This topic will explore the nuances and new features of Windows Server 2003’s resource access control. You will learn how to use the ACL editor to manage permissions templates, inheritance, special permissions, and how to evaluate resulting effective permissions for a user or group.
Configuring Permissions
Windows Explorer is the most common tool used to initiate management of resource access permissions, both on a local volume as well as on a remote server. Unlike shared folders, Windows Explorer can configure permissions locally and remotely.
The Access Control List Editor
As in earlier versions of Windows, security can be configured for files and folders on any NTFS volume by right-clicking the resource and choosing Properties (or Sharing And Security) then clicking the Security tab. The interface that appears has many aliases; it has been called the Permissions dialog box, the Security Settings dialog box, the Security tab or the Access Control List editor (ACL editor). Whatever you call it, it looks the same. An example can be seen in the Security tab of the Docs Properties dialog box, as shown in Below Figure
Prior to Windows 2000, permissions were fairly simplistic, but with Windows 2000 and later versions, Microsoft enabled significantly more flexible and powerful control over resource access. With more power came more complexity, and now the ACL editor has three dialog boxes, each of which supports different and important functionality.
The first dialog box provides a “big picture” view of the resource’s security settings or permissions, allowing you to select each account that has access defined and to see the permissions templates assigned to that user, group, or computer. Each template shown in this dialog box represents a bundle of permissions that together allow a commonly configured level of access. For example, to allow a user to read a file, several granular permissions are needed. To mask that complexity, you can simply apply the Allow:Read & Execute permissions template and, behind the scenes, Windows sets the correct file or folder permissions.
To view more details about the ACL, click Advanced, which exposes the second of the ACL editor’s dialog boxes, the Advanced Security Settings For Docs dialog box, as shown in below Figure. This dialog box lists the specific access control entries that have been assigned to the file or folder. The listing is the closest approximation in the user interface to the actual information stored in the ACL itself. The second dialog also enables you to configure auditing, manage ownership, and evaluate effective permissions.
If you select permission in the Permission Entries list and click Edit, the ACL editor’s third dialog box appears. This Permission Entry For Docs dialog box, shown in below Figure, lists the detailed, most granular permissions that comprise the permissions entry in the second dialog box’s Permissions Entries list and the first dialog box’s Permissions For Users list.
Adding and Removing Permission Entries
Any security principal may be granted or denied resource access permissions. In Windows Server 2003, the valid security principals are: users, groups, computers, and the special InetOrgPerson object class, which is used to represent users in certain cross-directory platform situations. To add permission, click the Add button on either the first or second ACL editor dialog box. The Select User, Computer Or Group dialog box will help you identify the appropriate security principal. Then select appropriate permissions. The interface has changed slightly from previous versions of Windows, but not enough to prevent an experienced administrator from mastering the new user interface quickly. You can remove an explicit permission that you have added to an ACL by selecting the permission and clicking Remove.
Modifying Permissions
A permission may be modified in the dialog box by selecting or clearing the Allow or Deny check boxes on the Security tab to apply permissions templates.
For a finer degree of control, click Advanced, select a permission entry and click Edit. Only explicit permissions may be edited. Inherited permissions are discussed later in this lesson.
The Permission Entry For Docs dialog box, shown in Figure (above), will allow you to modify permissions and specify the scope of the permissions inheritance, through the Apply Onto drop-down list.
New Security Principals
Windows Server 2003, unlike Windows NT 4, allows you to add computers or groups of computers to an ACL, thereby adding flexibility to control resource access based on the client computer, regardless of the user who attempts access. For example, you may want to provide a public computer in the employee lounge, but prevent a manager from exposing sensitive data during his or her lunch break. By adding the computer to ACLs and denying access permission, the manager who can access sensitive data from his or her desktop is prevented from accessing it from the lounge.
Windows Server 2003 also allows you to manage resource access based on the type of logon. You can add the special accounts, Interactive, Network, and Terminal Server User to an ACL. Interactive represents any user logged on locally to the console. Terminal Server User includes any user connected via remote desktop or terminal services.
Network represents a connection from the network, for example a Windows system running Client for Microsoft Networks.
