Vulnerability Instance

Security researchers Adam Barth and Collin
Jackson reported that when a file: resource is
loaded via the location bar it inherits the principal of the
previously loaded document.  This vulnerability can potentially give
the newly loaded document additional privileges to access the contents
of other local files that it wouldn't otherwise have permission to read.
A potential victim would first have to have downloaded the attackers
document to their local machine. Then the victim would have to open another
document in a directory of interest to the attacker before opening the
attacker's file in the same window.
Prior to version 3.0, Firefox (like browsers from other
vendors) treated all local files as having the same origin without
restriction. This vulnerability is a partial bypass of the restrictions
implemented in Firefox 3.0