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 within the setOuterText method of the Webkit htmlelement library. Due to a failure to properly track DOM manipulations made within the browser, it is possible to make use of a previously freed pointer and facilitate remote code execution under the context of the user running the browser process.
Patch Availability:
Apple has issued an update to correct this vulnerability. More details can be found at: http://support.apple.com/kb/HT4554