Microsoft has finally fixed the 5-year-old Windows Defender High CPU bug on Mozilla Firefox.


Bug on Windows Defender

The nearly half-decade-old bug seems to have been fixed by Microsoft and Mozilla. Issue Windows Defender and its Antimalware Service Executable (MsMpEng.exe) service was related to causing high CPU usage on Mozilla Firefox. Resource usage was significantly higher than Google Chrome and Microsoft Edge. For example, the image below (taken at the time of the bug’s initial reporting) shows the average CPU usage when reloading YouTube six times. As you can see, the spikes were significantly higher on Firefox.

Mozilla Firefox CPU Usage vs Edge and Chrome

This issue was recently resolved by the efforts of the Microsoft and Mozilla development team. Yannis Juglaret, a Firefox developer Confirmed This was about three weeks ago:

According to Microsoft, it will be deployed to all users as part of regular definition updates, packaged independently of OS updates. This also includes users of Windows 7 and 8.1, although those platforms should not have previously had a performance problem with Firefox because the ETW events that cause this do not exist on these older versions of Windows.

Later, Yannis Juglaret added that the recent Microsoft Defender March-2023 definition update (Platform: 4.18.2302.x | Engine: 1.1.20200.4) fixed the problem:

mpengine.dll version 1.1.20200.4 was released on April 4th, so this fix should now be available to everyone.

Here are the details for the protector update:

Mar-2023 (Platform: 4.18.2302.x | Engine: 1.1.20200.4)

  • Security Intelligence Update Version: 1.381.61.0
  • Release Date: April 4, 2023 (Engine) / April 11, 2023 (Platform)
  • Platform: 4.18.2302.x
  • Engine: 1.1.20200.4

Interestingly, it also found that Firefox has room for more improvement in processor usage compared to Chrome. Perhaps we will see such performance improvements in future browser updates and it won’t be exclusive to this one. Microsoft Defender alone.

through: Hayes


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