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Introduction
The cyber threat landscape has witnessed a significant escalation as FishMonger APT—a Chinese-aligned cyberespionage group operating under the Winnti umbrella—has ported its notorious SprySOCKS backdoor from Linux to Windows, introducing two previously undocumented variants with unprecedented stealth capabilities. This cross-platform expansion, active between 2023 and 2024 with victims across Honduras, Taiwan, Thailand, and Pakistan, represents a meaningful enhancement of the group’s operational arsenal, particularly targeting government organizations. The most concerning development is the WIN_DRV variant, which weaponizes a kernel driver to function as a rootkit, hiding network connections, processes, files, and registry keys from administrators and security products alike.
Learning Objectives
- Understand the technical architecture and infection chains of both WIN_DRV and WIN_PLUS SprySOCKS Windows variants
- Master detection and hunting techniques for kernel-level rootkit activity, including driver analysis and API hook detection
- Learn to implement defensive measures against print processor abuse, DLL side-loading, and C2 communication over TCP, UDP, and WebSocket protocols
You Should Know
- Understanding the SprySOCKS Windows Variants: WIN_DRV vs. WIN_PLUS
FishMonger APT developed two distinct Windows versions of SprySOCKS, both based on version 1.8 of the original Linux backdoor and retaining the same C&C message format, encryption keys, algorithms, and statically linked HP-Socket networking library.
WIN_DRV represents the advanced iteration, incorporating kernel drivers for rootkit-like stealth. The DriverLoader malware (stored as fsdiskbit.sys) decrypts and loads the RawWNPF kernel driver directly into memory. This driver, signed with a leaked certificate from the GitHub PastDSE project, enables the malware to:
– Hide processes via Windows API manipulation
– Conceal active network connections
– Mask malicious files from directory listings
– Obfuscate registry keys used for persistence
– Divert TCP traffic to the backdoor’s hidden listening port
WIN_PLUS is the driver-less variant that relies on Windows print processor abuse for execution and stealth, making it slightly less sophisticated but still highly capable.
Detection Strategy:
For Windows systems, hunt for the following indicators:
Check for unsigned or suspicious kernel drivers
Get-WindowsDriver -Online | Where-Object {$<em>.ProviderName -like "PastDSE" -or $</em>.Driver -like "RawWNPF"}
List recently loaded kernel drivers
Get-WinEvent -LogName "System" | Where-Object { $_.Id -eq 7045 } | Select-Object TimeCreated, Message
Check for Print Processor persistence (WIN_PLUS)
Get-ItemProperty -Path "HKLM:\SYSTEM\CurrentControlSet\Control\Print\Printers\" | Select-Object Name, DriverName
On Linux systems (original SprySOCKS target), hunt for anomalous processes and network connections:
Check for hidden processes using sysdig sudo sysdig -c topprocs_cpu Examine listening ports and associated processes sudo ss -tulpn | grep -v "127.0.0.1" Check for unusual kernel modules lsmod | grep -v "^Module"
2. WIN_DRV Infection Chain: From Deployment to Rootkit
The WIN_DRV variant employs a multi-stage infection chain designed for maximum stealth and persistence.
Step-by-Step Infection Process:
- Deployment: The malware package is deployed on the victim system, likely through exploitation of public-facing server vulnerabilities.
-
File Placement: A batch script copies malicious components into
%SystemRoot%\Fonts, a directory often overlooked by security tools. -
Persistence Creation: A scheduled task is created to ensure the malware survives reboots.
-
DLL Side-Loading: A legitimate signed executable is abused to load a malicious DLL, bypassing application whitelisting controls.
-
Backdoor Loading: The malicious DLL loads the SprySOCKS backdoor into memory.
-
Process Injection: The backdoor shellcode is injected into
svchost.exe, a trusted Windows process. -
Kernel Driver Loading: DriverLoader decrypts and loads the RawWNPF kernel driver.
-
Stealth Activation: RawWNPF hides malware activity and enables stealthy TCP traffic diversion, allowing operators to send commands through a random TCP port without exposing the real listening port.
