Mozilla is working on patching its Firefox browser after a hacker posted details of a flaw that could let criminals run unauthorized software on a victim’s machine.

The flaw lies in Firefox’s URL handler component, which was the source of another bug Mozilla disclosed Tuesday.

This second flaw was disclosed Tuesday by Billy Rios and Nathan McFeters, security consultants with Verisign and Ernst & Young respectively.

Like the first flaw, this one could be exploited by attackers to launch programs on the victim’s PC without authorization, said Tyler Reguly, a security research engineer at nCircle Network Security. “They’re both related to the URL handling process,” he said “It’s just different errors within that handling process.”

Even though the code posted by Rios and McFeters can only be used to launch software that is already installed on a victim’s PC, it could be very dangerous if used by criminals, Reguly said. “It’s still letting you run any program that exists on the user’s computer,” he said. “You can make it do some fairly bad things. For example, having it use command-line FTP to download a malicious file off a server somewhere and then execute that file.”

A victim would have to be tricked into clicking on a malicious link for the attack to work, Reguly said.

Mozilla’s security chief, Window Snyder, said that her team is working to verify and fix this latest flaw.

Firefox’s URL handler has been a headache for Mozilla ever since security researcher Thor Larholm showed that the way IE (Internet Explorer) and Firefox interact with each other could be exploited to launch software on a user’s machine without authorization. To make the attack work, IE would load malformed data from a Web site and would then send it to Firefox, which would launch the unauthorized software.

Microsoft and Mozilla disagreed about who was at fault, however. Snyder initially said that the attack wouldn’t work on Firefox alone and that Microsoft should change the way IE passes malformed data to other programs. Microsoft said that the problem lay with Firefox.

While disclosing details on the first URL handler bug on Tuesday, Snyder admitted that she was wrong. “We thought this was just a problem with IE. It turns out, it is a problem with Firefox as well,” she said. “We should have caught this scenario.”

Mozilla is planning to fix this issue in the upcoming 2.0.0.6 release of its browser. Snyder did not say when the Billy Rios bug would be patched.

Thor Larholm, A security researcher has found a security bug that could be attacked in Internet Explorer. Mozilla said it plans to patch the problem in its next Firefox software update. No, that’s not a typo, just the strange fall-out from an unusual bug that had security researchers debating the question this week: “Who’s to blame? Microsoft or Mozilla?”

Thor Larholm kicked off the controversy yesterday, claiming that he had discovered a flaw that would let an attacker run commands on a victim’s PC.

In his blog posting, Thor Larholm said the bug was similar to a flaw he’d discovered last month in Apple’s Safari 3.0 beta software, and he called it an “input validation flaw in Internet Explorer”. The problem is with a URL protocol handler component of Internet Explorer, he said. This software allows Internet Explorer users to launch applications such as Excel or Firefox by clicking on specially written links on web pages.

When Internet Explorer clicks on a link that launches the Firefox browser, however, the software does not properly check its syntax, and that, Larholm said, lets an attacker create a malicious link, that could be used in an attack. Security vendor Secunia ApS rates the flaw as ‘highly critical’.

So while the flaw affects Internet Explorer users, it appears to be a risk only to those who already have Firefox installed. And to make matters more complicated, if a Firefox user were to click on one of the specially-written links, he would not be affected.

Read more at Larholm.com.

A noted security researcher 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.

Source: PCWorld