A bad malware signature caused Sophos antivirus products to detect a critical Windows file as malicious on Sunday, preventing some users from accessing their computers.

The false positive detection flagged winlogon.exe, an important component of the Windows Login subsystem, as a Trojan program called Troj/FarFli-CT. Because the file was blocked, some users who attempted to log into their computers were greeted by a black screen.

Sophos issued an update to fix the problem within a few hours and said that the issue only affected a specific 32-bit version of Windows 7 SP1 and not Windows XP, Vista, 8 or 10.

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Source: Security