A Mac security firm has criticised Microsoft for warning Mac users of new malware, saying that the threat simply wasn't worth mentioning.

Late Monday, the Microsoft Malware Protection Center (MMPC), the group that researches malware and crafts signatures for the company's antivirus products, alerted users of a new Mac "backdoor," a program that, once installed, downloads additional attack code or lets hackers steal files from the compromised computer.

Microsoft malware engineer Meths Ferrer said that MMPC had found the backdoor, dubbed "Backdoor:MacOS X/Olyx.A," in an archived file that also contained a Windows backdoor called "Wolyx.A." According to Ferrer, Olyx.A disguises itself as a Google application support file when run by the user, then establishes a remote connection to an IP address hosted in Seoul, South Korea.

But Intego, a French antivirus company that focuses exclusively on the Mac, took exception to Ferrer's blog post.

"They're making it out like this is something serious, but it's not in the wild at all and not being installed," said Peter James, a spokesman for Intego. "This is no big deal."

A backdoor must either be manually installed by a user, perhaps after being tricked into running the file or packaged with other malware that exploits a vulnerability or uses social engineering tricks to get the victim to run the program, said James.

There's no evidence that Olyx is in wide circulation or being used by other malware, such as Mac-specific "scareware," the phony antivirus software that fools people into installing it after faking security alerts. "It could so stuff if it was in the wild, but it's not," argued James.

It's rare to see one antivirus firm bash another for issuing a warning or alerting customers to a possible threat. But that didn't stop Intego, which saw Ferrer's blog as counterproductive.

"We get criticised every time we issue a security alert," said James, adding that people accuse it of crying wolf about threats to the Mac, which has historically been relatively immune to attacks due to its small market share.

Cyber criminals with profit in mind are much more likely to target Windows simply because Microsoft's operating system powers nearly 90% of the world's personal computers.

"When something is a real threat, we'll say something," said James. "If it's not, we don't publicise [the malware] by issuing an alert. We've got other things to do."

Intego created an Olyx definition for its VirusBarrier product on June 30.

"It's kind of interesting that Microsoft took a month [to mention Olyx] after it started circulating," James said, taking another swipe. "Maybe this is a sign that they're going to be analysing more Mac malware in the future."

Other security companies have also made mention of Olyx, including Kaspersky Lab, which highlighted the backdoor in a malware report for June 2011.