Severe iPhone battery drain after iOS 27 update with Microsoft Defender

Apps like this are pretty much only good for wasting your battery. They simply can't protect you the way they suggest they can. iOS is a walled off (often referred to as sandbox) operating system, which no 3rd party app can get in or actually protect. Such apps are nothing more than hype and a revenue stream for the App Developer. You couldn't pay me to use one on my iPhone.

The best answer would be to totally remove this ineffectual and unneeded app from your iPhone. If you still feel you must use something which truly provides little to no benefit, reach out to Microsoft for help. Apple doesn't own Microsoft or any of their applications so whatever it's doing to your iPhone (clearly not good, btw) isn't something anyone here on this user to user only forum, nor at Apple can resolve.

Microsoft would initiate the background activities and for Defender that is to analyze all web traffic including those of other apps. Review the information provided here by Microsoft for battery consumption using Defender.

• To help ensure protection from web-based threats is active, Defender for Endpoint needs to run in the background at all times. This might lead to a minor increase in overall battery consumption of your device. In case you're seeing significant battery drain, send us feedback and we'll investigate.
• Network packets from any app go through this check and that causes the battery usage of Microsoft Defender for Endpoint to be computed inaccurately. The actual battery consumption of Microsoft Defender for Endpoint is lesser than what is shown on the Battery Settings page on the device.

https://learn.microsoft.com/en-us/defender-endpoint/ios-troubleshoot

Your device is still going to see more battery being used due to the installation of Defender, but the actually reported value attributed to Defender is because of the packet analysis when using any app on your device. Microsoft contends that the battery usage should be reported for the other app being used, but the usage is contributed to Defender because it is those background processes that are consuming your battery.

lobsterghost1 wrote:

Apps like this are pretty much only good for wasting your battery. They simply can't protect you the way they suggest they can.

Even Microsoft's App Store description of the Microsoft Defender Security app hints at this. Where the description mentions "antivirus scanning", there is a reference to footnote "[3]". That footnote says "Does not replace existing malware protection on iOS and Windows devices."

Then there's this:

Microsoft Learn – Microsoft Defender for Endpoint mobile threat defense on Android and iOS

which lists "Malware Protection" as being "Android-only".

As already pointed out, it’s useless software that is made and controlled by Microsoft.

Any questions about why it isn’t working well needs to be asked of Microsoft.

Thanks for the suggestion. In my case, I work at Microsoft and need Microsoft Defender as part of my device setup to access corporate resources, so uninstalling it isn't really an option for me.

I'm mainly trying to understand why Defender suddenly started showing several hours of background activity and unusually high battery usage after the iOS update.