Working with AppLocker rules

This article for IT professionals describes AppLocker rule types and how to work with them for your application control policies.

Rule collections

AppLocker policies are organized into rule collections, including executable files, scripts, Windows Installer files, packaged apps and packaged app installers, and DLL files. These collections give you an easy way to differentiate the rules for different types of apps. The following table lists the file formats that are included in each rule collection.

The DLL rule collection isn't enabled by default. To learn how to enable the DLL rule collection, see DLL rule collections.

Enforcement modes

AppLocker policies set an enforcement mode for each rule collection included in the policy. These enforcement modes are described in the following table.

When AppLocker policies are merged, the rules from all the policies are added to the effective policy and a single enforcement mode is selected for each rule collection. If multiple AppLocker policies are applied to a device through Group Policy, the enforcement mode setting applied is selected based on Group Policy precedence. If you apply an AppLocker policy locally using the Set-AppLockerPolicy PowerShell cmdlet with the -merge option, the more restrictive enforcement mode is chosen between the existing local policy and the policy being merged.

Rule conditions

Rule conditions are criteria that help AppLocker identify the apps to which the rule applies. The three primary rule conditions are publisher, path, and file hash.

• Publisher: Identifies an app based on its digital signature
• Path: Identifies an app by its location in the file system of the computer or on the network
• File hash: Represents the system computed cryptographic Authenticode hash of the identified file

Publisher

This condition identifies an app based on its digital signature and extended attributes when available. The digital signature contains info about the company that created the app (the publisher). Executable files, dlls, Windows installers, packaged apps and packaged app installers also include extended attributes, which are obtained from the binary resource. These attributes often include the name of the product, the original file name, and the version number of the file as defined by the publisher. If there are packaged apps and packaged app installers, these extended attributes contain the name and the version of the app package.

When you select a reference file for a publisher condition, the wizard creates a rule that specifies the publisher, product, file name, and version number. You can make the rule more generic by moving up the slider or by using a wildcard character (*) in the product, file name, or version number fields.

The File version and Package version control whether a user can run a specific version, earlier versions, or later versions of the app. You can choose a version number and then configure the following options:

• Exactly. The rule applies only to this version of the app
• And above. The rule applies to this version and all later versions.
• And below. The rule applies to this version and all earlier versions.

The following table describes how a publisher condition is applied.

Path

This rule condition identifies an application by its location in the file system of the computer or on the network.

AppLocker uses custom path variables for well-known paths, such as Program Files and Windows.

The following table details these path variables.

File hash

When you choose the file hash rule condition, the system computes the Authenticode cryptographic hash of the identified file. The advantage of this rule condition is that because each file has a unique hash, a file hash rule condition applies to only one file. The disadvantage is that each time the file is updated (such as a security update or upgrade) the file's hash changes. As a result, you must manually update file hash rules.

AppLocker default rules

AppLocker policies created using the AppLocker Group Policy editor can include default rules. Default rules are intended to help ensure that the files that are required for Windows to operate properly are allowed in an AppLocker rule collection. For background, see Understanding AppLocker default rules, and for steps, see Create AppLocker default rules.

Executable default rule types include:

• Allow members of the local Administrators group to run all apps.
• Allow members of the Everyone group to run apps that are located in the Windows folder.
• Allow members of the Everyone group to run apps that are located in the Program Files folder.

Script default rule types include:

• Allow members of the local Administrators group to run all scripts.
• Allow members of the Everyone group to run scripts that are located in the Program Files folder.
• Allow members of the Everyone group to run scripts that are located in the Windows folder.

Windows Installer default rule types include:

• Allow members of the local Administrators group to run all Windows Installer files.
• Allow members of the Everyone group to run all digitally signed Windows Installer files.
• Allow members of the Everyone group to run all Windows Installer files that are located in the Windows\Installer folder.

