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; does not properly handle just-in-time (JIT) compiled JavaScript stubs, which allows remote attackers to execute arbitrary code or cause a denial of service (application crash) via a crafted HTML document, related to a "reentrancy issue."