Cross-site scripting allows an attacker to inject code into a Web site so that certain actions execute the code. The result can be anything from a harmless pop-up window to exposure to malicious software.
Zulfikar Ramzan, a senior researcher with Symantec Security Response, said that any time a site allows users to submit content, there's a risk that someone may submit malicious code.
XSSed.com lists two of the vulnerabilities, one on each candidate's site, as "unfixed" as of 2 p.m. PST on Tuesday, April 22.
One of the vulnerabilities reported on XSSed.com surfaced over the weekend and has since been fixed. It allowed an unknown hacker to redirect visitors who viewed the Community Blogs section on Barack Obama's Web site to rival Hillary Clinton's Web site. Two other vulnerabilities now listed only on mirror sites also appear to have been fixed.
Security vendors Netcraft and Symantec have both published blog posts about the incident. And Zenophon "Zennie" Abraham, founder and CEO of Sports Business Simulations, has posted a video demonstrating the hack on YouTube.
A Barack Obama campaign spokesperson did not immediately respond to a request for comment.
However, a person posting under the name "Mox" on the Obama Community Blog took credit for hacking BarackObama.com.
It is "Mox" who also takes credit for posting three of the XSS vulnerabilities on BarackObama.com and the one on HillaryClinton.com. The fourth XSS hole on BarackObama.com was posted by someone using the name "C1c4Tr1Z."
Ramzan said that while the brief redirection of visitors to the Barack Obama site wasn't particularly serious, an XSS vulnerability of this sort could potentially be exploited to inflict more significant harm. He suggested that a fake fund-raising solicitation window could be launched this way and that it would probably fool a lot of people because it would appear to be part of the official Barack Obama Web site.