This indicates an attack attempt against a memory-corruption vulnerability in Apple Webkit.
The vulnerability is caused by an error when the vulnerable software handles a web page with a malformed Text Transformation. It may allow remote attackers to execute arbitrary code by sending a crafted web page.
Apple Safari 4.0.4 and the prior version
System Compromise: Remote attackers can gain control of vulnerable systems.
Refer to the vendor's web site for the suggested workaround: