Security researchers have stumbled upon a Windows trojan that hackers are using to help with the distribution of the infamous Mirai Linux malware, used to infect IoT devices and carry out massive DDoS attacks.

The Mirai malware was initially developed in late 2015 and early 2016, and only became a massive threat in the summer and autumn of 2016, when it spread to hundreds of thousands of routers and DVRs (deployed with smart cameras and CCTV systems).

After crooks used a botnet of Mirai-infected devices to launch DDoS attacks on the KrebsOnSecurity blog, increased attention from law enforcement forced the malware's author to dump the Mirai source code online.

This move resulted in tens of Mirai variants popping up everywhere, which in turn helped hide the author's tracks, or so the author thought, until this Brian Krebs exposé.

Mirai gets its first Windows version

One of the recent developments on the Mirai malware front was discovered by Russian cyber-security firm Dr.Web, whose experts came across a Windows trojan built with the sole purpose of helping Mirai spread to even more devices.

Standard Mirai versions work by infecting a device, selecting a random IP address and attempting to log in via the Telnet port using a list of default admin credentials. Subsequent versions added the option to launch these password guessing attacks via SSH ports.

In all of this process, the Mirai self-spreading behavior was contained to devices running various versions of the Linux operating systems only.

The trojan discovered by Dr.Web (detected as Trojan.Mirai.1) helps crooks launch the password-guessing attacks from Windows devices, even if Mirai itself (detected as Linux.Mirai) won't be able to run on Windows.

If the Windows trojan infects another Windows device, then that device is used as another point to launch the password-guessing attacks.

New Mirai Windows version targets even more ports

Under the hood, the Mirai Windows trojan works by infecting a device, where it contacts an online C&C server and downloads a list of IP addresses.

In the same way as the original Mirai trojan, the Windows variant attempts to log into the devices at the end of those IPs via a series of ports. Unlike the Linux version, the Windows trojan targets more ports.

22 - SSH
23 - Telnet
135 - DCE/RPC
445 - Active Directory
1433 - MSSQL
3306 - MySQL
3389 - RDP

When the Windows trojan manages to infect a new device, if the underlying platform runs Linux, it will execute a series of commands, which result in the creation of a new Mirai DDoS bot.

If the Windows trojan spreads to a new Windows device, it will drop a copy of itself there and continue to target new devices.

Additionally, researchers say that when the trojan infects a database, such as MSSQL and MySQL, the commands it receives tell it to create a new user with admin privileges, which attackers most likely use to steal data from infected devices.

The only case where the trojan doesn't do anything is if it manages to connect via RDP, in which case it just waits, most likely for a human operator to take control of the infected machine.

Dr.Web discovered the Mirai Windows version only this month, so it is currently unknown how this new development will affect the Mirai ecosystem.

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