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DCOM: Machine Access Restrictions in Security Descriptor Definition Language (SDDL) syntax

Published: November 15, 2012

Updated: November 15, 2012

Applies To: Windows 7, Windows Server 2003, Windows Server 2008, Windows Server 2008 R2, Windows Vista, Windows XP

This security policy reference topic for the IT professional describes the best practices, location, values, and security considerations for this policy setting.

This policy setting allows administrators to define additional computer-wide controls that govern access to all Distributed Component Object Model (DCOM)–based applications on a computer. These controls restrict call, activation, or launch requests on the computer. A simple way to think about these access controls is as an additional access check that is performed against a computer-wide access control list (ACL) on each call, activation, or launch of any COM-based server. If the access check fails, the call, activation, or launch request is denied. (This check is in addition to any access check that is run against the server-specific ACLs.) In effect, it provides a minimum authorization standard that must be passed to access any COM-based server. This policy setting controls access permissions to cover call rights.

These computer-wide ACLs provide a way to override weak security settings that are specified by an application through the CoInitializeSecurity function or application-specific security settings. They provide a minimum security standard that must be passed, regardless of the settings of the specific server.

These ACLs also provide a centralized location for an administrator to set a general authorization policy that applies to all COM-based servers on the computer.

This policy setting allows you to specify an ACL in two different ways. You can type the security descriptor in SDDL, or you can grant or deny Local Access and Remote Access permissions to users and groups. We recommend that you use the built-in user interface to specify the ACL contents that you want to apply with this setting. The default ACL settings vary, depending on the version of Windows you are running.

  • User-defined input of the SDDL representation of the groups and privileges

    When you specify the users or groups that are to be given permissions, the security descriptor field is populated with the Security Descriptor Definition Language representation of those groups and privileges. Users and groups can be given explicit Allow or Deny privileges for local access and remote access.

  • Blank

    This represents how the local security policy deletes the policy enforcement key. This value deletes the policy and then sets it as Not defined. The Blank value is set by using the ACL editor to empty the list, and then pressing OK.

GPO_name\Computer Configuration\Windows Settings\Security Settings\Local Policies\Security Options

The following table lists the actual and effective default values for this policy. Default values are also listed on the policy’s property page.

 

Server type or GPO Default value

Default Domain Policy

Blank

Default Domain Controller Policy

Blank

Stand-Alone Server Default Settings

Blank

DC Effective Default Settings

Not defined

Member Server Effective Default Settings

Not defined

Client Computer Effective Default Settings

Not defined

There are no differences in this policy between operating systems beginning with Windows Server 2008.

This section describes features and tools that are available to help you manage this policy.

None. Changes to this policy become effective without a computer restart when they are saved locally or distributed through Group Policy.

The registry settings that are created as a result of enabling the DCOM: Machine Access Restrictions in Security Descriptor Definition Language (SDDL) syntax policy setting take precedence over the previous registry settings when this olicy setting was configured. The Remote Procedure Call (RPC) service checks the new registry keys in the Policies section for the computer restrictions, and these registry entries take precedence over the existing registry keys under OLE. This means that previously existing registry settings are no longer effective, and if you make changes to the existing settings, computer access permissions for users are not changed. Use care in configuring the list of users and groups.

If the administrator is denied permission to access DCOM applications due to the changes made to DCOM in the Windows operating system, the administrator can use the DCOM: Machine Access Restrictions in Security Descriptor Definition Language (SDDL) syntax policy setting to manage DCOM access to the computer. The administrator can use this setting to specify which users and groups can access the DCOM application on the computer locally and remotely. This will restore control of the DCOM application to the administrator and users. To do this, open the DCOM: Machine Access Restrictions in Security Descriptor Definition Language (SDDL) syntax setting, and click Edit Security. Specify the users or groups you want to include and the computer access permissions for those users or groups. This defines the setting and sets the appropriate SDDL value.

This section describes how an attacker might exploit a feature or its configuration, how to implement the countermeasure, and the possible negative consequences of countermeasure implementation.

Many COM applications include some security-specific code (for example, to call CoInitializeSecurity), but they use weak settings that allow unauthenticated access to the process. Administrators cannot override these settings to force stronger security in earlier versions of Windows without modifying the application. An attacker could attempt to exploit weak security in an individual application by attacking it through COM calls.

Also, the COM infrastructure includes the Remote Procedure Call Services (RPCSS), a system service that runs during and after computer startup. This service manages activation of COM objects and the running object table and provides helper services to DCOM remoting. It exposes RPC interfaces that can be called remotely. Because some COM-based servers allow unauthenticated remote access, these interfaces can be called by anyone, including unauthenticated users. As a result, RPCSS can be attacked by malicious users who use remote, unauthenticated computers.

To protect individual COM-based applications or services, set the DCOM: Machine Access Restrictions in Security Descriptor Definition Language (SDDL) syntax setting to an appropriate computer-wide ACL.

Windows operating systems implement default COM ACLs when they are installed. Modifying these ACLs from the default may cause some applications or components that communicate by using DCOM to fail. If you implement a COM-based server and you override the default security settings, confirm that the application-specific call permissions that ACL assigns are the correct permissions for appropriate users. If it does not, you must change your application-specific permission ACL to provide appropriate users with activation rights so that applications and Windows components that use DCOM do not fail.

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