The Ubuntu AccountsService package before 0.6.14-1git1ubuntu1.1 does not properly drop privileges when changing language settings, which allows local users to modify arbitrary files via unspecified vectors.
https://euvd.enisa.europa.eu/vulnerability/EUVD-2011-4335
http://www.ubuntu.com/usn/USN-1351-1
http://people.canonical.com/~ubuntu-security/cve/2011/CVE-2011-4406.html