Requesting for refund of in app purchases.
How it works?
For most freemium games, we have in-app purchases but we only design it such that when the player purchases it, we will reward them with some in-game currency.
For the fraudsters, they buy some IAP and after which, request refunds from Google/Apple. Most developers do not check if past receipts are still valid. Hence the player actually gets free in-game currency.
How to fix it?
Check for validity of past receipts whenever the game starts and deduct in-game currency if there are any discrepancy. Allow your in-game currency to go into negative and after multiple times, you can choose to ban the player.
How to check for validity of purchases?
Google Play have provided a nice article and API for checking refund frauds:
For Apple App store, you can check the receipts: Receipt Validation
For those using Unity API, you can use Unity IAP API: Unity Receipt Validation
Hope this article helps some developer out there..