Hunting Commands:
Check for suspicious scheduled tasks
Get-ScheduledTask | Where-Object {$<em>.TaskPath -like "Fonts" -or $</em>.Actions -like ".bat"}
Enumerate DLLs loaded by svchost.exe
Get-Process -1ame svchost | ForEach-Object { $_.Modules } | Select-Object ModuleName, FileName
Check for Image File Execution Options (IFEO) hijacking
Get-ItemProperty -Path "HKLM:\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Image File Execution Options\" | Select-Object PSChildName, Debugger
Monitor for driver load events
Get-WinEvent -LogName "System" | Where-Object { $<em>.Id -eq 7045 -and $</em>.Message -like ".sys" } | Select-Object TimeCreated, Message
3. WIN_PLUS Infection Chain: Print Processor Abuse
WIN_PLUS takes a different approach to stealth, abusing Windows print processors—a lesser-known persistence mechanism.
Step-by-Step Infection Process:
- Deployment: Malicious components are placed on the victim system.
-
Print Processor Installation: `VSPMsg.dll` is registered as a Windows print processor, a technique that provides persistence without raising immediate suspicion.
-
Spoolsv.exe Execution: The Print Spooler service (
spoolsv.exe) executes the malicious print processor, granting the malware SYSTEM-level privileges. -
Loader Decryption: The first-stage loader decrypts the SprySOCKS loader from
config.dat. -
Process Injection: The loader injects SprySOCKS into
svchost.exe. -
Mutex Creation: The backdoor creates a mutex to prevent duplicate execution.
-
Final Decryption: The final backdoor payload is decrypted from an encrypted container.
-
Secondary Injection: The backdoor is injected into another `svchost.exe` process.
-
C2 Connection: SprySOCKS establishes communication with its hardcoded C2 infrastructure.
Detection Commands:
List all registered print processors
Get-ChildItem -Path "HKLM:\SYSTEM\CurrentControlSet\Control\Print\Environments\Windows x64\Print Processors" | ForEach-Object { Get-ItemProperty $_.PSPath }
Check for suspicious DLLs in spoolsv.exe
Get-Process -1ame spoolsv | ForEach-Object { $_.Modules } | Select-Object ModuleName, FileName
Monitor spoolsv.exe for unusual child processes
Get-WinEvent -LogName "Security" | Where-Object { $<em>.Id -eq 4688 -and $</em>.Message -like "spoolsv.exe" }
4. C2 Communication and Command Set
Both WIN_DRV and WIN_PLUS variants support communication over TCP, UDP, and WebSocket protocols, incorporating hardcoded C&C configurations. The backdoor implements over 30 commands for comprehensive remote control:
Capabilities Include:
- System information collection
- Process enumeration and management
- Service management (start, stop, modify)
- File operations (list, create, delete, upload, download, copy, rename, execute)
- SOCKS proxy functionality
- Keystroke logging
- Clipboard content capture
- Active window title monitoring
- TCP traffic diversion (WIN_DRV only)
Network Analysis Commands:
On Linux (for original SprySOCKS variants) sudo tcpdump -i any -1 'tcp port 443 or udp port 53' -v On Windows, monitor for unusual outbound connections netstat -ano | findstr ESTABLISHED Check for WebSocket traffic (port 80/443 with unusual headers) Use Wireshark filter: http or websocket
C2 Detection Using Zeek (Bro):
Detect potential SprySOCKS C2 traffic patterns
zeek -r capture.pcap 'http.request && /.(php|asp|aspx)\?[a-z0-9]{16,}/'
5. UEFI Bootkit Threat and CVE-2023-24932
ESET telemetry indicates limited indications that some SprySOCKS attack scenarios may involve a UEFI bootkit component, possibly exploiting CVE-2023-24932, a Secure Boot flaw previously exploited by the BlackLotus UEFI malware.
