WebKit in Apple Safari before 5.0.1 on Mac OS X 10.5 through 10.6 and Windows, and before 4.1.1 on Mac OS X 10.4; and webkitgtk before 1.2.6; accesses uninitialized memory during processing of the (1) :first-letter and (2) :first-line pseudo-elements in an SVG text element, which allows remote attackers to execute arbitrary code or cause a denial of service (application crash) via a crafted document.
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.