Apple has issued Safari 5.0.2 and Safari 4.1.2 to fix two issues for Mac where “Visiting a maliciously crafted website may lead to an unexpected application termination or arbitrary code execution.” (There is also a fix only for the Windows version.) The update also fixes a few bugs, notably using encrypted, authenticated connections to the new Apple Safari Extensions Gallery.
This is a 39 MB (for version 5.0.2) or 30 MB (version 4.1.2) update, and is available via Software Update or from Apple’s Downloads page. Full information about the security fixes is available here.