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Introduction
A sophisticated China-linked advanced persistent threat (APT) group has been targeting Indian organizations with a multi-stage phishing campaign that impersonates the Indian Income Tax Department to deliver the ValleyRat remote access trojan. The attack chain, uncovered during a Malware Research Diary session on ANY.RUN, demonstrates how modern APT groups are layering multiple evasion techniques—from spear-phishing and fake government portals to VHDX containers, DLL search order hijacking, and custom RC4-like payload decryption—to bypass traditional security controls and establish persistent remote access.
Learning Objectives
- Understand the complete attack chain of the China-1exus APT campaign targeting India, including the social engineering, delivery mechanisms, and payload execution phases.
- Analyze the technical implementation of DLL search order hijacking, custom RC4-like decryption, and in-memory execution techniques used by the malware.
- Learn how to detect, hunt for, and mitigate this threat using practical commands, threat hunting queries, and defensive strategies across Linux and Windows environments.
You Should Know
- The Multi-Stage Infection Chain: From Phishing Email to ValleyRat Deployment
The attack begins with a spear-phishing email sent to targets at cvcnetworkindia.com, containing a PDF attachment named 2026-0624-03.pdf. Rather than embedding malware directly, the PDF redirects victims to a fraudulent website (hxxps://kjfuwyce[.]love/) that impersonates the official Indian Income Tax Department portal. The fake website presents a tax assessment notice and instructs victims to download Tax_Assessment-T.zip.
The ZIP archive contains a VHDX virtual hard disk image (Tax_Assessment-T.vhdx). When mounted on a Windows system, the VHDX exposes three files:
– `Tax_Assessment.exe` (the legitimate-looking executable)
– `event.dll` (the malicious DLL)
– `event_original.dll` (the legitimate DLL, renamed)
The attackers abuse Windows DLL search order hijacking: when `Tax_Assessment.exe` executes, Windows loads `event.dll` from the current directory before searching system paths, causing the malicious library to load instead of the legitimate one. This technique preserves normal application behavior while achieving code execution.
Step-by-step guide to understanding VHDX mounting and DLL hijacking:
On a Windows system, VHDX files can be mounted natively:
Mount a VHDX file (requires administrator privileges) Mount-VHD -Path "C:\path\to\Tax_Assessment-T.vhdx" After mounting, the volume appears as a new drive letter (e.g., E:) The three files will be visible in the root of the mounted volume To unmount the VHDX after analysis Dismount-VHD -Path "C:\path\to\Tax_Assessment-T.vhdx"
For forensic analysts examining suspicious VHDX files in a safe environment, using a Linux system with `libguestfs-tools` provides a safer approach:
Install libguestfs-tools on Linux sudo apt-get install libguestfs-tools Inspect the VHDX contents without mounting (read-only) guestfish --ro -a Tax_Assessment-T.vhdx -i ls / Alternatively, use qemu to mount and inspect sudo modprobe nbd sudo qemu-1bd -c /dev/nbd0 Tax_Assessment-T.vhdx sudo mount /dev/nbd0p1 /mnt/vhdx ls -la /mnt/vhdx
Detection opportunities:
- Monitor for new VHDX/VHD file creation in user-writable directories
- Detect `Mount-VHD` PowerShell commands or `diskpart` commands involving VHDX
- Look for unusual DLL loads from non-system directories (e.g., `event.dll` loaded from a temporary or download folder)
- Technical Deep Dive: The Custom RC4-Like Decryptor and In-Memory Execution
The loader embedded within `Tax_Assessment.exe` employs multiple defense-evasion techniques that make static analysis challenging:
- Runtime string decryption: Critical strings are encrypted and only decrypted during execution, hindering static signature detection.
- API hashing: Windows API functions are resolved via custom hashing algorithms rather than direct imports, evading import table-based detection.
- Custom RC4-like payload decryption: The encrypted payload embedded in the executable is decrypted using a proprietary algorithm derived from RC4.
