Emotet Malware: How Does it Work and How Can it be Stopped?

By Ofir Yaakobi

Outlining the Emotet infection process:

Datto SaaS Defense has detected Emotet via spam emails arriving at our clients, that contained a malicious docm file.

Emotet Malware: How Does it Work and How Can it be Stopped?

To tempt the unassuming user to enable VBA macro execution, the infected Word document shows a message requesting to ‘enable content’ in the top bar and allows the macro to run. Doing so, the VBA code runs and utilizes cmd.exe to run Powershell code, which sends the HTTP request in an attempt to fetch the Emotet payload.

Emotet Malware: How Does it Work and How Can it be Stopped?

The VBA project contains a ‘Document Open’ function that runs as soon as the victim opens the Office file. The ‘Document Open’ function calls a variable “dgfjalfhkaugwikgfuol3wgnacoi3u5taboi3ut5roai3u5go3wugaolisdrgfso8i7wejwdoljgf” in another function that contains an obfuscated string. There are two variable assignments in this function—the first “s2” contains a list of 7 URLs of compromised websites hosting the Emotet payload and the second “s2” contains the command to download and execute the payload. The commands can be decoded by simply performing a replace on the string “Cew” (as we can see in the screenshot below).

Emotet Malware: How Does it Work and How Can it be Stopped?

After removing the string “Cew” we get the decoded Powershell command that executes -

Emotet Malware: How Does it Work and How Can it be Stopped?

The Infection traffic for Emotet is similar to what we saw before the takedown in January 2021. As of now, only one of the URLs was active.

Emotet Malware: How Does it Work and How Can it be Stopped?

The Emotet DLL was first stored as a random file name with a .dll extension under the C:\ProgramData directory. Then it was moved to a randomly-named directory under the infected user's AppData\Local folder.

Emotet gains persistence by modifying the autorun registry key “HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run” creating a new value with a random name and configuring the Rundll32 to execute the Emotet payload, as shown below. This enables the Emotet payload to be executed with each reboot of the victim host.

At this point of infection, the malware communicates with multiple C2 servers. Investigation of this communication showed HTTPS requests come from various places in the world (18 different IP addresses).

As the last execution step, Emotet waits for commands from the C2 servers.

This DLL has a unique export: “Control_RunDLL”. This makes it possible to detect Emotet infections. If the process rundll32.exe was started with a parameter that matches the export, the system would be infected.

Emotet Malware: How Does it Work and How Can it be Stopped?

Unpacking the malware:

Emotet attempts to hide its malicious code by using a customized packer. Packed programs are a subset of obfuscated programs in which the malicious program is compressed and cannot be analyzed.

Dynamic analysis can be used to unpack it, by letting the malware do the unpacking and grabbing the next stage out of memory. The process will need to allocate memory for the next stage, so it’s a good assumption that we will see a call to VirtualAlloc. After the call to VirtualAlloc, the EDX register (.text section) will contain the base address of the allocated region.

After seeing the memory has been populated, we can dump the memory to a file.

The screenshot below shows the address where the unpacked malware is located as well as the protection level that has been allocated to that area in memory: Read/Write. This is a great indicator that this is the unpacked malware as the memory allocated for the malware needs to be executable and writable.

Emotet Malware: How Does it Work and How Can it be Stopped?

After dumping the file we need to clean it up by looking for the ‘MZ’ signature and removing all of the hexadecimal code appended before it. This gives us the unpacked Emotet malware.

With detection in Datto SaaS Defense, we can see the full process tree starting from a malicious docm file that spawns Powershell via CMD to the rundll32 process execution. SaaS Defense is different from other third-party solutions in that it does not rely on third-party software, signatures, or malware reports. SaaS Defense was designed to analyze the composition of a safe email, chat, or document, making its detection rates and time to detection superior to those in the market today. SaaS Defense stops zero-day threats and their invasion techniques, such as Emotet, at first encounter, without manual interference or end-client disruption.

Don’t let an Emotet attack harm your business or your clients. With Datto SaaS Defense, you can proactively protect your clients’ data with our advanced threat protection solution that detects zero-day threats at the first encounter instead of days later.

Request a demo to discover a better way to secure clients from Emotet.

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