The network element must be configured from accepting any outbound IP packet that contains an illegitimate address in the source address field via egress ACL or by enabling Unicast Reverse Path Forwarding in an IPv6 enclave.

From Infrastructure L3 Switch Secure Technical Implementation Guide - Cisco

Part of IPv6 Egress Outbound Spoofing Filter

SV-15429r1_rule The network element must be configured from accepting any outbound IP packet that contains an illegitimate address in the source address field via egress ACL or by enabling Unicast Reverse Path Forwarding in an IPv6 enclave.

Vulnerability discussion

Unicast Reverse Path Forwarding (uRPF) provides a mechanism for IP address spoof protection. When uRPF is enabled on an interface, the router examines all packets received as input on that interface to make sure that the source address and source interface appear in the routing table and match the interface on which the packet was received. If the packet was received from one of the best reverse path routes, the packet is forwarded as normal. If there is no reverse path route on the same interface from which the packet was received, it might mean that the source address was modified. If Unicast RPF does not find a reverse path for the packet, the packet is dropped.If internal nodes automatically configure an address based on a prefix from a bogus Router Advertisement a dangerous situation may exist. An internal host may contact an internal server which responds with a packet that could be routed outside of the network via default routing (because the routers do not recognize the destination address as an internal address). To prevent this, filtering should be applied to network interfaces between internal host LANs and internal server LANs to insure that source addresses have valid prefixes.

Check content

IOS example interface FastEthernet 1/1 description downstream link to our network ipv6 address 2001:db8:60::f15:60c2 ipv6 verify unicast source reachable-via rx outbound-to-backbone ipv6 access-list outbound-to-backbone remark explicitly block all other packets deny ipv6 any any log

Fix text

The network element must be configured to ensure that an ACL is configured to restrict the router from accepting any outbound IP packet that contains an external IP address in the source field.

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