Nasty Firefox URI-Handling Bugs Crunched

Mozilla has fixed serious URI-handling holes in Firefox that, if left unpatched, leave a system open to hijacking.

Mozilla has fixed serious URI-handling holes in Firefox that, if left unpatched, leave a system open to hijacking. The maker of the open-source browser is "strongly" recommending that all Firefox users upgrade immediately.

Firefox isnt alone in suffering from these browser bugs—Netscape Navigator is also vulnerable.

July brought two sets of URI-handling headaches to Mozilla. First, security researcher Thor Larholm found a URI-handling issue in what he initially called a zero-day IE vulnerability when Firefox and Internet Explorer run together on a single system. Microsoft, of Redmond, Wash., earlier in July pointed the finger of blame at Firefox, and much blame-passing ensued.

Mozilla Security Chief Window Snyder owned up to the issue July 23, saying that Mozilla had found a new scenario over the preceding weekend in which Firefox could be used as an attack entry point in various ways. Specifically, while browsing with Firefox, Snyder said, a malicious URL could be used to pass along bad data to another application.

The problems arise from an input-validation error that can allow remote attackers to execute arbitrary commands on a victim system, through processes such as "cmd.exe," by employing various URI handlers.

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As enumerated by Billy Rios, the security researcher credited with finding the bug, the browsers register URL protocol handlers named "Mailto," "nntp," "news," "snews" and "telnet" upon installation. This facilitates application launch in the browser. But, because of a lack of input sanitation, attackers can execute applications on a vulnerable computer with the privileges of the targeted user.

Mozilla Firefox 2.0.0.5, 3.0a6 and Netscape Navigator 9 are other vulnerable versions of these browsers, and other vendors browsers may also be affected.

In a Deepsight alert to its customers July 31, Symantec, of Cupertino, Calif., outlined this possible attack scenario: First, an attacker constructs malicious links to pass arguments or parameters for an external application that will run when the URI is loaded. The attacker then plants the malicious link on a Web site or sends it through HTML e-mail or by other means.

If successful, the attacker then executes an arbitrary application. First, an attacker would launch the command line, then could pass arbitrary arguments to the command shell that would then launch other applications.

Symantec noted that a successful attack does rely on user interaction: a victim has to follow a link to a malicious site or must open a malicious e-mail.

Theres no shortage of proofs of concept on this one. Heres a Telnet PoC, for example.

If a user cant download the fix immediately, Symantec suggested avoiding following links from untrusted sources, not opening unsolicited HTML e-mail (or disable support for HTML e-mail), and just staying away from unfamiliar sites in general. Also, run software as a nonprivileged user with minimal access rights, and perform nonadministrative tasks—i.e., reading e-mail and browsing—as an unprivileged user with minimal access rights.

The Firefox update, Version 2.0.0.6, is available for Windows, Mac and Linux and can be downloaded here. Users on Firefox 2.0.0.x will be getting an automated update notification within 24 to 48 hours, or the update can be manually downloaded by selecting "check for updates" in Firefoxs Help menu.

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