- Process injection: The decrypted payload is injected into a newly created process.
- In-memory execution: The payload executes entirely in memory without writing to disk.
Step-by-step guide to analyzing the custom decryptor:
For reverse engineers analyzing the loader, here is a Python implementation that simulates the decryption routine (based on observed behavior):
def custom_rc4_decrypt(encrypted_data, key):
"""
Simulates the custom RC4-like decryption used by the loader.
Note: This is a reconstruction based on observed behavior.
"""
Initialize S-box
s_box = list(range(256))
j = 0
for i in range(256):
j = (j + s_box[bash] + key[i % len(key)]) & 0xFF
s_box[bash], s_box[bash] = s_box[bash], s_box[bash]
Decrypt the data
i = j = 0
decrypted = bytearray()
for byte in encrypted_data:
i = (i + 1) & 0xFF
j = (j + s_box[bash]) & 0xFF
s_box[bash], s_box[bash] = s_box[bash], s_box[bash]
k = s_box[(s_box[bash] + s_box[bash]) & 0xFF]
decrypted.append(byte ^ k)
return bytes(decrypted)
Example usage
encrypted_payload = bytes.fromhex("...")
key = b"ValleyRatKey" Example key (actual key may vary)
decrypted = custom_rc4_decrypt(encrypted_payload, key)
To extract the embedded encrypted payload from the executable:
On Linux, use objdump to find sections objdump -h Tax_Assessment.exe | grep -E ".data|.rdata" Use dd to extract a specific section (adjust offsets based on analysis) dd if=Tax_Assessment.exe of=encrypted_payload.bin bs=1 skip=0x1000 count=0x2000 Alternatively, use pev tools on Linux readpe -S Tax_Assessment.exe On Windows, use Process Monitor to observe the loader's behavior Filter for Process Name contains "Tax_Assessment.exe" Look for WriteProcessMemory and CreateRemoteThread events (indicators of injection)
Detection opportunities:
- Monitor for `WriteProcessMemory` and `CreateRemoteThread` API calls
- Detect anomalous process creation where a legitimate executable spawns a new process with injected code
- Use memory scanning to detect shellcode patterns or known ValleyRat signatures in process memory
3. Dynamic Module Retrieval and C2 Communication
After the initial payload is executed in memory, the malware communicates with its command-and-control (C2) server to retrieve additional encrypted modules. These modules are decrypted and executed dynamically in memory, allowing the threat actor to extend the malware’s capabilities on demand.
The modular architecture provides several advantages to the attacker:
– Reduced on-disk footprint: Only the initial loader is written to disk
– Dynamic capability extension: New functionality can be deployed without re-delivering the malware
– Evasion of signature-based detection: Each module is unique and only exists in memory
Step-by-step guide to analyzing C2 communication:
To hunt for ValleyRat C2 traffic in your environment, use the following approaches:
Network traffic analysis (Zeek/Bro):
Zeek script to detect suspicious HTTP POST requests with encrypted payloads
Add to your Zeek local.bro
event http_request(c: connection, method: string, original_URI: string,
unescaped_URI: string, version: string)
{
if (method == "POST" && /.php$/ in original_URI)
{
print fmt("Suspicious POST request: %s -> %s", c$id$orig_h, original_URI);
}
}
Suricata/Snort rule for ValleyRat C2 detection:
Suricata rule for detecting ValleyRat C2 beaconing
alert http $HOME_NET any -> $EXTERNAL_NET any (
msg:"VALLEYRAT C2 Beacon Detected";
flow:to_server,established;
http.method; content:"POST";
http.uri; content:".php"; depth:10;
http.user_agent; content:"Mozilla/5.0"; nocase;
pcre:"/\/[a-zA-Z0-9]{8,}.php$/";
classtype:trojan-activity;
sid:2026062801;
rev:1;
)
Windows-based threat hunting (Sysmon + PowerShell):
Query Sysmon Event Logs for suspicious network connections
Get-WinEvent -FilterHashtable @{
LogName='Microsoft-Windows-Sysmon/Operational'
ID=3 Network connection events
} | Where-Object {
$_.Message -match "DestinationIp.(?:8.8.8.8|1.1.1.1)" Replace with suspicious IPs
