Necurs, a nearly two-year-old rootkit, has been spreading quickly recently and was found on 83,427 unique machines during the month of November, according to researchers at Microsoft.

Necurs is distributed through drive-by download and and may occur anytime a user visits an infected website, said Tim Liu, a researcher at Microsoft's Malware Protection Center, in a blog.

As a rootkit, Necurs is particularly dangerous, Microsoft said. Once it infects a machine, it can hide itself at the root level, download additional malware, and even stop security applications from functioning.

"In addition, Necurs contains backdoor functionality, allowing remote access and control of the infected computer," Liu's blog said. "Necurs also monitors and filters network activity and has been observed to send spam and install rogue security software. Nefariousness aplenty."