Online bill pay giant CheckFree.com said the hijacking of its Web site this month affected an estimated 160,000 people, a disclosure that offers the most detailed account yet of the true size and scope of a brazen type of attack that experts say may become more common in 2009. In a filing with Wisconsin’s Office of Privacy Protection, CheckFree said at least 160,000 people may have visited the site during the nine-hour period it was hijacked, which had redirected visitors to a site in Ukraine. An analysis of that Ukranian site indicated that it was trying to exploit known security flaws in Adobe Acrobat and Adobe Reader, in an attempt to install a variant of the the Gozi Trojan, which is among the most sophisticated password-stealing programs in use today. CheckFree controls between 70 to 80 percent of the U.S. online bill pay market. Among the 330 kinds of bills

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