious files directly in memory, showcasing a sophisticated approach that underscores their adaptability and resourcefulness in the ever-changing landscape of cybersecurity threats. This adaptability not only enables them to bypass traditional defenses but also poses significant challenges for organizations seeking to protect themselves from such advanced tactics.

Recommendations

  • The initial breach may occur via spam emails. Therefore, it’s advisable to deploy strong email filtering systems to identify and prevent the dissemination of harmful attachments.  

  • When handling email attachments or links, particularly those from unknown senders, exercising caution is crucial. Verify the sender’s identity, particularly if an email seems suspicious.  

  • Consider disabling or limiting the execution of scripting languages on user workstations and servers if they are not essential for legitimate purposes. 

  • Restrict the execution of WerFaultSecure.exe to its designated location to prevent unauthorized execution from other directories.

  • Use a reputed anti-virus and internet security software package on your connected devices, including PC, laptop, and mobile.

  • Monitor the beacon on the network level to block data exfiltration by malware or TAs.

MITRE ATT&CK® Techniques

Tactic  Technique ID  Technique Name 
Initial Access (TA0027) Phishing (T1660) Malware distribution via phishing site
Execution  (TA0002)  User Execution (T1204) Manual execution by the user
Defense  Evasion  (TA0005) Masquerading (T1036.008) LNK file disguised as a legitimate PDF file  
Privilege  
Escalation 
(TA0004) 		 DLL Side-Loading (T1574.002)  Adversaries may execute their own  malicious payloads by side-loading DLLs.
Privilege  
Escalation 
(TA0004) 		 T1055 Process Injection
Discovery  
(TA0007) 		 System Information  Discovery (T1082) Queries the system information 
C&C 
(TA0011) 		 Application Layer Protocol 
(T1071) 		 Malware exe communicate to C&C server. 
Exfiltration (TA0010) Exfiltration Over C2 Channel (T1041)   Exfiltration Over C2 Channel 

Indicators of Compromise (IOCs)

Indicators    Indicator  
Type  		 Description  
d7b278d20f47203da07c33f646844e74cb690ed802f2ba27a74e216368df7db9 SHA256 Malicious LNK file
ba262c587f1f5df7c2ab763434ef80785c5b51cac861774bf66d579368b56e31 SHA256 Malicious DLL file
fe503708d7969e65e9437b56b6559bc9b6bb7f46f3be5022db9406579592670d SHA256 Decoy PDF
f6d171e79e2fb38b3919011835c8117a1c56788bcf634e69ae67a5e255fb9d58 14bbe421abe496531f4c63b16881eee23fb2c92b2938335dca1668206882201a c3805b8b37eb1ba34057cd6c882dc9bedcebc01ec90a6d4be8d0f6fc82859ecb SHA256 Lnk used to target Bhutan
c6398b5ca98e0da75c7d1ec937507640037ce3f3c66e074c50a680395ecf5eae SHA256 Lnk targeting Chinese entities
hxxps://shianchi[.]scapematic[.]info/jhgfd/jkhxvcf hxxps://jihang[.]scapematic[.]info/eqhgrh/uybvjxosg URL remote server
Iceandfire[.]xyz Domain C&C Server

Yara Rule

rule Nexe_Backdoor

{

  meta:

    author = “Cyble Research and Intelligence Labs”

    description = “Detects Malicious Backdoor used in the latest Patchwork APTcampaign”

    date = “2024-09-26”

    os = “Windows”

    reference_sample = “ba262c587f1f5df7c2ab763434ef80785c5b51cac861774bf66d579368b56e3”

  strings:

    $a = “WerSysprepCleanup”

    $b = “WerpSetReportFlags”

    $c = “WriteProcessMemory”

    $d = “VirtualAllocEx”

    $e = “Release\AESC.pdb” 

  condition:

    uint16(0) == 0x5A4D and all of them

}

References

https://medium.com/@knownsec404team/the-patchwork-group-has-updated-its-arsenal-launching-attacks-for-the-first-time-using-brute-ratel-175741987d87

https://xz-aliyun-com.translate.goog/t/15376?_x_tr_sl=zh-CN&_x_tr_tl=en&_x_tr_hl=en&_x_tr_pto=sc&u_atoken=0ce0739e487564fbf9e5b5ed29c0687a&u_asig=1a0c384b17265708412575151e0042&decode__2803=eqIxcD0DBD9Q0%3DXxGNne4mhOzdD%3D3hKH4D

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