Apple has banned a developer from its App Store after fraudulent purchases of his applications were made from around 400 accounts, it said in a statement.
Thuat Nguyen and his apps, which at one point over the weekend reportedly occupied 42 of the top 50 positions in the book sales chart, was "removed from the App Store for violating the developer Program License Agreement," Apple's Trudy Muller said by e-mail.
"The iTunes servers were not compromised," said Muller and Nguyen didn't receive any confidential information when his apps were downloaded.
Apple advised users who suspected fraudulent purchases were made to contact their bank and cancel the credit card in question. Apple also said users should change their iTunes password.
In response to the incident Apple is tightening security on App Store purchases and users can expect to be more frequently asked for the CCV number on their credit cards.
The CCV (card code verification) code is typically a three-digit number printed on the back of the card that doesn't make up part of the credit card number and isn't stored by Apple's servers. It is supposed to prove that the credit card is in the possession of the person making the purchase.