WebKit in Apple Safari before 5.0 on Mac OS X 10.5 through 10.6 and Windows, and before 4.1 on Mac OS X 10.4, does not properly perform ordered list insertions, which allows remote attackers to execute arbitrary code or cause a denial of service (memory corruption and application crash) via a crafted HTML document, related to the insertion of an unspecified element into an editable container and the access of an uninitialized element.
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.