InformationBy default, OSPF will advertise via multicast to solicit peers, and will listen for neighbor / peer advertisements on all OSPF configured interfaces.
If an interface is set to 'passive', then EIGRP will not advertise out of that interface or listen on that interface for EIGRP neighbors. I will however still advertise the networks associated with that interface to peers on other interfaces. By default, all interfaces advertise via multicast to solicit OSPF neighbors, and also listen for neighbor advertisements.
If an interface is set to the default (ie - not passive), then an attacker can pose as an OSPF peer, either to collect OSPF information from advertisements or to inject bogus routes into the table. Bogus routes can then be used to DOS that subnet, or to intercept traffic to or from that subnet either to eavesdrop on conversations or to modify data in transit.
Quite often the goal of an attack of this type is to collect login credentials from a malicious copy of the target website.
SolutionFor each routeable interface, if there is no facing peer on that interface set that interface to passive with the 'ip ospf passive-interface' configuration command.
Every routeable interface should be set to either passive-interface, or be configured with authentication keys.
switch(config)# int vlan 1
switch(config-if)# ip router ospf 10 area 0
switch(config-if)# ip ospf passive-interface
By default, passive interfaces are not configured - no OSPF configured interfaces are set to passive. This means that OSPF will advertise via multicast to solicit peers, and will listen for neighbor / peer advertisements on all OSPF configured interfaces.