The administrator must ensure ICMP unreachable notifications, mask replies, and redirects are disabled on all external interfaces of the premise router.

From Perimeter L3 Switch Security Technical Implementation Guide - Cisco

Part of Filter ICMP on external interface

SV-15318r1_rule The administrator must ensure ICMP unreachable notifications, mask replies, and redirects are disabled on all external interfaces of the premise router.

Vulnerability discussion

The Internet Control Message Protocol (ICMP) supports IP traffic by relaying information about paths, routes, and network conditions. Routers automatically send ICMP messages under a wide variety of conditions. Three ICMP messages are commonly used by attackers for network mapping and diagnosis: Host unreachable, Redirect, and Mask Reply.

Check content

Review the device configuration to determine if controls have been defined to ensure the router does not send ICMP unreachables, redirects, and mask replies out to any external interfaces. If ICMP unreachables notifications, mask replies, and redirects are enabled on external interfaces, this is a finding.

Fix text

Disable ICMP unreachable notifications, mask replies, and redirects on all external interfaces.

Pro Tips

Lavender hyperlinks in small type off to the right (of CSS class id, if you view the page source) point to globally unique URIs for each document and item. Copy the link location and paste anywhere you need to talk unambiguously about these things.

You can obtain data about documents and items in other formats. Simply provide an HTTP header Accept: text/turtle or Accept: application/rdf+xml.