Although Mozilla patched one more Firefox bug last week than first reported, the researcher whose work has plagued the open-source browser for weeks has released details about another flaw.

Firefox does not properly handle JavaScript "onUnload" events and can be tricked into taking the user to an unintended destination, said security researcher Michal Zalewski. "This flaw allows the attacker to track your footsteps and either redirect you to the URL you wanted to visit, which wouldn't be noticed at all, or to a similarly named phishing website when you choose to visit a target of some significance," Zalewski said.

The bug affects the just-released Firefox and updates, as well as Microsoft's Internet Explorer 7. JavaScript can be disabled in the browsers to block such redirects.

"The big difference in the two browsers is that Firefox displays the correct address for the redirected site in the address bar," Symantec said in a warning. "IE7, however, continues to display the URL that the user typed into the address bar, leading to a false sense of security."

Mozilla fixed 15 flaws last week in Firefox and, as opposed to the 14 that were first reported. An overlooked security update in the revised browsers patches another Zalewski vulnerability, Mozilla said yesterday.

"Firefox update includes fixes for the bugs that researcher Michael Zalewski reported last week, including the hostname vulnerability, cookie issue, and memory corruption issue," said Window Snyder, Mozilla's chief security executive.

"It was just a mistake," a Mozilla spokesman said regarding why Friday's list of patched bugs had originally omitted the 15th fixed flaw. The list has since been changed to reflect all the included patches.