Vulnerability Discussion

Source-routed packets allow the source of the packet to suggest that 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 IPv4 forwarding is enabled and the system is functioning as a router.

Check

Verify the system does not accept IPv4 source-routed packets by default.

# grep net.ipv4.conf.default.accept_source_route /etc/sysctl.conf /etc/sysctl.d/*
net.ipv4.conf.default.accept_source_route = 0

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

Check that the operating system implements the accept source route variable with the following command:

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

If the returned line does not have a value of "0", 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.conf.default.accept_source_route = 0

Issue the following command to make the changes take effect:

# sysctl --system