If you're running Windows Defender (quite a strong possibility if you're running Windows), make sure you've got all your updates. Microsoft issued a patch this past Monday for a vulnerability in its malware protection engine that could allow a remote attacker to gain control over any affected system simply by sending the victim a specially-crafted e-mail or instant message. The exploit can be activated when Windows Defender simply scans a piece of data, and doesn't require any action behalf of a local user to take effect.
The first public news of the flaw came from a series of tweets by one of the folks who discovered it. Tavis Ormandy and Natalie Silvanovich both work for Google's Project Zero security research team. The pair tweeted vaguely about the vulnerability on Friday night, saying it was "the worst Windows remote code execution flaw in recent memory." Despite requests for further details, the pair declined to provide any further information, saying that they would wait until it was patched.
Fortunately, Microsoft has already issued a patch for the bug. If you want to make sure your system is up-to-date, check your Engine version in "Windows Defender settings." Version 1.1.13704 is the patched version. Of course, people relying on third-party anti-malware software don't have to worry about it.