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 forwarding is enabled and the system is functioning as a router.

The sysctl --system command will load settings from all system configuration files. All configuration files are sorted by their filename in lexicographic order, regardless of which of the directories they reside in. If multiple files specify the same option, the entry in the file with the lexicographically latest name will take precedence. Files are read from directories in the following list from top to bottom. Once a file of a given filename is loaded, any file of the same name in subsequent directories is ignored.
/etc/sysctl.d/*.conf
/run/sysctl.d/*.conf
/usr/local/lib/sysctl.d/*.conf
/usr/lib/sysctl.d/*.conf
/lib/sysctl.d/*.conf
/etc/sysctl.conf

Based on the information above, if a configuration file that begins with "99-" is created in the "/etc/sysctl.d/" directory, it will take precedence over any other configuration file on the system.

Check

Verify RHEL 8 does not accept IPv4 source-routed packets by default.

Note: If IPv4 is disabled on the system, this requirement is Not Applicable.

Check the value of the accept source route variable with the following command:

$ sudo sysctl 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", a line is not returned, or the line is commented out, this is a finding.

Check that the configuration files are present to enable this network parameter.

$ sudo grep -r net.ipv4.conf.default.accept_source_route /etc/sysctl.d/*.conf

/etc/sysctl.d/99-sysctl.conf: net.ipv4.conf.default.accept_source_route = 0

If "net.ipv4.conf.default.accept_source_route" is not set to "0", is missing or commented out, this is a finding.

If the configuration file does not begin with "99-", this is a finding.

Fix

Configure RHEL 8 to not forward IPv4 source-routed packets by default.

Add or edit the following line in a system configuration file, which begins with "99-", in the "/etc/sysctl.d/" directory:

net.ipv4.conf.default.accept_source_route=0

Load settings from all system configuration files with the following command:

$ sudo sysctl --system