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, accesses uninitialized memory during the handling of a use element in an SVG document, which allows remote attackers to execute arbitrary code or cause a denial of service (application crash) via a crafted document containing XML that triggers a parsing error, related to ProcessInstruction.