Product Documentation

Security considerations and best practices

Apr 10, 2018

This document describes: 

Your organization may need to meet specific security standards to satisfy regulatory requirements. This document does not cover this subject, because such security standards change over time. For up-to-date information on security standards and Citrix products, consult http://www.citrix.com/security/.  

Security best practices

Keep all machines in your environment up to date with security patches. One advantage is that you can use thin clients as terminals, which simplifies this task.

Protect all machines in your environment with antivirus software.

Consider using platform-specific anti-malware software such as the Microsoft Enhanced Mitigation Experience Toolkit (EMET) for Windows machines. Some authorities recommend using the latest Microsoft-supported version of EMET within their regulated environments. Note that, according to Microsoft, EMET may not be compatible with some software, so it should be thoroughly tested with your applications before deployment in a production environment. XenApp and XenDesktop have been tested with EMET 5.5 in its default configuration. Currently, EMET is not recommended for use on a machine that has a Virtual Delivery Agent (VDA) installed.

Protect all machines in your environment with perimeter firewalls, including at enclave boundaries as appropriate.

If you are migrating a conventional environment to this release, you may need to reposition an existing perimeter firewall or add new perimeter firewalls. For example, suppose there is a perimeter firewall between a conventional client and database server in the data center. When this release is used, that perimeter firewall must be placed so that the virtual desktop and user device are on one side, and the database servers and Delivery Controllers in the data center are on the other side. Therefore, consider creating an enclave within your data center to contain the database servers and Controllers. Also consider having protection between the user device and the virtual desktop.

All machines in your environment should be protected by a personal firewall. When you install core components and VDAs, you can choose to have the ports required for component and feature communication opened automatically if the Windows Firewall Service is detected (even if the firewall is not enabled). You can also choose to configure those firewall ports manually. If you use a different firewall, you must configure the firewall manually.

Note: TCP ports 1494 and 2598 are used for ICA and CGP and are therefore likely to be open at firewalls so that users outside the data center can access them. Citrix recommends that you do not use these ports for anything else, to avoid the possibility of inadvertently leaving administrative interfaces open to attack. Ports 1494 and 2598 are officially registered with the Internet Assigned Number Authority (http://www.iana.org/).

All network communications should be appropriately secured and encrypted to match your security policy. You can secure all communication between Microsoft Windows computers using IPSec; refer to your operating system documentation for details about how to do this. In addition, communication between user devices and desktops is secured through Citrix SecureICA, which is configured by default to 128-bit encryption. You can configure SecureICA when you are creating or updating a Delivery Group.

Apply Windows best practice for account management. Do not create an account on a template or image before it is duplicated by Machine Creation Services or Provisioning Services. Do not schedule tasks using stored privileged domain accounts. Do not manually create shared Active Directory machine accounts. These practices will help prevent a machine attack from obtaining local persistent account passwords and then using them to log on to MCS/PVS shared images belonging to others.

Manage user privileges

Grant users only the capabilities they require. Microsoft Windows privileges continue to be applied to desktops in the usual way: configure privileges through User Rights Assignment and group memberships through Group Policy. One advantage of this release is that it is possible to grant a user administrative rights to a desktop without also granting physical control over the computer on which the desktop is stored.

When planning for desktop privileges, note:

  • By default, when non-privileged users connect to a desktop, they see the time zone of the system running the desktop instead of the time zone of their own user device. For information on how to allow users to see their local time when using desktops, see Change basic settings.
  • A user who is an administrator on a desktop has full control over that desktop. If a desktop is a pooled desktop rather than a dedicated desktop, the user must be trusted in respect of all other users of that desktop, including future users. All users of the desktop need to be aware of the potential permanent risk to their data security posed by this situation. This consideration does not apply to dedicated desktops, which have only a single user; that user should not be an administrator on any other desktop.
  • A user who is an administrator on a desktop can generally install software on that desktop, including potentially malicious software. The user can also potentially monitor or control traffic on any network connected to the desktop.

Some applications require desktop privileges, even though they are intended for users rather than for administrators. These users may not be as aware of security risks.

