Firefox leaks info useful to attackers

Some browser add-ons let hackers do preattack reconnaissance

Mozilla's head of security Tuesday confirmed a bug in Firefox that could be used by attackers to scout out a system prior to mounting a more thorough assault.

The flaw, said Window Snyder, Mozilla's chief security officer, is in the browser's chrome protocol -- "chrome" is the Firefox term for its user interface -- as she responded to reports of the vulnerability and the public posting of a proof-of-concept exploit.

Access to a user's machine would be through one of many Firefox extensions packaged in a flat file structure, rather than collected into a single Java archive, or .jar file, said Snyder. Several popular extensions, including Download Statusbar and Greasemonkey, use a flat file structure. "Users are only at risk if they have one of the 'flat' packaged add-ons installed," Snyder said on the Mozilla security blog.

By leading users to a tricked-out Web page, criminals could sniff for information that might be useful in more aggressive attacks, Snyder acknowledged. "A visited attacking page is able to load images, scripts or style sheets from known locations on the disk," she said. "Attackers may use this method to detect the presence of files which may give an attacker information about which applications are installed. This information may be used to profile the system for a different kind of attack."

Firefox developers are working on a patch, according to a thread on Bugzilla, Mozilla's bug-tracking and management site, but a fix has not yet been coded.

In the meantime, the authors of the two extensions that Snyder called out -- Download Statusbar and Greasemonkey -- have updated their works so that they cannot be exploited. "I just released a JARred version of Download Statusbar 0.9.5.3," said Devon Johnson on Bugzilla.

Firefox users can also deploy another add-on, the popular NoScript, to block exploits, no matter which extensions have been installed, updated or not. "[NoScript] block[s] chrome JavaScript load attempts," reported Giorgio Maone, NoScript's maker, on the same Bugzilla thread.

Although Snyder downplayed the threat posed by the bug, Gerry Eisenhauer, the researcher who uncovered the vulnerability, said there might be more to it. "This looks very interesting and may have bigger potential," he said Saturday in his original write-up. "But for now, it's just another information disclosure."

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Gregg Keizer

Computerworld
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