hacked IPhone

If running iOS 13 or earlier versions, check: Settings - Passwords & Accounts - Accounts - Any rogue entries here? If so, delete the rogue account. 

If running iOS 14 or higher, check: Settings - Calendar - Accounts - Any rogue entries here? If so, delete the rogue account. 

The Apple support article is below. 

Remove calendar spam

Video support

You have spam calendar entries.

You allowed a website or app to put these ads in your calendar.

They are trying to scare you into downloading useless software, steal your personal information.

You have not been hacked.

Removed the spam entries from your calendar.

elio118 Said:

"hacked IPhone: I think my iphone 12 is been hacked. Keep receiving message on Google calendar says so. What to do?"

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No one has hacked you iPhone.