cyberstrikeus CIS Ubuntu Linux 18.04 LTS Benchmark v2.2.0 - Control 3.4.2.8
Description
Base chain policy is the default verdict that will be applied to packets reaching the end of the chain.
Rationale
There are two policies: accept (Default) and drop. If the policy is set to accept, the firewall will accept any packet that is not configured to be denied and the packet will continue transversing the network stack.
It is easier to allow list acceptable usage than to deny list unacceptable usage.
Note: Changing firewall settings while connected over network can result in being locked out of the system.
Impact
If configuring nftables over ssh, creating a base chain with a policy of drop will cause loss of connectivity.
Ensure that a rule allowing ssh has been added to the base chain prior to setting the base chain's policy to drop.
Audit Procedure
Command Line
Run the following commands and verify that base chains contain a policy of DROP:
nft list ruleset | grep 'hook input'
nft list ruleset | grep 'hook forward'
nft list ruleset | grep 'hook output'
Expected Result
type filter hook input priority 0; policy drop;
type filter hook forward priority 0; policy drop;
type filter hook output priority 0; policy drop;
Remediation
Command Line
Run the following command for the base chains with the input, forward, and output hooks to implement a default DROP policy:
nft chain <table family> <table name> <chain name> { policy drop \; }
Example:
nft chain inet filter input { policy drop \; }
nft chain inet filter forward { policy drop \; }
nft chain inet filter output { policy drop \; }
Default Value
accept
References
- Manual Page nft
- NIST SP 800-53 Rev. 5: CA-9, SC-7
CIS Controls
Version 8
4.4 Implement and Manage a Firewall on Servers - Implement and manage a firewall on servers, where supported.
4.5 Implement and Manage a Firewall on End-User Devices - Implement and manage a host-based firewall or port-filtering tool on end-user devices.
Version 7
9.4 Apply Host-based Firewalls or Port Filtering - Apply host-based firewalls or port filtering tools on end systems, with a default-deny rule that drops all traffic except those services and ports that are explicitly allowed.
Profile Applicability
- Level 1 - Server
- Level 1 - Workstation
Assessment Status
Automated