} | Select-Object TimeCreated, Message
Query for suspicious process creation (Event ID 1)
Get-WinEvent -FilterHashtable @{
LogName='Microsoft-Windows-Sysmon/Operational'
ID=1
} | Where-Object {
$_.Message -match "Tax_Assessment|event.dll"
} | Select-Object TimeCreated, Message
YARA rule for ValleyRat detection:
rule ValleyRat_Loader {
meta:
description = "Detects ValleyRat loader based on API hashing and string patterns"
author = "CyberArmor Threat Research"
date = "2026-06-28"
strings:
$hash1 = { 8B 45 ?? 33 C9 8B 55 ?? 0F AF C1 } // API hashing routine
$hash2 = { 56 57 8B 7D ?? 8B F7 33 C0 39 06 }
$string1 = "event_original.dll" wide
$string2 = "Tax_Assessment" wide
condition:
(uint16(0) == 0x5A4D) and (any of ($hash)) and (any of ($string))
}
4. Persistence Mechanisms and Privilege Escalation
The ValleyRat loader establishes registry-based persistence to maintain access across system reboots. While the blog mentions registry-based persistence, additional analysis reveals the malware may also create scheduled tasks or use startup folder entries.
Step-by-step guide to detecting persistence mechanisms:
Check for suspicious registry entries (Windows):
Check common persistence locations
$persistencePaths = @(
"HKLM\Software\Microsoft\Windows\CurrentVersion\Run",
"HKCU\Software\Microsoft\Windows\CurrentVersion\Run",
"HKLM\Software\Microsoft\Windows\CurrentVersion\RunOnce",
"HKCU\Software\Microsoft\Windows\CurrentVersion\RunOnce",
"HKLM\SYSTEM\CurrentControlSet\Services"
)
foreach ($path in $persistencePaths) {
Get-ItemProperty -Path "Registry::$path" -ErrorAction SilentlyContinue
}
Check for suspicious scheduled tasks
Get-ScheduledTask | Where-Object {
$_.TaskName -match "Tax|Assessment|Update|Adobe"
} | Select-Object TaskName, State, Actions
Monitor for new service creation (Linux-based log analysis):
While the malware targets Windows systems, defenders can use Linux-based SIEM tools to analyze Windows event logs:
Using grep to search Windows Event Logs (EVTX) on a Linux analysis machine First, mount the Windows drive or copy EVTX files Use evtx_dump.py (from libevtx) to convert and search evtx_dump.py /mnt/windows/Windows/System32/winevt/Logs/Microsoft-Windows-Sysmon.evtx | \ grep -E "Tax_Assessment|event.dll|CreateProcess|RemoteThread"
5. Defensive Strategies and Hardening Recommendations
Organizations should strengthen defenses against this and similar threats by implementing the following measures:
1. Monitor virtual disk mounting activity:
- Alert on `Mount-VHD` commands from non-administrative users
- Detect VHDX/VHD file downloads and subsequent mounting events
- Restrict VHDX mounting to authorized administrators only
2. Detect DLL search order hijacking:
- Enable Sysmon to log DLL load events (Event ID 7)
- Monitor for DLLs loaded from unusual paths (Downloads, Temp, Recycle Bin)
- Implement Application Control (Windows Defender Application Control or AppLocker) to restrict DLL loading
3. Inspect suspicious PDF-to-download workflows:
- Deploy email filtering solutions to detect and quarantine PDFs with redirects
- Monitor for PDF files that contain external URLs or JavaScript
- Train users to recognize fake government portals
4. Implement behavioral detection:
- Deploy EDR solutions with behavior-based detection for process injection
- Monitor for anomalous `WriteProcessMemory` and `CreateRemoteThread` sequences
- Implement memory scanning for in-memory payloads
5. Network-level defenses:
- Block known malicious domains (e.g.,
kjfuwyce[.]love) - Implement SSL/TLS inspection to detect encrypted C2 traffic
- Use threat intelligence feeds to block C2 IP addresses and domains
Hardening commands (Windows):
Enable Sysmon with a configuration that logs DLL loads Download Sysmon from Microsoft Sysinternals Install with a configuration file sysmon64.exe -accepteula -i sysmon-config.xml Sample sysmon-config.xml should include: <DnsMonitor onmatch="exclude"> <ProcessAccess onmatch="exclude"> <ImageLoad onmatch="include"> <Image condition="end with">.dll</Image> </ImageLoad> Enable PowerShell script block logging for better detection Set-ItemProperty -Path "HKLM\SOFTWARE\Policies\Microsoft\Windows\PowerShell\ScriptBlockLogging" -1ame "EnableScriptBlockLogging" -Value 1 Enable Windows Defender Application Control (WDAC) Create a base policy New-CIPolicy -FilePath .\BasePolicy.xml -Level Publisher -Fallback Hash Convert to binary ConvertFrom-CIPolicy -XmlFilePath .\BasePolicy.xml -BinaryFilePath .\BasePolicy.p7b Deploy the policy Copy-Item .\BasePolicy.p7b C:\Windows\System32\CodeIntegrity\SiPolicy.p7b Reboot to apply
What Undercode Say
Key Takeaway 1: The campaign demonstrates that APT groups are moving beyond simple phishing attachments to complex, multi-stage delivery chains that leverage trusted Windows features like VHDX mounting and DLL search order hijacking to bypass traditional security controls.
Key Takeaway 2: The combination of custom RC4-like encryption, in-memory execution, and dynamically retrieved modules creates significant challenges for signature-based detection, requiring organizations to adopt behavioral and memory-based detection capabilities.
Analysis: This campaign represents a sophisticated evolution in APT tradecraft, combining social engineering with technical evasion at every stage. The use of the Indian Income Tax Department theme demonstrates careful targeting and reconnaissance by the threat actor. The staged delivery approach—using a PDF as a redirector rather than a direct malware carrier—reduces the likelihood of detection by email filters. The VHDX container adds another layer of obfuscation, as virtual disk images are often trusted by security tools. The DLL hijacking technique leverages a fundamental Windows design feature, making it difficult to prevent without application control policies. The custom RC4-like decryptor and in-memory execution make the malware difficult to analyze statically and evade traditional antivirus. Finally, the dynamic module retrieval capability allows the attacker to adapt the malware’s functionality post-compromise, making it a persistent and flexible threat. Defenders must adopt a defense-in-depth strategy that includes email filtering, user training, application control, behavioral monitoring, and threat intelligence to effectively detect and respond to such sophisticated attacks.
Prediction
+1 Expect to see an increase in VHDX-based malware delivery as attackers adopt this technique to bypass email and endpoint security controls. The use of virtual disk images as attack vectors will likely become more prevalent across various threat actor groups.
-1 The sophistication of this campaign indicates that the China-1exus APT group has significant resources and operational security, suggesting that this is not an isolated incident but part of a broader, sustained espionage effort targeting Indian organizations.
+1 The disclosure of this campaign and the accompanying technical analysis will empower the defensive community to develop better detection rules and hunting strategies, potentially disrupting future attacks using similar techniques.
-1 The modular architecture and dynamic module retrieval capability make this threat particularly dangerous, as the malware can be updated and extended without re-delivery, allowing the attackers to maintain persistence and adapt to defensive measures.
+1 Increased awareness of DLL search order hijacking will likely lead to more organizations implementing application control policies and monitoring for anomalous DLL loads, reducing the effectiveness of this technique over time.
-1 The use of legitimate Windows features (VHDX mounting, DLL search order) for malicious purposes highlights the challenge of distinguishing between legitimate and malicious activity, potentially leading to increased false positives and alert fatigue for security teams.
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