Hackers Weaponize MSI Packages & PNG Files to Deliver Multi-stage Malware

Researchers have reported a series of sophisticated cyber attacks aimed at organizations in Chinese-speaking regions, including Hong Kong, Taiwan, and mainland China.

These attacks employ a multi-stage loader known as PNGPlug to deliver a malware payload identified as ValleyRAT.

The attack chain begins with a phishing webpage that entices victims into downloading a malicious Microsoft Installer (MSI) package disguised as legitimate software.

Upon execution, this installer performs two significant actions:

It deploys a benign application to create an illusion of legitimacy.
It extracts an encrypted archive containing the malware payload.

The MSI package utilizes the Windows Installer’s CustomAction feature to execute malicious code.

This includes running an embedded malicious DLL that decrypts the archive (all.zip) using the hardcoded password hello202411, ultimately extracting core malware components such as:

libcef.dll: The loader, padded to inflate its size to 220MB, which helps it evade detection by security tools that often skip analyzing larger files.
down.exe: A legitimate application used to mask the malware’s activities.
aut.png and view.png: Files masquerading as PNG images that contain encoded malicious payloads.

Role of the PNGPlug Loader

According to the Intezer report, the primary function of the PNGPlug loader (libcef.dll) is to establish an environment conducive for malware execution through several steps:

The following table summarizes the key functionalities of the loader, including command-line argument parsing, memory injection capabilities, and anti-virus detection mechanisms.

Functionality	Description
Patching ntdll.dll	Enables memory injection capabilities.
Command-line Argument Parsing	– If the `/aut` argument is present: – Decrypts a registry path using XOR encryption. – Writes the path of `down.exe` to the registry. – Injects the contents of `aut.png` into memory. – If `/aut` is absent: – Executes `down.exe` with the specified argument.
Anti-Virus Detection	– Checks for the presence of specific security software (e.g., 360 Total Security). – If absent, maps `view.png` into memory to create a new process (`colorcpl.exe`), injecting its contents. – This process has been observed executing ValleyRAT malware.

The use of .png file extensions for malicious payloads is a key stealth tactic that inspired the name PNGPlug.

These PNG files contain additional data, specifically PE executables embedded at designated offsets, enhancing their ability to evade detection while executing their payloads.

ValleyRAT Malware Details

ValleyRAT is a sophisticated remote access trojan (RAT) attributed to the Silver Fox APT, known for its espionage and cybercrime activities targeting Chinese-speaking individuals and organizations. The malware employs advanced techniques such as:

Shellcode Execution: Running components directly in memory to minimize file footprints and evade detection.
Obfuscation and Privilege Escalation: Concealing malicious activities while gaining elevated access.
Persistence Mechanisms: Utilizing scheduled tasks and registry modifications to maintain control over infected systems.

The malware’s operational stages include initial execution, deployment of obfuscated shellcode, and a loader module that retrieves additional malicious components from command-and-control (C2) servers.

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