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 RHEL 8 ignores ICMP redirect messages.

Note: If either IPv4 or IPv6 is disabled on the system, this requirement only applies to the active internet protocol version.

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

$ sudo sysctl net.ipv4.conf.all.accept_redirects net.ipv6.conf.all.accept_redirects

net.ipv4.conf.all.accept_redirects = 0
net.ipv6.conf.all.accept_redirects = 0

If both of the returned lines do not have a value of "0", or a line is not returned, this is a finding.

Fix

Configure RHEL 8 to ignore ICMP redirect messages with the following commands:

$ sudo sysctl -w net.ipv4.conf.all.accept_redirects=0

$ sudo sysctl -w net.ipv6.conf.all.accept_redirects=0

If "0" is not the system's default value then add or update the following lines in the appropriate file under "/etc/sysctl.d":

net.ipv4.conf.all.accept_redirects = 0

net.ipv6.conf.all.accept_redirects = 0