This indicates an attack attempt to exploit a code-execution vulnerability in Apple Webkit.
This issue is caused by an error when the vulnerable software handles a particular tag used for embedding a foreign document. It may allow remote attackers to execute arbitrary code by sending a crafted web page.
Safari 4 (Mac OS X 10.4)
Safari 5 (Windows)
Safari 5 (Mac OS X 10.6)
Safari 5 (Mac OS X 10.5)
System Compromise: Remote attackers can gain control of vulnerable systems.
Refer to the vendor's web site for the suggested workaround: