Mozilla Foundation Security Advisory 2013-106

Character encoding cross-origin XSS attack

December 10, 2013
Masato Kinugawa
Firefox, SeaMonkey
Fixed in
  • Firefox 26
  • SeaMonkey 2.23


Security researcher Masato Kinugawa discovered that if a web page is missing character set encoding information it can inherit character encodings across navigations into another domain from an earlier site. Only same-origin inheritance is allowed according to the HTML5 specification. This issue allows an attacker to add content that will be interpreted one way on the victim site, but which may then behave differently, evading cross-site scripting (XSS) filtering, when forced into an unexpected character set. Web site authors should always explicitly declare a character encoding to avoid similar issues.

In general these flaws cannot be exploited through email in the Thunderbird and Seamonkey products because scripting is disabled, but are potentially a risk in browser or browser-like contexts.