Integer signedness error in the pixman_fill_sse2 function in pixman-sse2.c in Pixman, as distributed with Cairo and used in Mozilla Firefox before 20.0, Firefox ESR 17.x before 17.0.5, Thunderbird before 17.0.5, Thunderbird ESR 17.x before 17.0.5, SeaMonkey before 2.17, and other products, allows remote attackers to execute arbitrary code via crafted values that trigger attempted use of a (1) negative box boundary or (2) negative box size, leading to an out-of-bounds write operation.