Cisco IOS 2.2(18)EW, 12.2(18)EWA, 12.2(14)SZ, 12.2(18)S, 12.2(18)SE, 12.2(18)SV, 12.2(18)SW, and other versions without the "no service dhcp" command, keep undeliverable DHCP packets in the queue instead of dropping them, which allows remote attackers to cause a denial of service (dropped traffic) via multiple undeliverable DHCP packets that exceed the input queue size.
https://oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A5632
https://exchange.xforce.ibmcloud.com/vulnerabilities/18021
http://www.us-cert.gov/cas/techalerts/TA04-316A.html
http://www.kb.cert.org/vuls/id/630104
http://www.cisco.com/warp/public/707/cisco-sa-20041110-dhcp.shtml