Vulnerability Discussion

In order to prevent unauthorized connection of devices, unauthorized transfer of information, or unauthorized tunneling (i.e., embedding of data types within data types), organizations must disable or restrict unused or unnecessary physical and logical ports/protocols on information systems.

The Ubuntu operating system is capable of providing a wide variety of functions and services. Some of the functions and services provided by default may not be necessary to support essential organizational operations. Additionally, it is sometimes convenient to provide multiple services from a single component (e.g., VPN and IPS); however, doing so increases risk over limiting the services provided by any one component.

To support the requirements and principles of least functionality, the Ubuntu operating system must support the organizational requirements, providing only essential capabilities and limiting the use of ports, protocols, and/or services to only those required, authorized, and approved to conduct official business or to address authorized quality of life issues.

Check

Verify the Ubuntu operating system is configured to prohibit or restrict the use of functions, ports, protocols, and/or services as defined in the Ports, Protocols, and Services Management (PPSM) Category Assignments List (CAL) and vulnerability assessments.

Check the firewall configuration for any unnecessary or prohibited functions, ports, protocols, and/or services by running the following commands:
$ sudo ufw show before-rules
$ sudo ufw show user-rules
$ sudo ufw show after-rules

Ask the system administrator for the site or program PPSM Component Local Services Assessment (CLSA). Verify the services allowed by the firewall match the PPSM CLSA.

If there are any additional ports, protocols, or services that are not included in the PPSM CLSA, this is a finding.

If there are any ports, protocols, or services that are prohibited by the PPSM CAL, this is a finding.

Fix

Add all ports, protocols, or services allowed by the PPSM CLSA by using the following command:
$ ufw allow

where the direction is 'in' or 'out' and the port is the one corresponding to the protocol or service allowed.

To deny access to port, protocols or services, use:
$ ufw deny