MD5 Neighbor Authentication in Extended Interior Gateway Routing Protocol (EIGRP) 1.2, as implemented in Cisco IOS 11.3 and later, does not include the Message Authentication Code (MAC) in the checksum, which allows remote attackers to sniff message hashes and (1) replay EIGRP HELLO messages or (2) cause a denial of service by sending a large number of spoofed EIGRP neighbor announcements, which results in an ARP storm on the local network.