DLL default rule types:

• Allow members of the local Administrators group to run all DLLs.
• Allow members of the Everyone group to run DLLs that are located in the Program Files folder.
• Allow members of the Everyone group to run DLLs that are located in the Windows folder.

Packaged apps default rule types:

• Allow members of the Everyone group to install and run all signed packaged apps and packaged app installers.

AppLocker rule behavior

If no AppLocker rules are defined for a specific rule collection, all files covered by that rule collection are allowed to run. However, if any rule exists for a specific rule collection, then only those files matching at least one allow rule and not matching any deny rules run. For example, if you create an executable rule that allows .exe files in %SystemDrive%\FilePath to run, only executable files located in that path are allowed to run.

A rule can be configured to use allow or deny actions:

• Allow. You can specify which files are allowed to run in your environment, and for which users or groups of users. You can also configure exceptions to identify files that are excluded from the rule.
• Deny. You can specify which files aren't allowed to run in your environment, and for which users or groups of users. You can also configure exceptions to identify files that are excluded from the rule.

For a best practice, use allow actions with exceptions. Although you can use a combination of allow and deny actions, deny actions always win. You can't use any other rule to allow a file that matches a deny rule.

Rule exceptions

You can apply AppLocker rules to individual users or to a group of users. If you apply a rule to a group of users, the rule affects all users in that group. If you need to allow a subset of a user group to use an app, you can create a special rule for that subset. For example, the rule "Allow everyone to run Windows except Registry Editor" allows everyone in the organization to run the Windows operating system, but it doesn't allow anyone to run Registry Editor.

The effect of this rule would prevent users such as Help Desk personnel from running a program that is necessary for their support tasks. To resolve this problem, create a second rule that applies to the Help Desk user group: "Allow Help Desk to run Registry Editor." If you instead used a deny rule that blocks all users from running Registry Editor, the second rule wouldn't actually let the help desk users run Registry Editor.

DLL rule collection

Because the DLL rule collection isn't enabled by default, you must perform the following procedure before you can create and enforce DLL rules.

Membership in the local Administrators group, or equivalent, is the minimum required to complete this procedure.

To enable the DLL rule collection

1. Select Start, type secpol.msc, and then select ENTER.
2. If the User Account Control dialog box appears, confirm that the action it displays is what you want, and then select Yes.
3. In the console tree, double-click Application Control Policies, right-click AppLocker, and then select Properties.
4. Select the Advanced tab, select the Enable the DLL rule collection check box, and then select OK.

AppLocker wizards

You can create rules by using two AppLocker wizards:

1. The Create Rules Wizard enables you to create one rule at a time.
2. The Automatically Generate Rules Wizard allows you to create multiple rules at one time. You can select a folder and let the wizard create rules for any relevant files found. Or, for packaged apps, let the wizard create rules for all packaged apps installed on the computer. You can also specify the user or group to which to apply the rules. This wizard automatically generates allow rules only.

Other considerations

• By default, AppLocker rules don't allow users to open or run any files that aren't allowed. Administrators should maintain an up-to-date list of allowed applications.
• There are two types of AppLocker conditions that don't persist following an update of an app:
  • A file hash condition File hash rule conditions can be used with any app because a cryptographic hash value of the app file is generated at the time the rule is created. However, the hash value is specific to that exact version of the file. If you need to allow multiple versions of the file, you need individual file hash conditions for each version of the file.
  • A publisher condition with a specific product version set If you create a publisher rule condition that uses the Exactly version option, the rule can't persist if a new version of the app is installed. A new publisher condition must be created, or the version must be edited in the rule to be made less specific.
• If an app isn't digitally signed, you can't use a publisher rule condition for that app.
• If any rules are enforced for the EXE rule collection, you must create rules in the packaged apps and packaged app installers rule collection. Otherwise, all packaged apps and packaged app installers are blocked.
• A custom configured URL can be included in the message that is displayed when an app is blocked.
• Expect an increase in the number of Help Desk calls when users encounter apps that aren't allowed.

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