The Cisco perimeter switch must be configured to restrict it from accepting outbound IP packets that contain an illegitimate address in the source address field via egress filter or by enabling Unicast Reverse Path Forwarding (uRPF).

Overview

Finding ID	Version	Rule ID	IA Controls	Severity
V-221091	CISC-RT-000310	SV-221091r622190_rule		High

Description
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. DDoS 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.

Description

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. DDoS 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.

STIG	Date
Cisco NX-OS Switch RTR Security Technical Implementation Guide	2021-03-29

Details

Check Text ( C-22806r409762_chk )
Review the switch configuration to verify uRPF or an egress ACL has been configured on all internal interfaces to restrict the switch from accepting outbound IP packets that contain an illegitimate address in the source address field. uRPF example interface Ethernet2/4 description downstream link to LAN no switchport ip address 10.1.12.1/24 ip verify unicast source reachable-via rx Egress ACL example ip access-list EGRESS_FILTER 10 permit udp 10.1.15.0/24 any eq domain 20 permit tcp 10.1.15.0/24 any eq ftp 30 permit tcp 10.1.15.0/24 any eq ftp-data 40 permit tcp 10.1.15.0/24 any eq www 50 permit icmp 10.1.15.0/24 any 60 permit icmp 10.1.15.0/24 any echo 70 deny ip any any … … … interface Ethernet2/4 description downstream link to LAN no switchport ip access-group EGRESS_FILTER in ip address 10.1.12.1/24 If uRPF or an egress ACL to restrict the switch from accepting outbound IP packets that contain an illegitimate address in the source address field has not been configured on all internal interfaces in an enclave, this is a finding.

Check Text ( C-22806r409762_chk )

Review the switch configuration to verify uRPF or an egress ACL has been configured on all internal interfaces to restrict the switch from accepting outbound IP packets that contain an illegitimate address in the source address field.

uRPF example

interface Ethernet2/4
description downstream link to LAN
no switchport
ip address 10.1.12.1/24
ip verify unicast source reachable-via rx

Egress ACL example

ip access-list EGRESS_FILTER
10 permit udp 10.1.15.0/24 any eq domain
20 permit tcp 10.1.15.0/24 any eq ftp
30 permit tcp 10.1.15.0/24 any eq ftp-data
40 permit tcp 10.1.15.0/24 any eq www
50 permit icmp 10.1.15.0/24 any
60 permit icmp 10.1.15.0/24 any echo
70 deny ip any any
…
…
…
interface Ethernet2/4
description downstream link to LAN
no switchport
ip access-group EGRESS_FILTER in
ip address 10.1.12.1/24

If uRPF or an egress ACL to restrict the switch from accepting outbound IP packets that contain an illegitimate address in the source address field has not been configured on all internal interfaces in an enclave, this is a finding.

Fix Text (F-22795r409763_fix)
Configure the switch to ensure that an egress ACL or uRPF is configured on internal interfaces to restrict the switch from accepting any outbound IP packet that contains an illegitimate address in the source field. The example below enables uRPF. SW1(config)# int e2/4 SW1(config-if)# ip verify unicast source reachable-via rx SW1(config-if)# end