A number of Windows 7 and 8.1 users are encountering problems with Microsoft Security Essentials and Windows Defender. Users are seeing their automatic anti-malware protection turned off without their knowledge and are seeing out-of-date virus definitions. The problem is happening with some, but not all, users for the past several hours. Windows 10 users don't seem to be affected.
I just tried running a manual Security Essentials scan on my Windows 7 SP1 desktop machine and got error message 0x800106ba. I, like others reporting the issue, received a warning that my PC couldn't be scanned and my ant-malware service had stopped.
Microsoft Security Essentials provides a fuller range of protection against malicious software than Windows Defender. MSE is meant to protect against viruses, worms, Trojans, rootkits, spyware and more.
I have no idea how many users are affected, but saw early reports of this on AskWoody.com. There are more reports of the same issue on the Microsoft.com Answers site. Some System Center Endpoint Protection users also are reporting problems and have been guessing that a faulty virus definition could be the culprit.
I'm hearing from sources that an definition update that will fix the issue should be out in the next hour or so (by 3 pm ET or so). And that the problem, introduced in signatures 1.289.1521.0 could be mitigated in signatures 1.289.1587 or newer.
I've asked Microsoft for official comment. No word back so far.
Update (March 19, 8 pm ET): "We've resolved this issue, which appears to have been limited to Windows 7 and Windows Server 2008," said a Microsoft spokesperson via an emailed statement. The spokesperson didn't mention the updated definition that's now available, but here's information on 1.289.1588.0, posted by Microsoft on March 19. Just a reminder, Microsoft, that you've promised to support Windows 7 for free until January 14, 2020....
Yesterday, March 18, a number of IT administrators were reporting sync issues with Windows Server Update Services (WSUS). That issue also may have had something to do with virus definitions. (Thanks to @d_vickery on Twitter for that reminder.)