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Critical Security Control: 11

Critical Security Control: 11

Limitation and Control of Network Ports, Protocols, and Services

Manage (track/control/correct) the ongoing operational use of ports, protocols, and services on networked devices in order to minimize windows of vulnerability available to attackers.

Why Is This Control Critical?

Attackers search for remotely accessible network services that are vulnerable to exploitation. Common examples include poorly configured web servers, mail servers, file and print services, and domain name system (DNS) servers installed by default on a variety of different device types, often without a business need for the given service. Many software packages automatically install services and turn them on as part of the installation of the main software package without informing a user or administrator that the services have been enabled. Attackers scan for such issues and attempt to exploit these services, often attempting default user IDs and passwords or widely available exploitation code.

How to Implement This Control

ID # Description Category
CSC 11-1 Ensure that only ports, protocols, and services with validated business needs are running on each system. Quick win
CSC 11-2 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. Quick win
CSC 11-3 Perform automated port scans on a regular basis against all key servers and compared to a known effective baseline. If a change that is not listed on the organization's approved baseline is discovered, an alert should be generated and reviewed. Quick win
CSC 11-4 Keep all services up to date and uninstall and remove any unnecessary components from the system. Quick win
CSC 11-5 Verify any server that is visible from the Internet or an untrusted network, and if it is not required for business purposes, move it to an internal VLAN and give it a private address. Visibility/Attribution
CSC 11-6 Operate critical services on separate physical or logical host machines, such as DNS, file, mail, web, and database servers. Configuration/Hygiene
CSC 11-7 Place application firewalls in front of any critical servers to verify and validate the traffic going to the server. Any unauthorized services or traffic should be blocked and an alert generated. Advanced

CSC 11 Procedures and Tools

Port scanning tools are used to determine which services are listening on the network for a range of target systems. In addition to determining which ports are open, effective port scanners can be configured to identify the version of the protocol and service listening on each discovered open port. This list of services and their versions are compared against an inventory of services required by the organization for each server and workstation in an asset management system. Recently added features in these port scanners are being used to determine the changes in services offered by scanned machines on the network since the previous scan, helping security personnel identify differences over time.

CSC 11 Effectiveness Metrics

In order to test the effectiveness of the automated implementation of this control, organizations should measure the following:

1. How long does it take systems to identify any new unauthorized listening network ports that are installed on network systems (time in minutes)?

2. How long does it take for alerts to be generated about new services being installed (time in minutes)?

3. Are alerts then sent every 24 hours until the listening network port has been disabled or it has been authorized by change management (yes or no)?

4. Do alerts indicate the location, department, and other details about the system where authorized and unauthorized network ports are running (yes or no)?

CSC 11 Automation Metrics

In order to automate the collection of relevant data from these systems, organization should gather the following information with automated technical sensors:

1. What is the percentage of the organization's systems that are not currently running a host based firewall (by business unit)?

2. How many unauthorized services are currently running on the organization's business systems (by business unit)?

3. How many deviations from approved service baselines have been discovered recently on the organization's business systems (by business unit)?

CSC 11 Effectiveness Test

To evaluate the implementation of Control 11 on a periodic basis, the evaluation team must install hardened test services with network listeners on 10 locations on the network, including a selection of subnets associated with DMZs, workstations, and servers. The selection of these systems must be as random as possible and include a cross-section of the organization's systems and locations. The evaluation team must then verify that the systems generate an alert or e-mail notice regarding the newly installed services within 24 hours of the services being installed on the network. The team must verify that the system provides details of the location of all of the systems where test services have been installed.

CSC 11 System Entity Relationship Diagram

Organizations will find that by diagramming the entities necessary to fully meet the goals defined in this control, it will be easier to identify how to implement them, test the controls, and identify where potential failures in the system might occur.

A control system is a device or set of devices used to manage, command, direct, or regulate the behavior of other devices or systems. In this case, we are examining how active scanning systems gather information on network devices and evaluate that data against the authorized service baseline database. The following list of the steps in the above diagram shows how the entities work together to meet the business goal defined in this control. The list also delineates each of the process steps in order to help identify potential failure points in the overall control.

Step 1: Active scanner analyzes production systems for unauthorized ports, protocols, and services

Step 2: System baselines regularly updated based on necessary/required services

Step 3: Active scanner validates which ports, protocols, and services are blocked or allowed by the application firewall

Step 4: Active scanner validates which ports, protocols, and services are accessible on business systems protected with host-based firewalls.


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