A flaw in Mozilla's Firefox browser can help cybercriminals steal user information on sites where users create their own pages, such as MySpace.com.

The flaw lies in Firefox's Password Manager software, which can be tricked into sending password information to an attacker's site, said Robert Chapin, president of Chapin Information Services. For this attack to work, attackers need to be able to create HTML (Hypertext Markup Language) forms on the site, something that is allowed on blogging and social networking sites.

The attack was used in a MySpace phishing attack reported in late October. In that attack, users registered a MySpace account named login_home_index_html and used it to host a fake log-in page that exploited the flaw.

This page sent MySpace username and password information to another site, and MySpace users who visited the page using Firefox could have easily had their information compromised, said Chapin.

Firefox developers rate this bug critical, according to an entry in the project's Bugzilla database.

The flaw arises because Firefox's Password Manager does not perform a sufficiently strong check when it is deciding whether to send password information, and then does not ensure that password information is being sent to the server that requested it, Chapin said. In the MySpace attack, for example, Firefox would check to see if the form was coming from the MySpace.com domain, but did not make sure that the password information was being sent back to a MySpace server.

"From a programming point of view, this is almost like a typo," he said. "Ironically I think that's why it hasn't been discovered until now. It was just way too obvious."

Chapin has posted an analysis of this type of attack, which he has dubbed a Reverse Cross-Site Request, as well as a demonstration of how it works.

Internet Explorer (IE) is also susceptible to these types of attacks because, like Firefox, it does not ensure that password information is being sent to the same server that requests it, Chapin said.