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Linux Malware Could Run Undetected on Windows: Researchers

A new Windows 10 feature that makes the popular Linux bash terminal available for Microsoft’s operating system could allow for more malware families to target the operating system, Check Point researchers claim.

Called Windows Subsystem for Linux (WSL), the feature exited beta a couple of months ago and is set to become available to all users in the upcoming Windows 10 Fall Creators Update (FCU), set to be released by Microsoft in October 2017.

The feature brings the Linux command-line shell to Windows, thus allowing users to natively run Linux applications on Windows systems. Because of that, Check Point researchers argue, malware designed for Linux can slip undetected onto Windows computers.

Called Bashware, the new attack technique could be abused even by known Linux malware, because anti-malware solutions for Windows haven’t been configured to detect such threats, the security researchers argue.

“Existing security solutions are still not adapted to monitor processes of Linux executables running on Windows OS, a hybrid concept which allows a combination of Linux and Windows systems to run at the same time. This may open a door for cyber criminals wishing to run their malicious code undetected, and allow them to use the features provided by WSL to hide from security products that have not yet integrated the proper detection mechanisms,” Check Point says.

The security researchers claim they have already tested the attack technique on “most of the leading anti-virus and security products on the market,” and managed to successfully bypass all of them. Because of that, they claim, “Bashware may potentially affect any of the 400 million computers currently running Windows 10 PC globally.”

The risks posed by WSL, however, are mitigated by the fact that the user needs to manually enable the feature and reboot the system. Malware that wants to abuse the feature would need to enable developer mode on Windows, which is disabled by default, and even download and extract the Linux file system from Microsoft’s servers.

Check Point says the necessary features could be silently enabled in the background, thus setting up the necessary environment without user’s knowledge. Moreover, they say they were able to run Windows-based malware in the newly set up environment.

The researchers also point out that the newly discovered attack technique doesn’t leverage an implementation flaw, but that the lack of awareness by various security vendors is the actual issue here.

“However, we believe that it is both vital and urgent for security vendors to support this new technology in order to prevent threats such as the ones demonstrated by Bashware,” Check Point says.

According to Microsoft, however, the risks posed by such an attack are low, given that the features required to run Linux apps on Windows are disabled by default.

“We reviewed and assessed this to be of low risk. One would have to enable developer mode, then install the component, reboot, and install Windows Subsystem for Linux in order for this to be effective. Developer mode is not enabled by default,” a Microsoft spokesperson told SecurityWeek via email.

Contacted by SecurityWeek, anti-malware vendor Kaspersky Lab confirmed in an emailed statement that they are aware of the potential risks posed by WSL and that they are already working on the technology necessary to detect any malware that could abuse it.

“Kaspersky Lab is aware of the possibility to create malware for Windows Subsystem for Linux (WSL) and is working on technologies to detect this type of malware on user devices. In fact, in 2018, all Kaspersky Lab solutions for Windows will be updated with special technologies that detect behaviorally and heuristically and block any Linux and Windows threats when WSL mode is on. Currently, all Kaspersky Lab frontline solutions for Windows can detect downloaders and Windows parts of Linux malware,” Kaspersky Lab said.

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Ionut Arghire is an international correspondent for SecurityWeek.

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