Firefox 1.5.0.7 and 2.0, and Seamonkey 1.1b, allows remote attackers to cause a denial of service (crash) by creating a range object using createRange, calling selectNode on a DocType node (DOCUMENT_TYPE_NODE), then calling createContextualFragment on the range, which triggers a null dereference.  NOTE: the original Bugtraq post mentioned that code execution was possible, but followup analysis has shown that it is only a null dereference.