Microsoft’s latest biannual Security Intelligence Report (SIR) indicates that a three-fold increase in malware was linked to an app that was for some time classified as harmless by security companies.

Tim Rains, director of Microsoft’s Trustworthy Computing division, said that findings indicated that 5.8 Windows computers out of every 1,000 were infected with malware, in the third quarter of 2013. That figure jumped to 17 computers per 1,000 for the last quarter of the year.

the rise was attributed to malware called “Rotbrow.” Rotbrow was found on about 59 of every 1,000 computers using its security products. Known as a “dropper,” Rotbrow has capabilities to download other software on a computer.

“I would characterize it as a low and slow attack,” Rains said. “They were patient and waited a long time before they started to distribute malicious stuff. I think they gained a lot of people’s trust over time.”

Microsoft added detection for Rotbrow in its Malicious Software Removal Tool (MSRT) last December after it raised suspicion.