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ALMA-09-018830 - AlmaLinux OS 9 firewall must employ a deny-all, allow-by-exception policy for allowing connections to other systems.

Information

Failure to restrict network connectivity only to authorized systems permits inbound connections from malicious systems. It also permits outbound connections that may facilitate exfiltration of DOD data.

AlmaLinux OS 9 incorporates the "firewalld" daemon, which allows for many different configurations. One of these configurations is zones. Zones can be used to a deny-all, allow-by-exception approach. The default "drop" zone will drop all incoming network packets unless it is explicitly allowed by the configuration file or is related to an outgoing network connection.

Solution

Configure the "firewalld" daemon to employ a deny-all, allow-by-exception.

Start by adding the exceptions that are required for mission functionality to the "drop" zone. If SSH access on port 22 is needed for example, run the following command:

$ firewall-cmd --permanent --add-service=ssh --zone=drop

Set the default zone to the "drop" zone:

$ firewall-cmd --set-default-zone=drop

Note: This is a runtime and a permanent change.

Add any interfaces to the newly modified "drop" zone:

$ firewall-cmd --permanent --zone=drop --change-interface=enp1s0

Reload the firewall rules for changes to take effect:

$ firewall-cmd --reload

Check zones and interfaces:

$ firewall-cmd --get-active-zones

drop
 interfaces: enp1s0

Check new default zone's target is set to "DROP":

$ firewall-cmd --permanent --info-zone=drop | grep target

 target: DROP

The same outcome is achieved by creating a new zone, for example:

$ firewall-cmd --permanent --new-zone=stig
$ firewall-cmd --reload
$ firewall-cmd --permanent --change-interface=enp1s0 --zone=stig
$ firewall-cmd --permanent --add-service=ssh --zone=stig
$ firewall-cmd --permanent --set-target=DROP --zone=stig
$ firewall-cmd --set-default-zone=stig

See Also

https://dl.dod.cyber.mil/wp-content/uploads/stigs/zip/U_CL_AlmaLinux_OS_9_V1R6_STIG.zip

Item Details

Category: CONFIGURATION MANAGEMENT

References: 800-53|CM-6b., CAT|II, CCI|CCI-000366, Rule-ID|SV-269246r1050780_rule, STIG-ID|ALMA-09-018830, Vuln-ID|V-269246

Plugin: Unix

Control ID: 44cbe16b754d543212b7c6af694a5c126a2267c18fa3f075ce60df9a286f83cd