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.

Check

Verify the system will not accept IPv4 ICMP redirect messages.

# grep -r net.ipv4.conf.default.accept_redirects /run/sysctl.d/* /etc/sysctl.d/* /usr/local/lib/sysctl.d/* /usr/lib/sysctl.d/* /lib/sysctl.d/* /etc/sysctl.conf 2> /dev/null

If "net.ipv4.conf.default.accept_redirects" is not configured in the /etc/sysctl.conf file or in any of the other sysctl.d directories, is commented out, or does not have a value of "0", this is a finding.

Check that the operating system implements the value of the "accept_redirects" variables with the following command:

# /sbin/sysctl -a | grep net.ipv4.conf.default.accept_redirects
net.ipv4.conf.default.accept_redirects = 0

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

If conflicting results are returned, this is a finding.

Fix

Set the system to not accept IPv4 ICMP redirect messages by adding the following line to "/etc/sysctl.conf" or a configuration file in the /etc/sysctl.d/ directory (or modify the line to have the required value):

net.ipv4.conf.default.accept_redirects = 0

Issue the following command to make the changes take effect:

# sysctl --system