Information
A compromised host in an enclave can be used by a malicious platform to launch cyberattacks on third parties. This is a common practice in 'botnets', which are a collection of compromised computers using malware to attack other computers or networks. Denial-of-Service attacks frequently leverage IP source address spoofing to send packets to multiple hosts that, in turn, will then send return traffic to the hosts with the IP addresses that were forged. This can generate significant amounts of traffic. Therefore, protection measures to counteract IP source address spoofing must be taken. When uRPF is enabled in strict mode, the packet must be received on the interface that the device would use to forward the return packet, thereby mitigating IP source address spoofing.
Solution
Configure the firewall with an egress filter or uRPF on all internal interfaces to restrict the firewall from accepting any outbound packet that contains an illegitimate address in the source field.
URF Example:
ip verify reverse-path interface INSIDE
ACL Example:
Step 1: Configure an object group containing the allowed subnets as shown in the example below.
ASA(config)# object-group network LAN_SUBNETS
ASA(config-network-object-group)# network-object 10.1.10.0 255.255.255.0
ASA(config-network-object-group)# network-object 10.1.12.0 255.255.255.0
ASA(config-network-object-group)# network-object 10.1.13.0 255.255.255.0
ASA(config-network-object-group)# network-object 10.1.22.0 255.255.255.0
ASA(config-network-object-group)# exit
Step 2: Configure the ACL.
ASA(config)# access-list INSIDE_IN permit ip object-group LAN_SUBNETS any
ASA(config)# access-list INSIDE_IN deny ip any any
Note: Traffic that is permitted must be in compliance with the PPSM.
Step 3: Apply the ACL inbound on the inside interface.
ASA(config)# access-group INSIDE_IN in interface INSIDE