Apple said a hacker who manipulated sales data on its online App Store broke into the accounts of about 400 iTunes users in the process, according to a published report.

Clayton Morris, a blogger and host of Fox News' Fox and Friends, said Apple divulged the information to him in response to his queries about the matter. "Apple told me that an extremely small percentage of users, about 400 of the 150 million iTunes users—that is less than 0.0003% of iTunes users, were impacted," Morris wrote in a blog post Tuesday.

Apple plans to beef up security measures for its iTunes and App Store sites, Morris added. "Basically you'll have to enter your credit card's CCV code a little more often from now on," he said.

Hacker Thuat Nguyen over the weekend gamed the App Store's sales ranking system to make it appear as though his e-books accounted for 42 of the site's top 50 electronics books. "His apps were removed from the App Store for violating the developer Program License Agreement, including fraudulent purchase patterns," Apple said in a statement Tuesday.

Apple previously insisted App Store or iTunes users' information was not at risk as a result of the incident. "Developers do not receive any iTunes confidential customer data when an app is downloaded," the company said.

Still, Apple cautioned customers to be vigilant for suspicious transactions.

"If your credit card or iTunes password is stolen and used on iTunes we recommend that you contact your financial institution and inquire about cancelling the card and issuing a chargeback for any unauthorized transactions," Apple said.

"We also recommend that you change your iTunes account password immediately," Apple added.