jscript.dll in Microsoft Internet Explorer 6.0 SP1 and earlier allows remote attackers to cause a denial of service (application crash) via a Shockwave Flash object that contains ActionScript code that calls VBScript, which in turn calls the Javascript document.write function, which triggers a null dereference.
http://www.securityfocus.com/archive/1/425422/30/6890/threaded
http://www.securityfocus.com/archive/1/423675/100/0/threaded