Mozilla Foundation Security Advisory 2011-47

Potential XSS against sites using Shift-JIS

November 8, 2011
Yosuke Hasegawa
Firefox, SeaMonkey, Thunderbird
Fixed in
  • Firefox 3.6.24
  • Firefox 8
  • SeaMonkey 2.5
  • Thunderbird 3.1.16
  • Thunderbird 8


Yosuke Hasegawa reported that the Mozilla browser engine mishandled invalid sequences in the Shift-JIS encoding. When encountering an invalid pair Mozilla would turn the entire two-byte sequence into a single unknown character rather than an unknown character followed by a valid single-byte character. On some sites attackers may have been able to end their input with the first byte of a two byte sequence; when that input was later put into a page context it might cause the following delimiter (such as a double-quote) to be consumed, breaking the format of the page. Depending on the page this could potentially be used to steal data or inject script into the page.