rwho daemon rwhod in FreeBSD 4.2 and earlier, and possibly other operating systems, allows remote attackers to cause a denial of service via malformed packets with a short length.
https://exchange.xforce.ibmcloud.com/vulnerabilities/6229
http://www.securityfocus.com/bid/2473
http://archives.neohapsis.com/archives/freebsd/2001-03/0163.html