cyberstrikeus CIS Ubuntu Linux 20.04 LTS Benchmark v3.0.0 - Control 4.2.8
Profile
- Level: Level 1 - Server, Level 1 - Workstation
- Assessment Status: Automated
Description
A default deny policy on connections ensures that any unconfigured network usage will be rejected.
Note: Any port or protocol without a explicit allow before the default deny will be blocked
Rationale
With a default accept policy the firewall will accept any packet that is not configured to be denied. It is easier to allow list acceptable usage than to deny list unacceptable usage.
Impact
Any port and protocol not explicitly allowed will be blocked. The following rules should be considered before applying the default deny.
ufw allow out http
ufw allow out https
ufw allow out ntp # Network Time Protocol
ufw allow out to any port 53 # DNS
ufw allow out to any port 853 # DNS over TLS
ufw logging on
Audit Procedure
Command Line
Run the following command and verify that the default policy for incoming, outgoing, and routed directions is deny, reject, or disabled:
ufw status verbose | grep Default:
Example output:
Default: deny (incoming), deny (outgoing), disabled (routed)
Expected Result
Default: deny (incoming), deny (outgoing), disabled (routed)
Remediation
Command Line
Run the following commands to implement a default deny policy:
ufw default deny incoming
ufw default deny outgoing
ufw default deny routed
Default Value
Not applicable.
References
- NIST SP 800-53 Rev. 5: SC-7
CIS Controls
v8 - 4.4 Implement and Manage a Firewall on Servers v8 - 4.5 Implement and Manage a Firewall on End-User Devices v7 - 9.4 Apply Host-based Firewalls or Port Filtering
MITRE ATT&CK Mappings: T1562, T1562.004 | TA0011 | M1031, M1037