This vulnerability allows remote attackers to execute arbitrary code on vulnerable installations of Apple's Webkit. User interaction is required to exploit this vulnerability in that the target must visit a malicious page or open a malicious file.
The specific flaw exists due to the method by which the Webkit library renders right-to-left text. If a linebox has a width greater than it's container, Webkit frees an object that upon page destruction is freed again. An attacker can exploit this to code execute remote code under the context of the application.