5-Year-Old Windows Defender Bug That Caused CPU Spikes on Firefox Finally Fixed
It took over five years, but Microsoft has finally fixed a bug that caused Windows Defender to drain CPU resources when Mozilla’s Firefox browser is active.
In 2018, Mozilla staff reported that Windows 10’s built-in malware-detection program caused CPU consumption to top 30%. "This is considerably slowing me down and makes Firefox feel really slow,” Firefox designer Markus Jaritz said at the time.
According to Mozilla’s bug report(Opens in a new window), the company’s engineers reported the problem to Microsoft in June 2018, noting the slowdown issues seemed to involve a program called “MsMpEng.exe,” the malware protection engine for Defender. But it looks like Microsoft’s effort to fix the problem fizzled out—possibly because it needed more information on the bug—which resulted in no major update on the problem for years.
The problem only re-emerged in recent months after a user on Reddit again reported(Opens in a new window) Windows Defender had been causing their PC to consume 30% of its CPU resources while Firefox was active. Yannis Juglaret, a Mozilla senior software engineer, then posted in Mozilla’s original bug report, this time with screenshots showing Firefox triggering the CPU spikes through MsMpEng.exe when Google’s Chrome browser would not.
“The CPU time used overall by MsMpEng.exe seems indeed much higher (5x) with Firefox compared to Chrome,” he wrote.
He then found Firefox can trigger too many CPU-heavy calls to VirtualProtect, a Windows component that’s monitored over MsMpEng.exe.
“With a standard Firefox configuration, the amount of calls to VirtualProtect is currently very high, and that is what explains the high CPU usage with Firefox,” Juglaret added. “In Firefox, disabling JIT makes
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