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.
"Based on current case volume and customer feedback, we believe the number of impacted systems to be minimal and confined to a small number of cases," the company said in a support article.
One Twitter user who was affected by the issue said that he highly doubts only a small number of customers were affected, while another one reported that he's been on hold trying to reach Sophos Support by phone for over two hours.
"An email would have been nice," one user told Sophos via Twitter. "I cannot imagine the costly overtime $ and panicked IT staff like myself working on this."
Another user joked that this false positive actually removed some of his weekend.
In many cases administrators will only need to clear the bogus alerts from the Sophos Enterprise Console, Sophos Central or Sophos Home by marking them as resolved or acknowledged, the company said.
Users who encountered the black screen will have to reboot in Safe Mode, disable the Sophos Anti-Virus service from starting automatically and then boot into the OS normally. More detailed instructions are available in Sophos' support document.
Sophos is not the first antivirus company to issue a bad definition that affected users' computers.
To its credit, the company reacted promptly and fixed the issue in a timely manner during a weekend, but questions remain as to why in 2016 an antivirus program can still flag legitimate Windows system files as malicious.
False positive detections used to be much more frequent years ago, but most antivirus companies have since built whitelists of known software to avoid such incidents. Windows files in particular should be a top priority for whitelisting because blocking or deleting them can leave a computer unusable.