Microsoft fixes 5-year-old bug that caused Firefox to munch CPUs
Microsoft has fixed a five-year-old bug in Windows Defender that caused significant CPU usage spikes for users of the Mozilla Firefox browser, Neowin reports.
Firefox developer Yannis Juglaret confirmed that Microsoft rolled out a regular definition update for the anti-virus application that fixed the problem.
The culprit in the CPU spikes was Windows Defender’s Antimalware Service Executable service, which appears as MsMpEng.exe in the Task Manager.
The service is necessary for Windows Defender to run its Real-Time Protection feature, enabled by default, that protects users from installing malware on their system.
For several years, real-time protection consumed significantly more CPU resources with Firefox than Microsoft’s Edge browser or Google Chrome.
The graph below from Neowin compares how much CPU consumption occurred when opening YouTube six times on the three browsers when the bug was first reported.
CPU usage of Firefox, Edge, and Chrome compared at the discovery of the bug. Credit: Neowin
The bug was fixed a few weeks ago thanks to a collaboration between Microsoft and the Mozilla development team.
Juglaret said Mozilla expected the fix would mean the MsMpEng.exe service should consume around 75% less CPU power while monitoring Firefox than it did before.
Users running Windows versions as old as Windows 7 and Windows 8.1 have also received the update, despite the issue not impacting them.
Juglaret added that only users who explicitly rejected definition updates would not get the fix.
It is unlikely that many would have done this, as definition updates are critical to ensure that an anti-virus program can detect and neutralise the latest threats.
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