Outbound ICMP Time Exceed messages must be blocked to prevent network discovery by unauthorized users.

From Perimeter Router Security Technical Implementation Guide Cisco

Part of Outbound ICMP Time Exceed messages must be blocked.

SV-15376r4_rule Outbound ICMP Time Exceed messages must be blocked to prevent network discovery by unauthorized users.

Vulnerability discussion

The trace route tool will display routes and trip times on an IP network. An attacker can use trace route responses to create a map of the subnets and hosts behind the perimeter router, just as they could do with pings. The traditional trace route relies on TTL - time exceeded responses from routers along the path and an ICMP port-unreachable message from the target host. In some Operating Systems such as UNIX, trace route will use UDP port 33400 and increment ports on each response. Since blocking these UDP ports alone will not block trace route capabilities along with blocking potentially legitimate traffic on a network, it is unnecessary to block them explicitly. Because trace routes typically rely on ICMP Type 11 - Time exceeded message, the time exceeded message will be the target for implicitly or explicitly blocking outbound from the trusted network.

Check content

Review the device configuration to determine if ACLs block ICMP Type 11 - Time exceeded outbound to untrusted networks (e.g., ISP and other non-DoD networks). If ICMP Type 11 - Time Exceeded is not blocked outbound on the network device, this is a finding. Cisco IOS Example: interface FastEthernet 0/1 description LAN link ip address 10.0.0.0 255.255.255.0 ip access-group 100 in . access-list 100 deny icmp any any time-exceeded log access-list 100 permit icmp any any echo-request access-list 100 permit icmp any any packet-too-big access-list 100 permit icmp any any source-quench access-list 100 deny ip any any log

Fix text

Configure an ACL on the network device to block ICMP Type 11 - Time Exceeded outbound to untrusted networks (e.g., ISP and other non-DoD networks).

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