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Mozilla Foundation Security Advisory 2011-47

Title: Potential XSS against sites using Shift-JIS
Impact: High
Announced: November 8, 2011
Reporter: Yosuke Hasegawa
Products: Firefox, Thunderbird, SeaMonkey

Fixed in: Firefox 8.0
  Firefox 3.6.24
  Thunderbird 8.0
  Thunderbird 3.1.16
  SeaMonkey 2.5


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.