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5-year-old Windows Defender bug that caused CPU usage spikes when using Firefox patched

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People using Mozilla Firefox on Windows 11 or 10 can now rejoice as Microsoft has fixed a Windows Defender bug discovered in 2018 with the MsMpEng.exe (Antimalware Service Executable) process that was causing high CPU usage. While the bug’s impact wasn’t limited to Firefox, it was hit the hardest.

The issue was causing the Antimalware Service Executable process to hog CPU resources especially when using Firefox, which would generate around seven times the events — compared to Chrome or Edge — on the Windows Threat Intelligence ETW (Event Tracing for Windows) when someone was browsing the internet. These events were basically calls for Windows Defender to keep the device safe as Firefox is being used, except they were too many in comparison to other browsers, causing high CPU usage.

The fix should help decrease CPU usage when using Firefox by 75%.

Decrease in CPU usage after the fix, as recorded by Yannis Juglaret/Mozilla

While CPU usage has significantly decreased, Mozilla developers believe there’s more room for improvement.

“We should try to reduce the number of events that Firefox generates, which will reduce the CPU usage from AV software,” said Yannis Juglaret, senior software engineer, Mozilla. “We should also double check CPU usage from major AV products to check if some of them might have poorly optimized event analysis code, as this would currently impact our users more than users of competitor browsers.”

While the issue seems to be mostly fixed with Mozilla devs working to optimise things overall and with other anti-virus products, the fix has come five years after it was initially reported.

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CT Editorial Team

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