Terminal Server Security

This chapter is from the book

Windows Server 2003/2000 Terminal Server Solutions: Implementing Windows Terminal Services and Citrix MetaFrame Presentation Server 3.0, 3rd Edition

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This chapter is from the book

This chapter is from the book 

Windows Server 2003/2000 Terminal Server Solutions: Implementing Windows Terminal Services and Citrix MetaFrame Presentation Server 3.0, 3rd Edition

Learn More Buy

Administrative Delegation

It should be fairly obvious that granting a regular user administrative privileges to a Terminal Server will inevitably result in stability issues. It may not be quite so obvious that the exact same threat exists when legitimate administrators in your domain who are unfamiliar with managing a Terminal Server environment are also granted administrative access to your servers. In this situation, the administrator is just as likely as the regular user (if not more so) to cause server-wide problems based on a change made to the server or an application's configuration.

Historically it has been common to have only one administrative classification in a Windows environment without any further subdivision of privileges into more granular groupings. When someone requires administrative privileges to manage resources in a domain, they are likely added to the Domain Admins group, giving them full administrative privileges that span the entire domain. The result is an environment with multiple administrators with full control over all resources in the domain, yet they may lack the experience to properly administer many of the resources they have full control over. For example, someone skilled in maintenance of Windows file servers may not be qualified to manage Active Directory, a Microsoft SQL Server, or even a Terminal Server. While many argue that it is just "easier" to manage the resources with these privileges, allowing this practice in a Terminal Server environment can introduce a significant threat to your infrastructure's stability.

To help ensure a stable and secure Terminal Server environment, you must not give anyone administrative authority on your servers unless they are qualified to use it. The more Terminal Server administrators you have, the greater the risk of an undesirable change affecting your production environment.

Terminal Server Domain Group Creation

When configuring administrative delegations for your Terminal Server environment, a logical starting point is creation of the four global domain security groups listed in Table 16.1. These are the same groups discussed in Chapter 8 and used when demonstrating group policy object creation in Chapter 15. I review the specific roles of the Terminal Server Operators and User Support groups in more detail in Chapter 22, "Server Operations and Support."

Table 16.1 Terminal Server Administrative Domain Groups

Delegating Organizational Unit Authority

Once the domain groups are created, you need to delegate administrative privileges for the Terminal Server Administrators group to the Terminal Servers OU. There are two ways you can accomplish this: Either utilize the Delegation of Control wizard, accessible by right-clicking the OU and selecting Delegate Control; or directly update the access control entries from the Security tab found under the properties for the OU, as shown in Figure 16.3. If the Security tab is not visible on the Properties page, you will need to select the Advanced Features option from the View menu.

Figure 16.3 The Security tab for the Terminal Server organizational unit.

Once the Terminal Server Administrators group has been added, I suggest disabling inheritance of permissions from the OU's parent container. This ensures that permissions must be defined directly against the OU in order to affect delegation of authority, reducing the chances that an undesirable change performed in a parent container will negatively affect this OU's access control list (ACL).

Inheritance is disabled by deselecting the check box labeled "Allow inheritable permissions from parent to propagate to this object." In a Windows 2000 domain, this check box appears on the main Security tab (see Figure 16.3), while in a Windows 2003 domain, you must first click the Advanced button to bring up the dialog box containing this check box. When this option is deselected, you then must make a choice between whether you will keep a copy of inherited permissions or remove all inherited permissions and keep only permissions explicitly defined for the OU.

For this Terminal Server OU, I suggest removing all inherited permissions and keeping only those explicitly defined. In a typical Active Directory environment, your access control list would then look similar to the following:

• Account Operators

• Authenticated Users

• Domain Admins

• Print Operators

• ENTERPRISE DOMAIN CONTROLLERS (Windows 2003 Active Directory only)

• SYSTEM

• Terminal Server Administrators

Management of this OU should be restricted further by removing the Account Operators object and the Print Operators object from the ACL, leaving Terminal Server Administrators and Domain Admins as the two groups with full authority over the organizational unit. If you have any concerns that a member of the Domain Admins group might inadvertently modify properties of the Terminal Servers OU, you should consider restricting this group's access to the OU as well, either by completely removing the group from the ACL or by modifying the permissions to restrict activities the group can perform.

Managing Local Group Membership

Now that administrative permissions for the Terminal Server OU have been delegated accordingly, the next step is to create and populate the necessary local groups on each of your Terminal Servers. The idea is to use the local groups on a Terminal Server to assign all the necessary server privileges and then delegate these privileges by populating these groups with the appropriate domain groups. We begin by delegating administrative authority through these local groups and return later in this chapter to assign the appropriate user privileges.

When server security is implemented in this fashion it greatly simplifies administration by introducing a layer of abstraction between the domain and the server. When you know that all security on a server relates back to the local groups, you can verify who has access to what resources simply by viewing local group membership. If domain groups or individual users were directly assigned to the access control lists (ACL) of resources on the server, you would be required to trace back permissions from these resources to the appropriate domain group to ensure that security had not been compromised (intentionally or accidentally).

Before domain groups can be assigned to the local groups on a Terminal Server, we must ensure that these local groups exist. As I discussed in Chapter 8, I typically recommend that the following two additional local groups be created on a Terminal Server to allow for more granular delegation of administrative authority on a Terminal Server:

• Terminal Server Operators—Used to assign a subset of administrative privileges to those users responsible for managing the day-to-day operations of the Terminal Server. For example, members of this group would have the ability to terminate processes or restart the Terminal Server but would not have the ability to install/remove applications or modify configuration of the server.

