Vulnerability Discussion

Responding to broadcast (ICMP) echoes facilitates network mapping and provides a vector for amplification attacks.

Check

Verify the system does not respond to IPv4 ICMP echoes sent to a broadcast address.

# grep -r net.ipv4.icmp_echo_ignore_broadcasts /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.icmp_echo_ignore_broadcasts" 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 "1", this is a finding.

Check that the operating system implements the "icmp_echo_ignore_broadcasts" variable with the following command:

# /sbin/sysctl -a | grep net.ipv4.icmp_echo_ignore_broadcasts
net.ipv4.icmp_echo_ignore_broadcasts = 1

If the returned line does not have a value of "1", this is a finding.

If conflicting results are returned, this is a finding.

Fix

Set the system to the required kernel parameter 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.icmp_echo_ignore_broadcasts = 1

Issue the following command to make the changes take effect:

# sysctl --system