New Encrypted Downloader Delivers Metasploit Backdoor

A series of cyber-attacks targeting the Middle Eastern region use an encrypted downloader to deliver a Metasploit backdoor, AlienVault reports.

The attacks start with a malicious document containing parts of an article about the next Shanghai Cooperation Organization Summit, originally published at the end of May on a Middle Eastern news network.

The Office document contains malicious macro code designed to execute a Visual Basic script (stored as a hexadecimal stream) and launch a new task in a hidden Powershell console. This attack stage is meant to serve a .NET downloader that uses a custom encryption method to obfuscate process memory and evade antivirus detection.

Dubbed GZipDe, the downloader appears based on a publicly available reverse-tcp payload to which the malware author added a new layer of encryption payload.

“It consists of a Base64 string, named GZipDe, which is zip-compressed and custom-encrypted with a symmetric key algorithm, likely to avoid antivirus detection,” AlienVault reveals.

A new memory page with execute, read and write privileges is created, then a decrypted payload is executed. Courtesy of a special handler that controls process’ access to system resources, only one instance of the malware can run at the same time.

Shellcode in the downloader connects to a server at 175.194.42[.]8 to deliver the final payload. The server wasn’t up during analysis, but it was previously recorded serving a Metasploit payload, the security researchers note.

Metasploit has become a popular choice among threat actors, and was previously seen being used in targeted attacks associated with the Turla hackers.

The Metasploit payload delivered from 175.194.42[.]8, AlienVault says, contains a shellcode to bypass system detection, as well as a Meterpreter payload. This malicious program is a powerful backdoor capable of gathering information from the system. The malware also stays in contact with the command and control server to receive further commands.

The shellcode, the researchers explain, loads the entire DLL into memory, meaning that it works without writing information to the disk.

Called reflective DLL injection, this technique allows the attacker to “transmit any other payload in order to acquire elevated privileges and move within the local network,” AlienVault concludes.

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