Microsoft has rolled out a rapid fix after Windows Defender users found themselves unable to perform a full scan. Reports on the official forums, as well as by AskWoody and Bleeping Computer, suggest the anti-virus crashed service and threw up an error when some users tried to check their entire PC for malware.
“The Windows Defender Antivirus Service service terminated unexpectedly. It has done this 1 time(s). The following corrective action will be taken in 100 milliseconds: Run the configured recovery program,” said the error in Event Viewer.
In the antivirus program itself, users were told that their anti-virus service had stopped and prompted them to restart it. Such errors can naturally raise fears of a malware infection, so many will be relieved to hear it was Microsoft's mistake.
The fault, according to a Reddit user, had roots in how Windows Defender handled two dots. Users were able to resolve the issue by checking their event viewer to find the offending file and renaming it.
Thankfully, that's no longer necessary, as Microsoft has now rolled out an updated definition (1.313.1687) within a day. It didn't list the exact cause of the issue and investigations from the community haven't been conclusive so far.
Either way, you can manually update the software via “Windows Security > Virus & threat protection > Check for updates”. Though this marks a major mistake on Microsoft's part, the security issue was at least resolved quickly despite the global pandemic.