A noted security researcher Monday disclosed four new zero-day vulnerabilities in Microsoft and Mozilla's browsers, including a critical flaw in Internet Explorer (IE) and a "major" bug in Firefox.
Michael Zalewski, who regularly publishes browser flaw findings, posted details on the Full-disclosure mailing list for cookie-stealing, keystroke-snooping, malicious downloading and site-spoofing bugs.
The most serious of the four, said Zalewski, is an IE6 and IE7 flaw he rated "critical." Dubbing it a "bait-and-switch" vulnerability, he said that the Microsoft browser gives hackers a window of opportunity to run malicious JavaScript to hijack the PC.
"The entire security model of the browser collapses like a house of cards and renders you vulnerable to a plethora of nasty attacks," Zalewski claimed in notes that accompanied a demonstration of the IE bug. Up-to-date IE6 and IE7 are both at risk, he said, although Firefox is not.
But Mozilla's browser also suffered at Zalewski's hands. A new IFrame vulnerability in Firefox 2.0 can let attackers plant keyloggers or drop malicious content into a legitimate Web site. The flaw, rated as "major," is related to a similar bug discovered last year; although Mozilla patched that problem, Zalewski said the fix hadn't plugged all the holes.
Zalewski posted information about two other bugs, both rated "medium." A Firefox vulnerability could lead to unauthorized downloads, while IE6 is open to yet another address bar-spoofing flaw. "IE7 is not affected because of certain high-level changes in the browser," Zalewski said of the fourth vulnerability.
Mozilla is aware of both Firefox bugs -- they have been posted to its Bugzilla management system -- and a Microsoft spokeswoman said the company's security team is looking into Zalewski's claims. "Upon completion of this investigation, Microsoft will take the appropriate action, which may include issuing a security advisory or providing a security update," she added.
Microsoft also said it knows of no ongoing attacks using the vulnerabilities.