Microsoft's free anti-virus product Security Essentials, for the second time in a row, failed to win quality certification by AV-Test Institute.

AV-Test is a Germany-based test firm that evaluates both free and paid anti-virus tools and issues certifications for the most reliable anti-virus software programs.

According the latest test report by AV-Test, the firm tested 25 security products for home users, including popular products by AVG, Kaspersky, and McAfee. Microsoft Security Essential 4.1 was one of three products that failed to gain certification in the evaluation test.

Security Essential missed a considerably higher number of zero-day and new malware samples as compared to its rivals, resulting in poor performance in tests.

AV-Test's industrial average score for detecting zero-day malware samples is 92 percent. However, Security Essentials got just 71 percent in November. Microsoft's security bet showed improvements in the December test, detecting 78 percent of zero-day malware samples, but still lower than the industrial average score.

Additionally, out of a set of 215,999 malware samples discovered in the past three months, Security Essentials missed about nine percent of new malware arrivals.

Microsoft, on the other hand, defended its software and challenged AV-Test's results, saying that 94 percent of the missed malware samples "were never encountered by any of our customers."

"Our review showed that 0.0033 percent of our Microsoft Security Essentials and Microsoft Forefront Endpoint Protection customers were impacted by malware samples not detected during the test. In addition, 94 percent of the malware samples not detected during the test didn't impact our customers," wrote Joe Blackbird, Program Manager, Microsoft Malware Protection Center. "We know from telemetry from hundreds of millions of systems around the world that 99.997 percent of our customers hit with any 0-day did not encounter the malware samples tested in this test."

Microsoft is trying to assure its customers that AV-Test's test was invalid, based on what its customers encounter. However, the second certification test failure in a row has ruined the reputation of the anti-virus tool.

In the September/October tests, Security Essentials failed to detect even more zero-day threats and AV-Test removed it from the certification list for the first time since 2010.