Treat these applications as highly-sensitive applications, even if their data is not sensitive. Consider these approaches to reduce security risk:
  • Enforce two-factor authentication and disable any single sign-on mechanism for the application
  • Enforce contextual access policies
  • Publish the application to a dedicated desktop. If the application must be published to a shared hosted desktop, do not publish any other applications to that shared hosted desktop
  • Ensure the desktop privileges are only applied to that desktop, and not to other computers
  • Enable Session Recording for the application. Also enable other security logging capabilities in the application, and within Windows itself.
  • Configure XenApp and XenDesktop to limit features used with the application (for example, clipboard, printer, client drive, and USB redirection)
  • Enable any security features of the application. Limit it to match strictly the users' requirements - no more
  • Configure security features of Windows to match strictly the users' requirements. This will be a simpler configuration if only that single application is published to the desktop; for example, a restrictive AppLocker configuration can be used. Control access to the file system.
  • Plan to reconfigure, upgrade, or replace the application so that desktop privileges are not required in future

These approaches will not remove all security risk from applications that require desktop privileges.

Manage logon rights

Logon rights are required for both user accounts and computer accounts.  As with Microsoft Windows privileges, logon rights continue to be applied to desktops in the usual way: configure logon rights through User Rights Assignment and group memberships through Group Policy.

The Windows logon rights are: log on locally, log on through Remote Desktop Services, log on over the network (access this computer from the network), log on as a batch job, and log on as a service.

For computer accounts, grant computers only the logon rights they require. The logon right "Access this computer from the network" is required:

  • At VDAs, for the computer accounts of Delivery Controllers
  • At Delivery Controllers, for the computer accounts of VDAs. See Active Directory OU-based Controller discovery.
  • At StoreFront servers, for the computer accounts of other servers in the same StoreFront server group

For user accounts, grant users only the logon rights they require.  

According to Microsoft, by default the group Remote Desktop Users is granted the logon right "Allow log on through Remote Desktop Services" (except on domain controllers).

Your organization's security policy may state explicitly that this group should be removed from that logon right.  Consider the following approach:

  • The Virtual Delivery Agent (VDA) for Server OS uses Microsoft Remote Desktop Services.  You can configure the Remote Desktop Users group as a restricted group, and control membership of the group via Active Directory group policies.  Refer to Microsoft documentation for more information.
  • For other components of XenApp and XenDesktop, including the VDA for Desktop OS, the group Remote Desktop Users is not required.  So, for those components, the group Remote Desktop Users does not require the logon right "Allow log on through Remote Desktop Services"; you can remove it. Additionally:
    • If you administer those computers via Remote Desktop Services, ensure that all such administrators are already members of the Administrators group.
    • If you do not administer those computers via Remote Desktop Services, consider disabling Remote Desktop Services itself on those computers.

Although it is possible to add users and groups to the login right "Deny logon through Remote Desktop Services", the use of deny logon rights is not generally recommended.  Refer to Microsoft documentation for more information.

Configure user rights

Delivery Controller installation creates the following Windows services:

