The Cisco perimeter switch must be configured to deny network traffic by default and allow network traffic by exception.

Overview

Finding ID	Version	Rule ID	IA Controls	Severity
V-221087	CISC-RT-000240	SV-221087r622190_rule		High

Description

A deny-all, permit-by-exception network communications traffic policy ensures that only connections that are essential and approved are allowed. This requirement applies to both inbound and outbound network communications traffic. All inbound and outbound traffic must be denied by default. Firewalls and perimeter switches should only allow traffic through that is explicitly permitted. The initial defense for the internal network is to block any traffic at the perimeter that is attempting to make a connection to a host residing on the internal network. In addition, allowing unknown or undesirable outbound traffic by the firewall or switch will establish a state that will permit the return of this undesirable traffic inbound.

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

Details

Check Text ( C-22802r409750_chk )

Review the switch configuration to verify that the inbound ACL applied to all external interfaces is configured to allow specific ports and protocols and deny all other traffic. Step 1: Verify that an inbound ACL is applied to all external interfaces as shown in the example below: interface Ethernet1/2 no switchport ip access-group EXTERNAL_ACL in ip address x.11.1.2 255.255.255.254 Step 2: Review inbound ACL to verify that it is configured to deny all other traffic that is not explicitly allowed. ip access-list EXTERNAL_ACL 10 permit tcp x.11.1.1/32 eq bgp x.11.1.2/32 20 permit tcp x.11.1.1/32 x.11.1.2/32 eq bgp 30 permit icmp x.11.1.1/32 x.11.1.2/32 echo 40 permit icmp x.11.1.1/32 x.11.1.2/32 echo-reply 50 permit tcp any x.11.2.3/32 eq www 60 permit … … … … 90 deny ip any any log If the ACL is not configured to allow specific ports and protocols and deny all other traffic, this is a finding. If the ACL is not configured inbound on all external interfaces, this is a finding.

Fix Text (F-22791r409751_fix)

Step 1: Configure an inbound ACL to deny all other traffic by default as shown in the example below: SW1(config)# ip access-list EXTERNAL_ACL SW1(config-acl)# permit tcp host x.11.1.1 eq bgp host x.11.1.2 SW1(config-acl)# permit tcp host x.11.1.1 host x.11.1.2 eq bgp SW1(config-acl)# permit icmp host x.11.1.1 host x.11.1.2 echo SW1(config-acl)# permit icmp host x.11.1.1 host x.11.1.2 echo-reply SW1(config-acl)# permit tcp any x.11.2.3/32 eq www SW1(config-acl)# permit … … … … SW1(config-acl)# deny ip any any log Step 2: Apply the ingress filter to all external interfaces. SW1(config)#int e1/2 SW1(config-if)#ip access-group EXTERNAL_ACL in