Security researcher echo reported that a web page could open a window with
an about:blank location and then inject an <isindex> element into that page
which upon submission would redirect to a chrome: document. The effect of
this defect was that the original page would wind up with a reference to a
chrome-privileged object, the opened window, which could be leveraged for
privilege escalation attacks.

Mozilla security researcher moz_bug_r_a4 provided proof-of-concept code
demonstrating how the above vulnerability could be used to run arbitrary
code with chrome privileges.