A recent update to the built-in antivirus software in Windows 10 has taught the program a new trick—how to download files through a command line tool, including nefarious ones (trojans, spyware, ransomware, and other malware).
Downloading malware is not the intended purpose, at least I presume that's not the case. But the new function could potentially be abused in such a manner. Fortunately, this is not something the typical home user needs to worry about, not unless they're a PC masochist (more on that in a moment).
"Well, you can download a file from the internet using Windows Defender itself. In this example, I was able to download Cobalt Strike beacon using the binary 'MpCmdRun.exe' which is the 'Microsoft Malware Protection Command Line'," Askar stated on Twitter.
This effectively allows a local attacker to leverage Defender as what is called a living-off-the-land binary (LOLBin). That's when legitimate software is used for something malicious—in this case, using an antivirus program to download a virus.
It appears this new ability was added to Defender with the 4.18.2007.8 update in July, so the functionality has been there for nearly two months. Bleeping Computer tested the new download switch in the command line tool and was able to download the same WastedLocker ransomware that recently caused a ruckus with Garmin's infrastructure, which prompted the company to reportedly pay a multi-million dollar ransom.
This is not quite as careless as it may seem at first glance. For one, Defender will still scan files download through this method, so in theory it should still protect against malware. And secondly, this would need to be initiated by a local user.
Nevertheless, this is something system administrators should be aware of, so they can take the proper precautions. It's not unheard of for a rogue employee to cause mischief, whether they are disgruntled, are on the verge of being fired, or any other reason.