This help article explains why you received a notification from Yahoo! that your Yahoo! Contacts information is being shared with dzhang, and provides information on how to prevent this from occuring.

This issue occurs when an app on your mobile device has been given permission, either explicit or implied in the app's Terms of Service, to access your Yahoo! Contacts information.

To stop your Yahoo! Contacts information from being shared, you will need to remove the external application's permission to access your Yahoo! Account.

If you continue to receive the same notifcation that your information is being shared, or you notice the external application is still listed in your Account Information, this indicates that there is an app installed on your mobile device that has been connected to your Yahoo! account. To prevent this from occurring, you will need to remove the app that is causing this. There could be a number of apps that are causing this. A common one some users have reported is "Samsung Social Hub." If you do not have this particular app installed, you will need to determine further which app is causing this issue and remove it. You may need to contact your mobile device's manufacturer, or your wireless carrier for further assistance with this.

NOTE: If you are using a third party app for access to Yahoo! Mail, we recommend using the official app provided by your phone manufacturer, or the official Yahoo! Mobile app.