This time, it is "phishing" <https://en.wikipedia.org/wiki/Phishing> on the iPhone--attempts to get you to give them your Apple account credentials. For clarity: This is not malware residing on your device; it is a message that came in piggyback with something else.
Occasionally, iPhone (or any iOS device) will need your credentials (ID and password), and will make its request by putting up a "slip" containing the notification. You may be able to dismiss without complying, but the request would return the next time the OS needed access to Apple services. We become accustomed to just doing what is needed without thinking about context--and this is what the jerks are counting on. They display such a request (looking more or less identical to the legitimate one), and we pass them our credentials (username and password).
The article below suggests that such request be dismissed immediately, but then one should go to System Settings to see if the request is there--and if so only there to supply the password being requested.
This would be similar to our _not_ using the convenient link in e-mail from [someone posing as] a bank; instead we use the links previously stored in our Contacts address books.
I hope this helps.
Mark_
15 October 2017 21:46