A cross-site scripting vulnerability in the social networking section of Sen. Barack Obama's campaign site was exploited over the weekend to redirect users to the URL of his rival, Sen. Hillary Clinton, (D-N.Y.), researchers claimed Monday.

According to the UK-based anti-fraud company Netcraft, someone identified only as "Mox" confessed to the hack in an entry on the Community Blogs section on the Obama site Sunday. Obama, an Illinois Democrat, leads Clinton in the race for the party's presidential nomination. The site exploit occurred just before this week's big Pennsylvania primary.

"You may also be wondering, how did you get Hillary's site to appear where Obama's should be?" wrote Mox. "The answer to that is, through the magical world of Cross Site Scripting."

Cross-site scripting vulnerabilities, which are most commonly exploited by identity thieves and phishers, let attackers inject their own malicious code into legitimate pages.

An Obama supporter captured the cross-site scripting hack and the resulting redirect to Clinton's campaign site on video Saturday, and posted it on YouTube. Clicking on the "Community Blogs" link, the video showed, sent users to hillaryclinton.com.

The cross-site scripting bug has been patched, Mox said Sunday.

The Community Blogs section of the Obama site lets supporters create their own blogs, and read other supporters' postings. Users must register on the site to access Community Blogs.

The Obama site isn't out of the clear, however. "While Mox states that the original issue has now been fixed, a number of similar vulnerabilities have since been identified and remain unfixed," said Netcraft's Paul Mutton in an alert on the security company's site Monday.

The additional vulnerabilities mentioned by Mutton were spelled out by Dimitris Pagkalos, a 22-year-old security researcher who co-manages an online archive of sites vulnerable to cross-site scripting attacks. According to Pagkalos, Obama's site harbors two still-unpatched bugs.

Pagkalos also provided more detail on the redirect that Mox implemented over the weekend, noting that the attack used an IFRAME injected into the title parameter of a personal group -- another social networking feature of the Obama site -- that then let Mox remotely call some malicious JavaScript.

The bug, said Pagkalos, could have been used to infect Obama's supporters and site visitors with malware, adware or identity-stealing spyware.

Just over a week ago, Oliver Friedrichs, director of emerging technologies at Symantec and a noted researcher on electoral cybercrime, said the US presidential candidates' campaigns were clueless about the threat to their Web sites. "There's just a general lack of awareness," said Friedrichs in an interview after a presentation he gave no the subject at the RSA Conference.

Obama's campaign did not reply to a request for comment.