Vulnerability Discussion

ICMP redirect messages are used by routers to inform hosts that a more direct route exists for a particular destination. These messages modify the host's route table and are unauthenticated. An illicit ICMP redirect message could result in a man-in-the-middle attack.

This feature of the IPv4 protocol has few legitimate uses. It must be disabled unless absolutely required.

Check

Verify RHEL 9 will not accept IPv4 ICMP redirect messages.

Check the value of the default "accept_redirects" variables with the following command:

$ sudo sysctl net.ipv4.conf.default.accept_redirects

net.ipv4.conf.default.accept_redirects = 0

If the returned line does not have a value of "0", a line is not returned, or the line is commented out, this is a finding.

Check that the configuration files are present to enable this network parameter.

$ sudo /usr/lib/systemd/systemd-sysctl --cat-config | egrep -v '^(#|;)' | grep -F net.ipv4.conf.default.accept_redirects | tail -1

net.ipv4.conf.default.accept_redirects = 0

If "net.ipv4.conf.default.accept_redirects" is not set to "0" or is missing, this is a finding.

Fix

Configure RHEL 9 to prevent IPv4 ICMP redirect messages from being accepted.

Add or edit the following line in a single system configuration file, in the "/etc/sysctl.d/" directory:

net.ipv4.conf.default.accept_redirects = 0

Load settings from all system configuration files with the following command:

$ sudo sysctl --system