Cisco IOS 12.0 through 12.4 allows remote attackers to cause a denial of service (device crash) via (1) "abnormal" MGCP messages, aka CSCsd81407; and (2) a large facsimile packet, aka CSCej20505.
https://oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A5801
https://exchange.xforce.ibmcloud.com/vulnerabilities/35907
http://www.vupen.com/english/advisories/2007/2816
http://www.securityfocus.com/bid/25239
http://www.cisco.com/en/US/products/products_security_advisory09186a0080899653.shtml
http://securitytracker.com/id?1018533