The line-breaking implementation in 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 on Windows does not properly handle long strings, which allows remote attackers to execute arbitrary code via a crafted document.write call that triggers a buffer over-read.
The product performs operations on a memory buffer, but it reads from or writes to a memory location outside the buffer's intended boundary. This may result in read or write operations on unexpected memory locations that could be linked to other variables, data structures, or internal program data.