Mozilla Firefox before 3.5.16 and 3.6.x before 3.6.13, Thunderbird before 3.0.11 and 3.1.x before 3.1.7, and SeaMonkey before 2.0.11 do not properly validate downloadable fonts before use within an operating system's font implementation, which allows remote attackers to execute arbitrary code via vectors related to @font-face Cascading Style Sheets (CSS) rules.