• Terminal Server User Support—Used to assign privileges to those users who would perform typical user support functions. Among other privileges, members of this group would have the ability to shadow other users—something a regular user can't do.

While local groups can be added to a Terminal Server by manually creating them through the Microsoft Management Console (MMC) Computer Management snap-in, I prefer scripting their creation using the Net LocalGroup command. The script is run locally on a Terminal Server during the initial configuration phase to create the desired local groups. The following code sample demonstrates how these two groups could be created using a script:

@ECHO OFF
REM Create the desired local groups on the Terminal Server
net localgroup "Terminal Server Operators" /add
net localgroup "Terminal Server User Support" /add

Once the necessary local groups have been created, we are ready to assign the appropriate domain groups. The most common means of assigning a domain group to a local server group is directly through the MMC Computer Management snap-in, as shown in Figure 16.4. While easy enough, this method is still prone to error when the task must be duplicated across a number of Terminal Servers. Another option to consider is scripting the local group assignment, similar to what I suggested for creation of the local groups. Unfortunately this is really effective only if the local group memberships will remain static. In a production Terminal Server environment, it is likely that local group membership assignments will change over time. In a large Terminal Server environment it would not be practical to adjust and rerun a membership script on all servers. As a result, an alternative allowing more dynamic control over local group membership is necessary.

The preferred method of managing local group membership is through a group policy object (GPO) in an Active Directory domain. Through a single GPO we can define a local group membership standard that is consistent across all Terminal Servers in the domain. This method not only allows for consistent definition of the local group members but also provides dynamic control over the membership we are looking for.

Figure 16.4 Assigning domain groups to a local group through the Local Users and Groups container.

To demonstrate how this is done, I use the "Terminal Server Machine Policy" created in Chapter 15. This GPO contains all the base computer configuration settings for all the Terminal Servers in the OU. There are no user configuration options defined in this GPO. The local group membership is controlled from within the Restricted Groups policy, located under Computer Configuration\Windows Settings\Security Settings.

Managing the local group membership is a two-stage process. First the local groups that will be populated are added to the Restricted Groups policy, and then the desired domain members are added to each of these groups. The local groups are added to the policy by right-clicking Restricted Groups and selecting the Add Group menu option. The dialog box shown in Figure 16.5 opens, where you can enter the desired local group name.

Figure 16.5 Accessing the "Terminal Server Machine Policy."

Note that there is no validation performed on any groups that you add to the Restricted Groups policy. If the group name you provide does not exist on the Terminal Server, the group name is simply ignored and is not automatically created by the GPO.

Typically I define the following local groups within the Restricted Groups policy:

• Administrators—Members of this group will have full administrative control over the Terminal Server. Because of this, access should be very tightly controlled.

• Terminal Server Operators—This group name is used to assign a subset of administrative privileges to those users responsible for managing the day-to-day operations of the Terminal Server.

• Terminal Server User Support—Those users responsible for performing typical user support functions such as shadowing or session logoffs would be members of this group.

• Remote Desktop Users (Windows Server 2003 only)—To remotely log on to a Windows 2003 Terminal Server, a user must be a member of this existing local server group.

• Users—All regular users who require access to run one or more applications on the Terminal Server are assigned to this local group.

• Power Users, Guests—These two groups are included in the Restricted Groups policy to ensure that no users are members of either of them.

• Anonymous (MetaFrame servers only)—When MetaFrame is installed on a Terminal Server, it adds support for anonymous logons through the special Anonymous group. Just as with the Power Users and Guests groups, this group is included in the Restricted Groups policy to ensure that no users belong to this group. Of course, if your MetaFrame server will allow this type of support you will need to omit this entry from the GPO.

After all the local groups have been defined within the Restricted Groups policy, the next step is to assign the appropriate domain groups to these local groups. This is done by right-clicking the desired local group in the right-hand pane and selecting the Security option from the pull-down menu. The Configure Membership dialog box appears, as shown in Figure 16.6. By default this group contains no members and belongs to no other group. If these options are left as-is, then when the policy is applied to the server, all members who may have been manually added to the local group are removed. Domain groups are added by clicking the Add button on the upper half of the dialog box for the Members of This Group option. Since we're configuring only local groups on the Terminal Server, there is no need to modify the lower portion of the dialog box, labeled This Group Is a Member Of.

Figure 16.6 The Configure Membership dialog box for a listed Restricted Group.

Once the Add button has been clicked, an Add Members dialog box appears. This dialog box lets you type in the name of the domain group or click the Browse button to find the desired group within the domain. Multiple groups can be entered at once by separating them with a semicolon. When manually typing in the group name, be sure to precede it with the domain name and a backslash. For example, to add the domain group Terminal Server Operators of the domain called PRODUCTION, you would type the following:

PRODUCTION\Terminal Server Operators

When you have completed entering the desired domain groups for a given local group, click OK to close the Configure Membership dialog box. Each of the local groups listed in the Restricted Groups policy now appears in the Members column when the Details view is active (see Figure 16.7, which shows the defined groups in a Windows 2000 domain). When the GPO is applied to all servers in the Terminal Server OU, all the listed local groups are updated with the corresponding domain groups. As I mentioned, groups with no explicit memberships will have any existing memberships removed on the affected Terminal Servers.

Figure 16.7 Viewing local groups in Restricted Groups.

Table 16.2 summarizes my suggestions for local group membership in a Terminal Server Restricted Groups policy. The domain groups listed in this table were discussed in the "Terminal Server Domain Group Creation" section, earlier in this chapter.

Table 16.2 Terminal Server Restricted Group Membership Suggestions