WebKit in Apple Safari before 5.0.3 on Mac OS X 10.5 through 10.6 and Windows, and before 4.1.3 on Mac OS X 10.4, does not properly perform a cast of an unspecified variable during processing of Cascading Style Sheets (CSS) boxes, which allows remote attackers to execute arbitrary code or cause a denial of service (application crash) via a crafted web site.