"During the incident, users would have seen a blank page if they were redirected to the non-CheckFree site. Those with up-to-date security software would likely have received a message indicating a malware download attempt had occurred," the company said. "If the user's anti-virus software was out of date or they did not have anti-virus software installed, they may have been subject to a malware software download."

The attack targeted flaws in Adobe Acrobat and Adobe Reader, CheckFree said.

CheckFree is advising its customers to download antivirus software and the latest updates to Adobe Reader, used to view pdf files.

CheckFree has about 12,000 locations in the U.S. where customers can walk in and pay bills in person. It accepts payments for services such as utilities, credit cards and mobile phones. The company's Web site can be used to pay bills too, however.

This isn't the first time Network Solutions' account credentials have been used to seize control of a Web site. In May, hackers used a similar technique to knock Comcast.net off-line for several hours.