Researchers at Kaspersky Lab have uncovered new malware spreading through Pirate Bay, the popular torrent tracker.

The malware, which aims to infect users' PCs with adware and tools for additional malware installation, has a multi-layered structure. Due to its hidden, seemingly endless stack of functionality, the threat has been named PirateMatryoshka, after the traditional Russian stacking doll.

For the most part, torrent sites are used for the distribution of pirated content that is illegal in the majority of countries, as it infringes intellectual property rights.

They are also a popular target for those looking to distribute malware, because users looking for illegal content often disconnect their online security solutions, or ignore system notifications in order to install the content.

PirateMatryoshka malware carries a Trojan-downloader disguised as a hacked version of legitimate, everyday software.

Although a significant majority of malware found on torrent sites is spread through newly set up user accounts (seeders), PirateMatryoshka is spread using established seeders with no known history of malicious activity.

The latter makes an effective distribution process, because due to the good reputation of the seeder, potential victims have no reason to doubt the file to be downloaded is safe.

"Once a user clicks on the installer, PirateMatryoshka's infection process begins," says Kaspersky Lab.

"First, it shows the victim a copy of the Pirate Bay page that is in fact a phishing page, asking them to enter their credentials to continue the installation. Later, this malware uses these credentials to create new seeders distributing more of PirateMatryoshka."

The phishing link has been accessed around 10 000 times. Kaspersky Lab detects this malware as Trojan-Downloader.Win32.PirateMatryoshka, and Trojan.Win32.InstClick.

The infection process is designed to continue even if user credentials are not entered, with the malware unpacking further malicious modules, including a malicious clicker that can tick the 'agree' box that triggers the adware installer, flooding a victim's device with unsolicited software.

The researchers describe PirateMatryoshka as a combination of sophisticated, multi-functional malware, with highly effective distribution functionality for its own and other malicious code.

Anton Ivanov, security researcher at Kaspersky Lab, says the company often sees multi-layered malware, such as droppers, and finds malicious installers that install more than one program on a user's device.

"However, with PirateMatryoshka, this process is more sophisticated, as the malware that reaches a victim's computer through the adware can introduce additional installers that are meant to bring more malware.

"This is quite advanced considering this is an un-targeted, mass attack that carries a phishing component for wider onward distribution."