Let Multicast source = Foobarnews which is streaming media to listeners. Multicasting's only check thusfar is RPF (reverse path forwarding). Routers check that TTL is set to one, and the source is coming in from the correct interface. Those are the only checks I can see... This means someone on network 2 will never be able to reach someone on network 5. The TTL would expire, or would be ignored. Any TTL over one, is dropped... Attacker and 3 are synonomous, which according to all logic should be able to spoof a Multicast router's information. Provided their TTL being passed to network 4 is set to 1, and they're coming from the same interface as network 1.

RFC 4732...

2.2.2.  IP Multicast-based DoS Attacks

   There are essentially two forms of IP multicast: traditional Any-
   Source Multicast (ASM), as specified in RFC 1112 [4] where multiple
   sources can send to the same multicast group, and Source-Specific
   Multicast (SSM) where the receiver must specify both the IP source
   address and the group address.  The two forms of multicast provide
   rather different DoS possibilities.

   ASM protocols such as PIM-SM [6], MSDP [32], and DVMRP [12] typically
   cause some routers to instantiate routing state at the time a packet
   is sent to a multicast group.  They do this to ensure that the
   traffic goes to the group receivers and not to non-receivers.  Such
   protocols are particularly vulnerable to DoS attacks, as an attacker
   that sends to many multicast groups may cause both multicast routing
   table explosion (and hence control processor memory exhaustion) and
   multicast forwarding table exhaustion (and hence forwarding card
   memory exhaustion or thrashing).

   ASM also permits an attacker to send traffic to the same group as
   legitimate traffic, potentially causing network congestion and
   denying service to the legitimate group.

   SSM does not permit senders to send to arbitrary groups unless a
   receiver has requested the traffic.  Thus, sender-based attacks on
   multicast routing state are not possible with SSM.  However, as with
   ASM, a receiver can still join a large number of multicast groups
   causing routers to hold a large amount of multicast routing state,
   potentially causing memory exhaustion and hence denial-of-service to
   legitimate traffic.

   With IPv6, hosts are required to send ICMP Packet Too Big or
   Parameter Problem messages under certain circumstances, even if the
   destination address is a multicast address.  If the attacker can
   place himself in the appropriate position in the multicast tree, a
   packet with an unknown but mandatory Destination Option, for
   instance, could generate a very large number of responses to the
   claimed sender.

   With IPv4, the same problem exists with multicast ICMP Echo Request
   packets, but these are somewhat easier to filter.

   The examples above should not be taken as exhaustive.  These are
   actually specific cases of a general problem that can happen when a
   multicast/broadcast request solicits a reply from a large number of
   nodes.