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I wanted to sign in Telegram on my iPod Touch running iOS 11 with another account.

I deleted the app and install it again. On the sign up screen Telegram filled the phone number field with the number I was using 5 minutes before. So I deleted the app again, cleared the advertising identifier, turned off the keychain, erased the pre filled text in iCloud, and restarted. After I reinstalled the app the phone number was still there on sign up.

How can a new installed Telegram app find the phone number I was using? I though that the latest iOS privacy features were preventing an app to track a user after reinstalling it.

Rocapot
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    Maybe it's keeping track of your device ID, and stored your phone number against it when you installed the app for the first time. Not sure about iOS permissions, hence commenting. – pri Mar 14 '18 at 07:11
  • Are you sure that you deinstalled the app properly? – Tom K. Mar 14 '18 at 10:23
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    Properly? You long click on the icon, then on the X. Or from the setting you can do it too. That it is what I have done. – Rocapot Mar 15 '18 at 01:44
  • @PriyankGupta Either that or it is stored in some place locally. The UDID is not supposed not to be working anymore. So I wonder what they can be using as a fingerprint – Rocapot Mar 15 '18 at 01:47
  • Had a similar issue today with my [O'Reilly Safari](https://www.safaribooksonline.com/) iPhone app. Had to uninstall it b/c of some problems. On iPhone this is done by pressing down the icon for 2-3 sec and then clicking on the "x" in the upper left corner of the icon; the system shows a "Delete ..." prompt that says "Deleting this app will also delete its data". I clicked on "Delete" on the confirmation prompt and after it was deleted, I reinstalled it. The funny thing was, I did not have to login to my O'Reilly Safari account after re-installing. It looks like my user data was never deleted. – x457812 Mar 15 '18 at 18:15

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From my understanding iOS 11 doesn't delete the keychain keys for an app when you delete the app, I'm talking about the keychain on the device so cloud settings won't affect this. No other apps can access another app's keychain so you don't need to worry about it from a security point of view but it's entirely possible the app is using the keychain store to persist the phone number information.

Michael Hudson
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