Permissions Templates and Special Permissions
Permissions templates, visible on the Security tab in the first dialog box are bundles of special permissions, which are fully enumerated in the third dialog box, Permissions Entry For Docs. Most of the templates and special permissions are self-explanatory, while others are beyond the scope of this book. However, the following points are worth noting:
* Read & Execute: This permissions template is sufficient to allow users to open and read files and folders. Read & Execute will also allow a user to copy a resource, assuming they have permission to write to a target folder or media. There is no permission in Windows to prevent copying. Such functionality will be possible with Digital Rights Management technologies as they are incorporated into Windows platforms.
*Write and Modify: The Write permissions template applied to a folder allows users to create a new file or folder (when applied to a folder) and, when applied to a file, to modify the contents of a file as well as its attributes (hidden, system, read-only) and extended attributes (defined by the application responsible for the document). The Modify template adds the permission to delete the object.
*Change Permissions: After modifying ACLs for a while, you might wonder who can modify permissions. The answer is, first, the owner of the resource. Ownership will be discussed later in this lesson. Second, any user who has an effective permission that allows Change Permission can modify the ACL on the resource. The Change Permission must be managed using the ACL editor’s third dialog box, Permission Entry For Docs. It is also included in the Full Control permission template.
Inheritance
Windows Server 2003 supports permissions inheritance, which simply means that per-missions applied to a folder will, by default, apply to the files and folders beneath that folder. Any change to the parent’s ACL will similarly affect all contents of that folder. Inheritance enables you to create single points of administration, managing a single ACL on a branch or resources under a folder.
Understanding Inheritance
Inheritance is the result of two characteristics of a resource’s security descriptor. First, permissions are, by default, inheritable. As previously shown in Figure, the permission Allow Users to Read & Execute is specified to Apply to: This folder, subfolders, and files. That alone, however, is not enough to make inheritance work. The other half of the story is that new objects, when created, are set by default to “Allow Inheritable Permissions From The Parent To Propagate To This Object...” the check box visible in the same figure.
So a newly created file or folder will inherit the inheritable permissions from its parent, and any changes to the parent will affect the child files and folders as well. It is helpful to understand this two-step implementation of inheritance because it gives us two ways to manage inheritance: from the parent and from the child.
Inherited permissions are displayed differently in each dialog box of the ACL editor. The first and third dialog boxes (Security tab and Permissions Entry For Docs) show inherited permissions as dimmed check marks, to distinguish them from permissions that are set directly on the resource, called explicit permissions, which are not dimmed. The second dialog box (Advanced Security Settings) shows, for each permission entry, from what folder the permission entry is inherited.
Overriding Inheritance
Inheritance allows you to configure permissions high in a folder tree. Such initial per-missions, and any changes to those permissions, will propagate to all the files and folders in that tree that are, by default, configured to allow inheritance.
Occasionally, however, you might need to modify permissions on a subfolder or file, to provide additional access or restrict access to a user or group. You cannot remove inherited permissions from an ACL. You can override an inherited permission by assigning an explicit permission. Alternatively, you can block all inheritance and create an entirely explicit ACL.
To override an inherited permission by assigning an explicit permission, simply check the appropriate permissions box. For example, if a folder has an inherited Allow Read permission assigned to the Sales Reps group, and you do not want Sales Reps to access the folder, you can select the box to Deny Read.
To override all inheritance, open the resources Advanced Security Settings dialog box and clear Allow Inheritable Permissions From The Parent To Propagate To This Object... You will block all inheritance from the parent. You will then have to manage access to the resource by assigning sufficient explicit permissions.
To help you create an explicit permissions ACL, Windows gives you a choice when you choose to disallow inheritance. You are asked whether you want to Copy or Remove permissions entries, as shown in Below Figure.
Copy will create explicit permissions identical to what was inherited. You can then remove individual permissions entries that you do not want to affect the resource. If you choose Remove, you will be presented with an empty ACL, to which you will add permissions entries. The result is the same either way; an ACL populated with explicit permissions. The question is whether it is easier to start with an empty ACL and build it from scratch or start with a copy of the inherited permissions and modify the list to the desired goal. If the new ACL is wildly different than the inherited permissions, choose Remove. If the new ACL is only slightly different than the result of inherited permissions, it is more efficient to choose Copy.
