Security researchers found a remote access Trojan (RAT) that has been hiding for nearly two years. It uses novel techniques to do its malfeasance: Malicious macros in Excel documents will compile embedded C Sharp (C#) source code into an executable file that actually downloads the RAT.

Starting With the Carp Downloader

Palo Alto Networks named Carp Downloader as the malicious document payload in question. The name stems from how its originators make use of a specific technique of compiling and executing embedded C# — or C shARP.

Carp generates two paths, and one is to a randomly named executable and C# file in the %APPDATA%\\Microsoft folder. Next, it base64-decodes the embedded C# source code in the Carp Downloader and writes it to the previously generated C# file path. The payload then downloads a file on port 443. In a final routine, it will decrypt and run the downloaded file using AES-128.

The Trojan has survived undetected for so long due to a low instance of infections. The researchers found only 27 total samples extending to December 2015.

About the Remote Access Trojan

Researchers named the RAT Cardinal, based on internal notations that were contained within the Microsoft .NET Framework executables. When launched, Cardinal checks the current working directory. If it doesn’t find what it expects, there is an installation routine or two.

Once installed, it attempts to inject itself into a process. It will attempt to use an installed executable for the newly spawned process, selecting from a list that includes cvtres.exe, vbc.exe, AppLaunch.exe, csc.exe, RegSvs.exe and RegAsm.exe.

Cardinal RAT will then parse the current configuration and try to connect to the command-and-control (C&C) server. There are two pieces of data that will be sent here: First is a DWORD that specifies the data length, and the second is the data itself, which is transmitted and hidden.

The RAT will use its own scheme to encrypt the data. First, it leverages a series of XOR and addition operations, followed by decompression using the ZLIB library. This transmittal method is the way the RAT’s stolen files would be exfiltrated.

A Major Threat

SecurityWeek said the RAT collects victim information, updates itself and manipulates settings, becomes a reverse proxy, executes commands and even uninstalls itself. Additionally, it can obtain and launch new files, keylog, take screen shots and erase browser cookies.

Ultimately, RAT is a major problem if it hits. So far, the low known incidence of this specific remote access Trojan has kept it on the back burner. But given how it uses a novel method in the downloader’s construction, security professionals must expect other cybercriminals to take advantage of this same technique as they develop new malware.

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