  • Citrix AD Identity Service (NT SERVICE\CitrixADIdentityService): Manages Microsoft Active Directory computer accounts for VMs.
  • Citrix Analytics (NT SERVICE\CitrixAnalytics): Collects site configuration usage information for use by Citrix, if this collection been approved by the site administrator.  It then submits this information to Citrix, to help improve the product.
  • Citrix App Library (NT SERVICE\CitrixAppLibrary): Supports management and provisioning of AppDisks, AppDNA integration, and management of App-V.
  • Citrix Broker Service (NT SERVICE\CitrixBrokerService): Selects the virtual desktops or applications that are available to users.
  • Citrix Configuration Logging Service (NT SERVICE\CitrixConfigurationLogging): Records all configuration changes and other state changes made by administrators to the site.
  • Citrix Configuration Service (NT SERVICE\CitrixConfigurationService): Site-wide repository for shared configuration.
  • Citrix Delegated Administration Service (NT SERVICE\CitrixDelegatedAdmin): Manages the permissions granted to administrators.
  • Citrix Environment Test Service (NT SERVICE\CitrixEnvTest): Manages self-tests of the other Delivery Controller services.
  • Citrix Host Service (NT SERVICE\CitrixHostService): Stores information about the hypervisor infrastructures used in a XenApp or XenDesktop deployment, and also offers functionality used by the console to enumerate resources in a hypervisor pool.
  • Citrix Machine Creation Service (NT SERVICE\CitrixMachineCreationService): Orchestrates the creation of desktop VMs.
  • Citrix Monitor Service (NT SERVICE\CitrixMonitor): Collects metrics for XenApp or XenDesktop, stores historical information, and provides a query interface for troubleshooting and reporting tools.
  • Citrix Storefront Service (NT SERVICE\ CitrixStorefront): Supports management of StoreFront.  (It is not part of the StoreFront component itself.)
  • Citrix Storefront Privileged Administration Service (NT SERVICE\CitrixPrivilegedService): Supports privileged management operations of StoreFront. (It is not part of the StoreFront component itself.)
  • Citrix Config Synchronizer Service (NT SERVICE\CitrixConfigSyncService): Propagates configuration data from the main site databse to the Local Host Cache.
  • Citrix High Availability Service (NT SERVICE\CitrixHighAvailabilityService): Selects the virtual desktops or applications that are available to users, when the main site database is unavailable.

Delivery Controller installation also creates the following Windows services.  These are also created when installed with other Citrix components:

  • Citrix Diagnostic Facility COM Server (NT SERVICE\CdfSvc): Supports the collection of diagnostic information for use by Citrix Support.
  • Citrix Telemetry Service (NT SERVICE\CitrixTelemetryService): Collects diagnostic information for analysis by Citrix, such that the analysis results and recommendations can be viewed by administrators to help diagnose issues with the site.

Delivery Controller installation also creates the following Windows service. This is not currently used. If it has been enabled, disable it.

  • Citrix Remote Broker Provider (NT SERVICE\XaXdCloudProxy)

Delivery Controller installation also creates these following Windows services.  These are not currently used, but must be enabled. Do not disable them.

  • Citrix Orchestration Service (NT SERVICE\CitrixOrchestration)
  • Citrix Trust Service (NT SERVICE\CitrixTrust)

Except for the Citrix Storefront Privileged Administration Service, these services are granted the logon right Log on as a service and the privileges Adjust memory quotas for a process, Generate security audits, and Replace a process level token. You do not need to change these user rights. These privileges are not used by the Delivery Controller and are automatically disabled.

Configure service settings

Except for the Citrix Storefront Privileged Administration service and the Citrix Telemetry Service,  the Delivery Controller Windows services listed above in the Configure user rights section are configured to log on as the NETWORK SERVICE identity. Do not alter these service settings.  

The Citrix Storefront Privileged Administration service is configured to log on Local System (NT AUTHORITY\SYSTEM).  This is required for Delivery Controller StoreFront operations that are not normally available to services (including creating Microsoft IIS sites).  Do not alter its service settings.  

The Citrix Telemetry Service is configured to log on as its own service-specific identity.

You can disable the Citrix Telemetry Service. Apart from this service, and services that are already disabled, do not disable any other of these Delivery Controller Windows services.

Configure registry settings

It is no longer necessary to enable creation of 8.3 file names and folders on the VDA file system. The registry key NtfsDisable8dot3NameCreation can be configured to disable creation of 8.3 file names and folders.  You can also configure this using the fsutil.exe behavior set disable8dot3 command.

Deployment scenario security implications

Your user environment can contain either user devices that are unmanaged by your organization and completely under the control of the user, or user devices that are managed and administered by your organization. The security considerations for these two environments are generally different.

Managed user devices

Managed user devices are under administrative control; they are either under your own control, or the control of another organization that you trust. You may configure and supply user devices directly to users; alternatively, you may provide terminals on which a single desktop runs in full-screen-only mode. Follow the general security best practices described above for all managed user devices. This release has the advantage that minimal software is required on a user device.

A managed user device can be configured to be used in full-screen-only mode or in window mode:

  • Full-screen-only mode: Users log on to it with the usual Log On To Windows screen. The same user credentials are then used to log on automatically to this release.
  • Users see their desktop in a window: Users first log on to the user device, then log on to this release through a web site supplied with the release.