When you disallow inheritance by deselecting the Allow Inheritable Permissions option, you block inheritance. All access to the resource is managed by explicit per-missions assigned to that file or folder. Any changes to the ACL of its parent folder will not affect the resource; although the parent permissions are inheritable, the child does not inherit. Block inheritance sparingly because it increases the complexity of managing, evaluating, and troubleshooting resource access.
Reinstating Inheritance
Inheritance can be reinstated in two ways: from the child resource or from the parent folder. The results differ slightly. You might reinstate inheritance on a resource if you disallowed inheritance accidentally or if business requirements have changed. Simply re-select the Allow Inheritable Permissions option in the Advanced Security Settings dialog box. Inheritable permissions from the parent will now apply to the resource. All explicit permissions you assigned to the resource remain, however. The resulting ACL is a combination of the explicit permissions, which you might choose to remove, and the inherited permissions. Because of this dynamic, you might not see some inherited permissions in the first or third ACL editor dialog boxes. For example, if a resource has an explicit permission, Allows Sales Reps Read & Execute, and the parent folder has the same permission, when you choose to allow inheritance on the child, the result will be that the child has both an inherited and an explicit permission. You will see a check mark in the first and third dialog boxes; the explicit permission obscures the inherited permission in the interface. But the inherited permission is actually present, which can be confirmed in the second dialog box, Advanced Security Settings.
The second method for reinstating inheritance is from the parent folder. In the Advanced Security Settings dialog box of a folder, you may select the check box, Replace Permission Entries On All Child Objects With Entries Shown Here That Apply To Child Objects. The result: all ACLs on subfolders and files are removed. The permissions on the parent are applied. You might see this as “blasting through” the parent’s permissions. After applying this option, any explicit permission that had been applied to subfolders and files is removed, unlike the method used for reinstating inheritance on the child resources. Inheritance is restored, so any changes to the parent-folder ACL are propagated to its subfolders and files. At this point, you might set new, explicit per-missions on subfolders or files. The Replace Permissions option does its job when you apply it, but does not continuously enforce parent permissions.
Effective Permissions
It is common for users to belong to more than one group, and for those groups to have varying levels of resource access. When an ACL contains multiple entries, you must be able to evaluate the permissions that apply to a user based on his or her group memberships. The resulting permissions are called effective permissions.
Understanding Effective Permissions
The rules that determine effective permissions are as follows:
*File permissions override folder permissions: This isn’t really a rule, but it is often presented that way in documentation, so it is worth addressing. Each resource maintains an ACL that is solely responsible for determining resource access. Although entries on that ACL may appear because they are inherited from a parent folder, they are nevertheless entries on that resource’s ACL. The security subsystem does not consult the parent folder to determine access at all. So you may interpret this rule as: The only ACL that matters is the ACL on the resource.
*Allow permissions are cumulative: Your level of resource access may be determined by permissions assigned to one or more groups to which you belong. The Allow permissions that are assigned to any of the user, group, or computer IDs in your security access token will apply to you, so your effective permissions are fundamentally the sum of those Allow permissions. If the Sales Reps group is allowed Read & Execute and Write permissions to a folder, and the Sales Managers group is allowed Read & Execute and Delete permissions, a user who belongs to both groups will have effective permissions equivalent to the Modify permissions template: Read & Execute, Write and Delete.
*Deny permissions take precedence over Allow permissions: A permission that is denied will override a permission entry that allows the same access. Extending the example above, if the Temporary Employees group is denied Read permission, and a user is a temporary sales representative, belonging to both Sales Reps and Temporary Employees, that user will not be able to read the folder.
*Explicit permissions take precedence over inherited permissions: A per-mission entry that is explicitly defined for a resource will override a conflicting inherited permission entry. This follows common-sense design principles: A parent folder sets a “rule” through its inheritable permissions. A child object requires access that is an exception to the rule, and so an explicit permission is added to its ACL. The explicit permission takes precedence.
