Assessment Design Procedures
2. Determine Assessment Server Location
When determining the location of the assessment servers on the network, the following factors should be considered:
•The type of assessment: agent‐less or agent‐based.
Agent‐less assessment consumes more bandwidth than agent‐based assessment during the scan of an end‐system. However, it is important to understand that the amount of bandwidth consumed by agent‐less assessment should only be considered when a large number of end‐ systems are being assessed over a severely bandwidth‐restricted link. For example, if 1000 end‐systems are connected to a branch office over a 512 Kbps connection that is also carrying latency‐sensitive VoIP and other real‐time applications, it is recommended to position an assessment server at the branch office to execute assessment for connecting devices and avoid congestion on the bandwidth restricted link.
•End‐system configuration for the associated Security Domain.
If agent‐less assessment is implemented and connecting end‐systems are running personal firewalls, the assessment server location may be relevant to the effectiveness of the assessments. For example, Microsoft XP SP2 is enabled by default with a personal firewall that denies all unsolicited inbound connection attempts. Therefore, a Microsoft XP SP2 personal firewall will prevent the successful execution of an end‐system assessment unless the firewall is configured to allow specific types of unsolicited inbound connections, such as from specific IP addresses or over specific protocols as defined in the Exceptions list. This may be configured by the end user via web‐based remediation or through a Windows domain controller group policy definition.
3.Identify Assessment Server Configuration
An assessment server utilizes third‐party assessing software to execute scans against connecting end‐systems, and this software must be locally configured with the security assessment parameters. The third‐party assessing software on all assessment servers belonging to the same Security Domain must be configured identically so that consistency is maintained in the assessment of all connecting end‐systems within that domain. The selection of the vulnerabilities assessed by the assessment servers is based solely on the enterprise security policy. Here are some examples of vulnerability assessment configuration:
•Remote scans that utilize a locally configured account on the end‐system can evaluate virtually any configuration details of the end‐system within the rights of the account. For an administrative account, any end‐system parameters can be checked including the registry settings and the running services. Examples include the date of antivirus definition files, installation of antivirus software, status of antivirus protection, installed patches, and personal firewall status and configuration information.
•Remote scans that do not utilize a locally configured account on the end‐system can evaluate a more limited set of vulnerabilities through the assessment of remotely accessible services on the end‐system. Examples include OS‐specific vulnerabilities accessed through open TCP/ UDP ports and vulnerabilities of remotely‐accessible services running on the end‐system (FTP server, HTTP server).
The SANS Top 20 security vulnerabilities provide a suggested base guideline configuration for enterprises initially deploying NAC with end‐system assessment (http://www.sans.org/top20/).
Third‐party assessing software regularly releases updates to assess newly identified security vulnerabilities. A subscription to such a service is recommended to maintain an up‐to‐date assessment engine on the network. New vulnerabilities only need to be included in the assessment