Mitigation Steps:
Check Secure Boot status Confirm-SecureBootUEFI Verify UEFI firmware integrity (requires manufacturer tools) Example for Dell systems: Get-CimInstance -1amespace root\dcim\sysman -ClassName DCIM_BIOSService | Invoke-CimMethod -MethodName GetBIOS Apply Microsoft's CVE-2023-24932 mitigation (requires careful testing) https://support.microsoft.com/en-us/topic/kb5025885-how-to-manage-the-windows-boot-manager-revocations-for-secure-boot-changes-associated-with-cve-2023-24932-41a975df-beb2-40c1-99a3-b3ff139f832d
Linux UEFI Bootkit Detection:
Check for UEFI firmware modifications sudo efibootmgr -v Examine UEFI variables for anomalies sudo ls -la /sys/firmware/efi/efivars/ Verify kernel signatures sudo mokutil --list-enrolled
6. Defense and Mitigation Strategies
Organizations should implement the following defensive measures against SprySOCKS Windows variants:
Endpoint Detection and Response (EDR) Tuning:
- Monitor for `svchost.exe` process injection attempts
- Alert on unsigned kernel driver loads
- Detect suspicious print processor registrations
- Flag DLL side-loading from `%SystemRoot%\Fonts`
Network Defenses:
- Implement egress filtering to block unknown C2 destinations
- Deploy SSL/TLS inspection to detect encrypted C2 traffic
- Monitor for WebSocket connections to suspicious domains
System Hardening Commands:
Disable Print Spooler if not required Stop-Service -1ame Spooler -Force Set-Service -1ame Spooler -StartupType Disabled Restrict DLL loading to known paths Enable DLL search order mitigation Set-ItemProperty -Path "HKLM:\SYSTEM\CurrentControlSet\Control\Session Manager" -1ame "SafeDllSearchMode" -Value 1 Enable Windows Defender Application Control (WDAC) Create a base policy New-CIPolicy -FilePath C:\WDAC\BasePolicy.xml -UserPEs Enforce driver signing Set-ItemProperty -Path "HKLM:\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System" -1ame "EnableDriverSigning" -Value 1
Linux-Specific Hardening (for cross-platform environments):
Restrict kernel module loading echo "install /sbin/modprobe --deny" > /etc/modprobe.d/deny-all.conf Enable SELinux/AppArmor in enforcing mode sudo setenforce 1 Monitor for unusual scheduled tasks (cron) sudo cat /etc/crontab /etc/cron./ | grep -v "^"
What Undercode Say
- Cross-Platform Expansion Signals Strategic Shift: FishMonger’s porting of SprySOCKS from Linux to Windows demonstrates a deliberate strategy to broaden its target surface, enabling the group to compromise mixed-OS environments common in government and enterprise networks. This is not a simple code recompile—it represents a significant investment in Windows-specific stealth capabilities.
-
Kernel-Level Stealth Raises the Bar: The WIN_DRV variant’s use of a kernel driver for rootkit functionality marks a concerning evolution in APT tradecraft. By hiding at the kernel level, the malware can evade traditional EDR solutions that operate in user mode, forcing defenders to adopt memory forensics and kernel-mode monitoring.
The discovery of Windows SprySOCKS variants, active since 2023-2024, reveals that FishMonger has been operating undetected in Windows environments for over two years. The group’s ability to maintain operational security while deploying kernel-level rootkits suggests a mature, well-resourced adversary. The use of a leaked certificate from the GitHub PastDSE project to sign the DriverLoader component indicates that the group is leveraging publicly available tools and techniques, making attribution more challenging while lowering the barrier to entry for other threat actors.
The combination of print processor abuse, DLL side-loading, and kernel driver stealth across two distinct variants demonstrates a deep understanding of Windows internals. Defenders must now assume that FishMonger has a permanent foothold in Windows environments and adjust their threat hunting accordingly.
Prediction
- +1 The discovery of Windows SprySOCKS variants will accelerate the development of kernel-mode detection frameworks and memory forensics tools, ultimately improving the security community’s ability to detect rootkit-level threats across all platforms.
-
-1 FishMonger’s cross-platform capability will likely inspire copycat APT groups to port their Linux toolkits to Windows, leading to a surge in multi-OS malware campaigns targeting government and enterprise networks.
-
-1 The use of kernel drivers signed with leaked certificates will become a more prevalent technique among APT groups, as the barrier to entry for rootkit development continues to decrease with publicly available tools like PastDSE.
-
+1 Increased awareness of print processor abuse as a persistence mechanism will prompt Microsoft to implement additional security controls around the Print Spooler service, potentially closing this attack vector in future Windows releases.
-
-1 The potential integration of UEFI bootkit components with SprySOCKS suggests that FishMonger is moving toward firmware-level persistence, which would make detection and removal exponentially more difficult for defenders.
-
+1 Organizations that implement comprehensive EDR solutions with kernel-mode monitoring capabilities will be better positioned to detect and respond to WIN_DRV-style attacks, driving innovation in the endpoint security market.
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