Windows NT 4.0 allows remote attackers to cause a denial of service (crash) via extra source routing data such as (1) a Routing Information Field (RIF) field with a hop count greater than 7, or (2) a list containing duplicate Token Ring IDs.
http://www.iss.net/security_center/static/1399.php
http://support.microsoft.com/support/kb/articles/Q179/1/57.asp