User interaction is required in that a target must be coerced into visiting a malicious page. The specific flaw exists within Webkit's process for destructing attribute objects via the removeChild method. If an attribute's child object is accessed after the attribute was removed from the document, an invalid pointer is referenced. This can be exploited by an attacker to execute remote code under the context of the user running the browser.
Patch Availability:
Apple has issued an update to correct this vulnerability. More details can be found at: http://support.apple.com/kb/HT4196