One Layer 2 exploit is a content-addressable memory (CAM) table flood, which allows an attacker to make a switch act like a hub. A hub forwards all packets to all ports. A switch learns about Ethernet MAC addresses at each of its ports so that it can forward packets only to the port that provides a link to the destination address of the packet. In a heavily switched environment, an attacker receives only packets destined for the attacker. By exploiting a CAM table flood, the attacker can cause the switch to forward all packets to all destinations, allowing the attacker to sniff all traffic.
The mapping of each MAC address to each physical port is contained in the CAM table within the switch. However, the CAM table has a limited number of entries, which means an attacker can cause the table to overflow by sending the switch a flood of Ethernet packets containing random spoofed source addresses. As a result, the switch might discard old, but valid, entries from the table to accommodate the flood of new mappings from the hacker. In this attack mode, the hacker causes the switch to "push out" valid CAM table entries. When a switch attempts to forward a packet, if the MAC address of the packet is not in the CAM table, the switch acts like a hub and forwards the packet to all ports on the switch. Attackers can use CAM table flooding to force a switch to act like a hub, allowing the attacker to sniff packets that would normally go only to a different port.
Solution: Port security is a feature on Cisco switches that limits the number of allowable source MAC addresses per port. Port security can statically assign a list of MAC addresses per port, or it can limit the total number of MAC addresses allowed per port.
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