Evaluating Effective Permissions
Complexity is a possibility, given the extraordinary control over granular permissions and inheritance that NTFS supports. With all those permissions, users and groups, how can you know what access a user actually has?
Microsoft added a long-awaited tool to help answer that question. The Effective Per-missions tab of the Advanced Security Settings dialog box, shown in Below Figure, provides a reliable approximation of a user’s resulting resource access.
To use the Effective Permissions tool, click Select and identify the user, group, or built-
in account to analyze. Windows Server 2003 then produces a list of effective permis
sions. This list is an approximation only. It does not take share permissions into
account, nor does it evaluate the account’s special memberships, such as the following:
i. Anonymous Logon
ii. Batch
iii. Creator Group
iv. Dialup
v. Enterprise Domain Controllers
vi. Interactive
vii. Network
viii. Proxy
ix. Restricted
x. Remote Interactive Logon
xi. Service
xii. System
xiii. Terminal Server User
xiv. Other Organization
xv. This Organization
An ACL can contain entries for the Network or Interactive accounts, for example, which would provide the opportunity for a user to experience different levels of resource access depending on whether the user was logged on to the machine or using a net-work client. Because the user in question is not logged on, logon-specific permissions entries are ignored. However, as an extra step, you can evaluate effective permissions for a built-in or special account such as Interactive or Network.
Resource Ownership
Windows Server 2003 includes a special security principal called Creator Owner, and an entry in a resource’s security descriptor that defines the object’s owner. To fully manage and troubleshoot resource permissions, you must understand these two parts of the security picture.
Creator Owner
When a user creates a file or folder (which is possible if that user is allowed Create Files/Write Data or Create Folders/Append Data, respectively), the user is the creator and initial owner of that resource. Any permissions on the parent folder assigned to the special account Creator Owner are explicitly assigned to the user on the new resource.
As an example, assume that a folder allows users to create files (allow Create Files/ Write Data), and the folder’s permissions allows users to Read & Execute, and Creator Owner Full Control. This permission set would allow Maria to create a file. Maria, as the creator of that file, would have full control of that file. Tia can also create a file, and would have full control of her file. However, Tia and Maria would only be able to read each other’s files. Tia could, however, change the ACL on the file she created. Full Control includes the Change Permission.
Ownership
If for some reason Tia managed to modify the ACL and deny herself Full Control, she could nevertheless modify the ACL, because an object’s owner can always modify its ACL, preventing users from permanently locking themselves out of their files and folders.
It is best practice to manage object ownership so that an object’s owner is correctly defined. This is partly because owners can modify ACLs of their objects, and also because newer technologies, such as disk quotas, rely on the ownership attribute to calculate disk space used by a particular user. Prior to Windows Server 2003, managing ownership was awkward. Windows Server 2003 has added an important tool to simplify ownership transfer.
An object’s owner is defined in its security descriptor. The user who creates a file or folder is its initial owner. Another user can take ownership, or be given ownership of the object using one of the following processes:
*Administrators can take ownership: A user who belongs to the Administrators group of a system, or who has otherwise been granted the Take Ownership user right, can take ownership of any object on the system.
To take ownership of a resource, click the Owner tab of the Advanced Security Settings dialog box, as shown in Below Figure. Select your user account from the list and click Apply. Select the Replace Owner On Subcontainers And Objects check box to take ownership of subfolders and files.
*Users can take ownership if they are allowed Take Ownership permission: The special permission Take Ownership can be granted to any user or group. A user with an Allow Take Ownership permission can take ownership of the resource and then, as owner, modify the ACL to provide sufficient permissions.
*Administrators can facilitate the transfer of ownership: An administrator can take ownership of any file or folder. Then, as owner, the administrator can change permissions on the resource to grant Allow Take Ownership permission to the new owner, who then can take ownership of the resource.
*Restore Files And Directories user right enables the transfer of ownership: A user with the Restore Files And Directories rights may transfer owner-ship of a file from one user to another. If you have been assigned the Restore Files And Directories right, you can click Other Users Or Groups and select the new owner. This capability is new in Windows Server 2003, and makes it possible for administrators and backup operators to manage and transfer resource ownership without requiring user intervention.
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