The Cisco router must be configured to drop all fragmented Internet Control Message Protocol (ICMP) packets destined to itself.

Overview

Finding ID	Version	Rule ID	IA Controls	Severity
V-96525	CISC-RT-000140	SV-105663r1_rule		Medium

Description
Fragmented ICMP packets can be generated by hackers for DoS attacks such as Ping O' Death and Teardrop. It is imperative that all fragmented ICMP packets are dropped.

STIG	Date
Cisco IOS Router RTR Security Technical Implementation Guide	2019-07-25

Details

Check Text ( C-95361r1_chk )

Review the external and internal ACLs to verify that the router is configured drop all fragmented destined to itself. ip access-list extended EXTERNAL_ACL permit tcp host x.11.1.1 eq bgp host x.11.1.2 permit tcp host x.11.1.1 host x.11.1.2 eq bgp deny icmp any host x.11.1.2 fragments permit icmp host x.11.1.1 host x.11.1.2 echo … … deny ip any any ! ip access-list extended INTERNAL_ACL deny icmp any host 10.1.12.2 fragments permit icmp any any Note: Ensure the statement to deny ICMP fragments is before any permit statements for ICMP. If the router is not configured to drop all fragmented ICMP packets destined to itself, this is a finding.

Fix Text (F-102201r1_fix)

Configure the external and internal ACLs to drop all fragmented ICMP packets destined to itself as shown in the example below. R1(config)#ip access-list extended EXTERNAL_ACL R1(config-ext-nacl)#deny icmp any host x.11.1.2 fragments R1(config)#ip access-list extended INTERNAL_ACL R1(config-ext-nacl)#deny icmp any host 10.1.12.2 fragments Note: Ensure the above statement is before any permit statements for ICMP.