Hackers Exploit Legitimate Microsoft Utility to Deliver Malicious DLL Payload

Hackers are now exploiting a legitimate Microsoft utility, mavinject.exe, to inject malicious DLLs into unsuspecting systems.

This utility, intended for injecting DLLs in Application Virtualization (App-V) environments, has become a tool of choice for cyber attackers due to its signed nature by Microsoft, which makes it appear benign to security systems.

The Mechanism of Exploitation

mavinject.exe facilitates DLL injection into running processes through the use of several Windows APIs, including OpenProcess, VirtualAllocEx, WriteProcessMemory, and CreateRemoteThread.

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According to the Report, this sequence of operations allows attackers to execute malicious code within trusted application contexts, typically avoiding detection due to its trusted status.

Here’s how it functions:

• OpenProcess retrieves a handle to the target process, enabling manipulation with necessary permissions.
• VirtualAllocEx allocates memory in the target process’s virtual address space for the DLL.
• WriteProcessMemory copies the DLL’s path into the allocated memory, setting the stage for loading.
• CreateRemoteThread initiates a thread in the target process that triggers the LoadLibraryW function, loading and executing the DLL.

Real-World Attack Scenarios

Two notable cases illustrate the severity of this exploitation:

• Case 1: Earth Preta (Mustang Panda) – This Chinese government-supported APT group has been reported by Trend Micro to use mavinject.exe for injecting malicious DLLs into normal processes like waitfor.exe. The attackers gain initial access through phishing, distributing a seemingly legitimate file which then leverages mavinject.exe to inject a backdoor, allowing communication with a Command and Control (C2) server undetected.

• Case 2: Lazarus Group – Known for their sophisticated attacks, Lazarus employs mavinject.exe to inject malware into explorer.exe. This method exploits the process’s benign reputation with security tools, making it an ideal vector for hiding malicious activities under the guise of a legitimate operation.

Identifying and neutralizing these threats requires careful monitoring:

• Detection: Security tools should watch for unusual API calls associated with mavinject.exe execution, particularly the sequence used in DLL injection.
• Response Measures: When not using App-V, blocking mavinject.exe can be a policy. Furthermore, establishing rules to detect and log DLL injections, along with regular audits for abnormal DLL behavior, can significantly enhance security.

The exploitation of mavinject.exe underscores the double-edged nature of system utilities.

While they serve legitimate purposes, their capabilities can be turned against users by threat actors.

Security professionals must remain vigilant, recognizing that even trusted system components can be weaponized in an attack, highlighting the need for comprehensive monitoring and strategic threat detection mechanisms.

This case serves as a stark reminder of the importance of understanding attack vectors and the continuous evolution of cybersecurity threats.

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