Vulnerability Discussion

Source-routed packets allow the source of the packet to suggest routers forward the packet along a different path than configured on the router, which can be used to bypass network security measures. This requirement applies only to the forwarding of source-routed traffic, such as when IP forwarding is enabled and the system is functioning as a router.

Accepting source-routed packets has few legitimate uses. It must be disabled unless it is absolutely required.

Check

Verify AlmaLinux OS 9 will not accept source-routed packets.

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

$ sysctl -a | grep accept_source_route

net.ipv4.conf.all.accept_source_route = 0
net.ipv4.conf.default.accept_source_route = 0
net.ipv4.conf.enp1s0.accept_source_route = 0
net.ipv4.conf.lo.accept_source_route = 0
net.ipv6.conf.all.accept_source_route = 0
net.ipv6.conf.default.accept_source_route = 0
net.ipv6.conf.enp1s0.accept_source_route = 0
net.ipv6.conf.lo.accept_source_route = 0

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

Fix

Configure AlmaLinux OS 9 to ignore source-routed packets.

Create a numbered *.conf file in /etc/sysctl.d/ with the following content:

net.ipv4.conf.all.accept_source_route = 0
net.ipv4.conf.default.accept_source_route = 0
net.ipv4.conf.lo.accept_source_route = 0
net.ipv6.conf.all.accept_source_route = 0
net.ipv6.conf.default.accept_source_route = 0
net.ipv6.conf.lo.accept_source_route = 0

The system configuration files must be reloaded for the changes to take effect. To reload the contents of the files, run the following command:

$ sysctl –system