Emotet, the notorious trojan behind a number of botnet-driven spam campaigns and ransomware attacks, has found a new attack vector: using already infected devices to identify new victims that are connected to nearby Wi-Fi networks.

According to researchers at Binary Defense, the newly discovered Emotet sample leverages a “Wi-Fi spreader” module to scan Wi-Fi networks, and then attempts to infect devices that are connected to them.

The cybersecurity firm said the Wi-Fi spreader has a timestamp of April 16, 2018, indicating the spreading behavior has been running “unnoticed” for close to two years until it was detected for the first time last month.

The development marks an escalation of Emotet’s capabilities, as networks in close physical proximity to the original victim are now susceptible to infection.

How Does Emotet’s Wi-Fi Spreader Module Work?

The updated version of the malware works by leveraging an already compromised host to list all the nearby Wi-Fi networks. To do so, it makes use of the wlanAPI interface to extract the SSID, signal strength, the authentication method (WPA, WPA2, or WEP), and mode of encryption used to secure passwords.

On obtaining the information for each network this way, the worm attempts to connect to the networks by performing a brute-force attack using passwords obtained from one of two internal password lists. Provided the connection fails, it moves to the next password in the list. It’s not immediately clear how this list of passwords was put together.

But if the operation succeeds, the malware connects the compromised system on the newly-accessed network and begins enumerating all non-hidden shares. It then carries out a second round of brute-force attack to guess the usernames and passwords of all users connected to the network resource.

After having successfully brute-forced users and their passwords, the worm moves to the next phase by installing malicious payloads — called “service.exe” — on the newly infected remote systems. To cloak its behavior, the payload is installed as a Windows Defender System Service (WinDefService).

In addition to communicating with a command-and-control (C2) server, the service acts as a dropper and executes the Emotet binary on the infected host.

The fact that Emotet can jump from one Wi-Fi network to the other puts onus on companies to secure their networks with strong passwords to prevent unauthorized access. The malware can also be detected by actively monitoring processes running from temporary folders and user profile application data folders.

Emotet: From Banking Trojan to Malware Loader

Emotet, which was first identified in 2014, has morphed from its original roots as a banking Trojan to a “Swiss Army knife” that can serve as a downloader, information stealer, and spambot depending on how it’s deployed.

Over the years, it has also been an effective delivery mechanism for ransomware. Lake City’s IT network was crippled last June after an employee inadvertently opened a suspicious email that downloaded the Emotet Trojan, which in turn downloaded TrickBot trojan and Ryuk ransomware.

Although Emotet-driven campaigns largely disappeared throughout the summer of 2019, it made a comeback in September via “geographically-targeted emails with local-language lures and brands, often financial in theme, and using malicious document attachments or links to similar documents, which, when users enabled macros, installed Emotet.”

“With this newly discovered loader-type used by Emotet, a new threat vector is introduced to Emotet’s capabilities,” Binary Defense researchers concluded. “Emotet can use this loader-type to spread through nearby wireless networks if the networks use insecure passwords.”