Most Internet traffic is unicast, a single source sending to a single destination. Multicast permits members of a group to communicate with each other by sending and receiving data to and from the group's multicast address. Hosts join multicast groups by notifying multicast-enabled routers that they want to receive packets sent to that group's address. The router joins the group on behalf of the original client and multicast packets are transmitted only to the requesting nodes. Securing unicast traffic isn't easy, even with the years of research and experience backing IPSec.
Attackers can spoof a multicast stream and replace a business meeting's video feed with a fake.
By configuring their system to pretend to be a multicast traffic originator, attackers can inject their multicast stream into local routers. Or, they can simply join a group without authorization. Because multicast traffic can be widely distributed, it offers a tantalizing target for DoS attacks against any or all members of a multicast group. Multicast means that you're sending a single packet out which the multicast routers, by definition, multiply and send to multiple users. If an attacker wanted to crash lots of routers and networks, he could just send huge volumes of malicious data with a relatively small effort.