Software Engineering Institute

On Microsoft Windows systems, Mozilla Firefox installs protocol handlers for Mozilla-specific protocols, such as the FirefoxURL and FirefoxHTML protocols. If Microsoft Windows encounters a URL protocol it cannot handle natively it searches the Windows registry for the appropriate protocol handler. When the correct protocol handler is found, Windows passes it the URL string. By design Windows passes the URL string to the protocol handler as it was received without performing any sanitization.

Mozilla Firefox URL protocol handlers are constructed in a way that may allow a remote attacker to execute arbitrary JavaScript with elevated (chrome) privileges. Mozilla Firefox security restrictions detect and prevent the execution of such JavaScript. However, if a remote attacker can persuade a user with Firefox installed to access a specially crafted web page using Internet Explorer, and perhaps other Windows applications, the malicious JavaScript will be executed. Reports claim this vulnerability is introduced when Firefox versions 2.0.0.2 and later are installed.

Exploit code is publicly available for this vulnerability.

This vulnerability may allow a remote, unauthenticated attacker to execute JavaScript with elevated (chrome) privileges. This essentially means that the attacker can execute arbitrary code with the privileges of the user.

Update
Mozilla has addressed this vulnerability in Firefox 2.0.0.5.

Unregister the Firefox protocols

Disabling the Mozilla Firefox protocol handlers will mitigate this vulnerability. To unregister the protocol handlers, delete or rename the following registry keys:

HKEY_CLASSES_ROOT\FirefoxURL
HKEY_CLASSES_ROOT\FirefoxHTML
Modifying the Windows registry may have unintended consequences and should be done with care.