Apple put new security measures in place on iTunes on Wednesday, one day after barring a Vietnamese applications developer for fraud.
Apple said users of the hugely popular online store would be asked to make more frequent entries of the CCV code on their credit cards when making purchases or accessing iTunes from a new computer.
The CCV code is a three- or four-digit number on the back of a credit card.
The new security measures were announced after Apple said it had barred a Vietnamese program developer from its iTunes application store for fraudulent activity.
"Developer Thuat Nguyen and his apps were removed from the App Store for violating the developer Program License Agreement, including fraudulent purchase patterns," Apple said.
Apple did not provide any further details about the incident involving the App Store, which offers free and paid applications for the iPhone, iPod Touch and iPad.
But the Cupertino, California-based company said Wednesday that iTunes servers "were not compromised."
"An extremely small percentage of users, about 400 of the 150 million iTunes users -- that is less than 0.0003 percent of iTunes users, were impacted," the company said.
Apple also advised users whose "credit card or iTunes password is stolen and used on iTunes" to contact their financial institution and change their iTunes password.
According to the technology blog Engadget.com, applications credited to the unknown Nguyen grabbed 42 of the top 50 sales positions in the App Store's book category at one point.
Engadget said it had received reports from a number of people that hundreds of dollars had been spent from their iTunes accounts to buy books from Nguyen's company.
Explore further: Apple bars developer from App Store