A three-fold increase in Microsoft Windows computers infected with malicious software in late 2013 came from an application that was for some time classified as harmless by security companies.
The finding comes as part of Microsoft's latest biannual Security Intelligence Report (SIR), released on Wednesday, which studies security issues encountered by more than 800 million computers using its security tools.
In the third quarter of 2013, an average of 5.8 Windows computers out of every 1,000 were infected with malware, said Tim Rains, director of Microsoft's Trustworthy Computing division, which tracks security trends targeting the company's widely used products. That jumped to about 17 computers per 1,000 for the last quarter of the year.
Rains attributed the rise to malware called "Rotbrow." The program masquerades as a browser add-on called "Browser Protector" and is supposedly a security product, Rains said by phone Wednesday. Rotbrow was found on about 59 of every 1,000 computers using its security products, he said.
For some time, computer security companies didn't classify Rotbrow as malicious software. Rotbrow is known as a "dropper," with capabilities to download other software on a computer. It didn't initially download malware to computers it was installed on, Rains said.
But then Rotbrow started downloading malicious browser extensions. Microsoft noticed the change and alerted other security companies, which then began blocking it.
The tactic, which had been used by fake antivirus programs in the past, meant that Rotbrow was already installed on a huge number of computers.
"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."
Rotbrow often distributes Sefnit, a type of malicious botnet code, which can subsequently download other harmful programs to a computer such as those involved in click fraud. Sefnit has also been linked to "ransomware," which is malware that encrypts a person's files and demands payment.
Microsoft added detection for Rotbrow in its Malicious Software Removal Tool (MSRT) last December after it raised suspicion.
Overall, Microsoft's latest report concluded that security improvements in Windows such as ASLR (Address Space Layout Randomization) and DEP (Data Execution Prevention) have made it much more difficult to exploit known vulnerabilities. The report also said the number of vulnerabilities in Microsoft products that can be remotely exploited has fallen by 70 percent between 2010 and 2013.
"We are really trying to raise the cost of exploitation," Rains said. "It's not impossible to exploit, just hard. They have to put in the extra time, extra cost.
As a result, attackers are increasingly trying to just trick people into downloading their malware by bundling it with legitimate programs or music, he said.
The latest report does not include data on the zero-day vulnerability in Internet Explorer that Microsoft released an emergency patch for on Monday. The flaw, which affects IE 6 through IE 11, could allow attackers to execute code remotely on a compromised computer if the user views an infected web page using the browser.
Rains said "time will tell" if its next report shows a rise in infections due to the bug. But Microsoft believes the quick release of a patch and fact users have to be lured to a malicious website mitigates the risk.
"I don't think we will see an uptick [in infections] given the quick response and the type of vulnerability that is," Rains said.
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