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Joe Blackbird, program manager at Microsoft Malware Protection Center, said the company would improve the performance of both products. He said the majority of the malware samples the software was tested against don't represent what Windows users encounter.
"When we explicitly looked for these files, we could not find them on our customers' machines," Blackbird wrote in a blog. "When we did our review, we found that our customer-focused processes had already added signatures that protected against 4 percent of the missed samples. These files affected 0.003 percent of our customers," Blackbird wrote in response to the antivirus tests, posted on the Microsoft Protection Center blog.
Nonetheless, Blackbird called the tests meaningful and said improvements would be made to regain the firm's seal of approval.
"We continually evaluate and look at ways to improve our processes. We know from feedback from customers that industry testing is valuable, and their tests do help us improve," Blackbird said. "We're committed to reducing our 0.003 percent margin to zero."
AV-Test's Marx said he agrees with Microsoft's response and indicated that the testing firm develops its processes to meet the growing number of malware strains in the wild. Antivirus software is typically good at detecting known malware, but virus writers release new malware samples quickly and only target very few users at the same time.
"Microsoft actually confirms our results," Marx said. "In our tests we are focusing on the prevalence of unique malware families. We are getting a lot of different viruses every day so we tried to cover as many malware families as possible."