Unexpected use of mobile data for software updates

If you’re disconnected from Wi-Fi or it’s too slow, iOS will put up a prompt seeking permission to use cellular data. It should not do it without permission.

You should turn off Automatic Download as a basic prevention measure , in Settings, General, Software updates.

Remember this is a total since you last reset statistics. Does it show on your mobile account monthly bill?

iOS 18.6.2 is 5+ months out of date. Update to iOS 18.7.2 if you want to stay on iOS 18,, or update to iOS 26.1 if you want to install the latest iOS version.

I suspect it has downloaded 26.1 using mobile data!

If 26.1 has been downloaded, then you will see it if you Tap Settings > General > Storage and scroll down to look for iOS 26.1. If you don't see the file there, it has not been downloaded.

Unfortunately, there is no way to know whether a download occurred using Wi-Fi or Cellular Data. If Wi-Fi is enabled, the iPhone will always use Wi-Fi as a priority. Data would only be used if Wi-Fi was not available at the time and you have also enabled downloads of updates using cellular data.

grahamchiswick wrote:

Thanks LD150. So something else was downloaded using my mobile data allowance. Maybe iOS 18.7.2. ?

I believe all settings were set correctly to prevent use of mobile data for downloading software (iOS upgrades and app upgrades).

Since no one else has chipped in to say they have the same problem, I will just disable Automatic Updates on the Settings > General > Software Update screen.

Turn off Automatic Downloads as well, to avoid data usage, but as i say it should ask you to use cellular data, unless ,maybe you have the setting to allow more data on 5G turned on (I cant remember the exact wording)

Not sure if this is related, but I had a problem with with massive excessive mobile data usage used for "System Services > General" (about 1 GB every 3-4 days). I tried numerous suggestions from this forum, and only one worked.

I completely reset the phone to its iOS 18 baseline, then installed iOS 26. Then I downloaded new copies of each app from the app store. I have had no abnormal data usage since doing this 2 months ago.

I talked with a corporate I.T. security person, and he said that the cause of my iPhone’s massive abnormal data usage was most likely malware. He said the main symptom of malware on an iPhone is unexplained consumption of large amounts of data. While malware is rare on iPhones, it does occasionally occur. It's unfortunate that Apple provides no way to scan their iPhones for malware, and independent apps also cannot scan the phone.

This would explain why my phone was affected, while 4 friends using the same phone and mostly the same apps have not experienced it.

I just wonder whether iOS updates use this particular setting, or is controlled separately

By default, updates will occur only when a good Wi-Fi connection is present.

IF......Wi-Fi is not available......AND.....you have authorized updates to be downloaded using cellular data, that would be the only way that a download over cellular would occur.

Based on the information that you have provided, I do not think that an update was downloaded over cellular data.

BTW ios26 would be 12-14 GB