Unmanaged user devices

User devices that are not managed and administered by a trusted organization cannot be assumed to be under administrative control. For example, you might permit users to obtain and configure their own devices, but users might not follow the general security best practices described above. This release has the advantage that it is possible to deliver desktops securely to unmanaged user devices. These devices should still have basic antivirus protection that will defeat keylogger and similar input attacks.

Data storage considerations

When using this release, you can prevent users from storing data on user devices that are under their physical control. However, you must still consider the implications of users storing data on desktops. It is not good practice for users to store data on desktops; data should be held on file servers, database servers, or other repositories where it can be appropriately protected.

Your desktop environment may consist of various types of desktops, such as pooled and dedicated desktops. Users should never store data on desktops that are shared amongst users, such as pooled desktops. If users store data on dedicated desktops, that data should be removed if the desktop is later made available to other users.

Mixed-version environments

Mixed-version environments are inevitable during some upgrades. Follow best-practice and minimize the time that Citrix components of different versions co-exist. In mixed-version environments, security policy, for example, may not be uniformly enforced.

Note: This is typical of other software products; the use of an earlier version of Active Directory only partially enforces Group Policy with later versions of Windows.

The following scenario describes a security issue that can occur in a specific mixed-version Citrix environment. When Citrix Receiver 1.7 is used to connect to a virtual desktop running the VDA in XenApp and XenDesktop 7.6 Feature Pack 2, the policy  setting Allow file transfer between desktop and client is enabled in the Site but cannot be disabled by a Delivery Controller running XenApp and XenDesktop 7.1. It does not recognize the policy setting, which was released in the later version of the product. This policy setting allows users to upload and download files to their virtual desktop, which is the security issue. To work around this, upgrade the Delivery Controller (or a standalone instance of Studio) to version 7.6 Feature Pack 2 and then use Group Policy to disable the policy setting. Alternatively, use local policy on all affected virtual desktops.

Remote PC Access security considerations

Remote PC Access implements the following security features:

  • Smart card use is supported.
  • When a remote session connects, the office PC's monitor appears as blank.
  • Remote PC Access redirects all keyboard and mouse input to the remote session, except CTRL+ALT+DEL and USB-enabled smart cards and biometric devices.
  • SmoothRoaming is supported for a single user only.
  • When a user has a remote session connected to an office PC, only that user can resume local access of the office PC. To resume local access, the user presses Ctrl-Alt-Del on the local PC and then logs on with the same credentials used by the remote session. The user can also resume local access by inserting a smart card or leveraging biometrics, if your system has appropriate third-party Credential Provider integration. This default behavior can be overridden by enabling Fast User Switching via Group Policy Objects (GPOs) or by editing the registry.

Note: Citrix recommends that you do not assign VDA administrator privileges to general session users.

Automatic assignments

By default, Remote PC Access supports automatic assignment of multiple users to a VDA. In XenDesktop 5.6 Feature Pack 1, administrators could override this behavior using the RemotePCAccess.ps1 PowerShell script. This release uses a registry entry to allow or prohibit multiple automatic remote PC assignments; this setting applies to the entire Site.

Caution: Editing the registry incorrectly can cause serious problems that may require you to reinstall your operating system. Citrix cannot guarantee that problems resulting from the incorrect use of Registry Editor can be solved. Use Registry Editor at your own risk. Be sure to back up the registry before you edit it.

To restrict automatic assignments to a single user:

On each Controller in the Site, set the following registry entry:

HKEY_LOCAL_MACHINE\Software\Citrix|DesktopServer

Name: AllowMultipleRemotePCAssignments

Type: REG_DWORD

Data: 0 = Disable multiple user assignment, 1 = (Default) Enable multiple user assignment.

If there are any existing user assignments, remove them using SDK commands for the VDA to subsequently be eligible for a single automatic assignment.

  • Remove all assigned users from the VDA: $machine.AssociatedUserNames | %{ Remove-BrokerUser-Name $_ -Machine $machine
  • Remove the VDA from the Delivery Group: $machine | Remove-BrokerMachine -DesktopGroup $desktopGroup

Restart the physical office PC.