How to prevent unauthorized in-app purchase?

To prevent unauthorized purchases, you can require authentication for all purchases, including in-app purchases, made through this Google Play account on this device. This is important if you share your device with others or want to prevent purchases in kid-friendly apps. Authentication means certain information, like a password, must be entered on your device to make a purchase.

Change authentication settings on your mobile device

  1. Open the Google Play Store app Google Play.  
  2. Tap Menu Menuand then Settings.
  3. Tap Require authentication for purchases.
  4. Choose a setting.
  5. Follow the on-screen instructions.

Note: To change authentication settings you will need your Google password. Visit account recovery if you don't remember your password. 

Reference: Google Play Help