Microsoft scrambled earlier today to revise an antivirus definition file that deleted Google's Chrome browser from users' PCs.
"That's certainly one way to win the browser war," said Andrew Storms, director of security operations at nCircle Security.
Google told Chrome users that Microsoft incorrectly marked the browser as malware.
Storms was referring to the battle between Microsoft's Internet Explorer (IE) and rivals, including Chrome, for usage share. According to data from one Web metrics firm, Chrome will pass Mozilla's Firefox as the second-most-popular browser by the end of this year, pitting Google and Microsoft for the top spot.
Chrome users began reporting the specious detection of the browser early Friday in a quickly-growing thread on a Google support forum.
"This morning, after I started up the PC, a Windows Security box popped up and said I had a Security Problem that needed to be removed," said someone identified as "chasd harris" in the first message of the thread. "I clicked the Details button and saw that it was 'PWS:Win32/Zbot.' I clicked the Remove button and restarted my PC. Now I do not have Chrome. It has been removed or uninstalled."
Scores of others reported the same behavior on their Windows PCs running Microsoft's Security Essentials -- its free, consumer-grade antivirus software -- as well as Forefront, the antivirus product designed for enterprises.
Microsoft issued a corrected definition file around 10 a.m. PT Friday, about three hours after users began reporting the false positive on Google's support forum. Several bloggers, including ZDNet's Ryan Naraine and Ed Bott , reported the problem.
Microsoft has acknowledged the gaffe, and said approximately 3,000 users were affected.
"An incorrect detection for PWS:Win32/Zbot was identified and as a result, Google Chrome was inadvertently blocked and in some cases removed from customers PCs," Microsoft said in a statement posted to the Facebook page of its malware research center. "We have already fixed the issue..., but approximately 3,000 customers were impacted."
Microsoft told users to update Security Essentials with the new definition file, then reinstall Chrome.
For its part, Google slapped a red warning banner at the top of its Google support pages that read, "Alert: Google Chrome has been incorrectly marked as malware by Microsoft security software."
Chrome was fingered as Zbot, better known as Zeus, a widespread botnet Trojan that focuses on stealing online banking credentials, which criminals then use to vacuum money from accounts.
Microsoft isn't the only security software maker to have screwed up in this way: All three of the world's largest antivirus companies -- Symantec, McAfee and Trend Micro -- have issued defective definitions in the past. In some cases, those mistakes have wreaked much more havoc.
In April 2010, for example, a McAfee antivirus update crippled an unknown number of corporate PCs worldwide when it quarantined a crucial Windows XP system file.
Some users who had seen Chrome vanish when they accepted Security Essentials' advice and let the software delete "chrome.exe," said that their bookmarks had also been eliminated, and that they were not restored after reinstalling Chrome.
Others, however, said that their bookmarks had not been affected.
Computerworld replicated Security Essentials' error by manually deleting chrome.exe, but had trouble reinstalling Chrome in Windows 7 using IE to download Google's browser. Only after uninstalling the remainder of Chrome, then using Firefox to download and save the Chrome setup file was Computerworld able to restore Google's browser.
In that test, Chrome's bookmarks were reinstated.
Microsoft did not immediately reply to questions about whether the Security Essentials' snafu permanently eradicated Chrome bookmarks, as some users attested.