The URL parser in Microsoft Internet Information Services (IIS) 5.1 on Windows XP Professional SP2 allows remote attackers to execute arbitrary code via multiple requests to ".dll" followed by arguments such as "~0" through "~9", which causes ntdll.dll to produce a return value that is not correctly handled by IIS, as demonstrated using "/_vti_bin/.dll/*/~0".  NOTE: the consequence was originally believed to be only a denial of service (application crash and reboot).