Using Software Restriction Policies to Protect Against Unauthorized Software

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Introduction

Software Restriction Policies—An Overview

Software Restriction Policy Architecture

Software Restriction Policy Options

Software Restriction Policy Design

Step-by-Step Guide for Designing a Software Restriction Policy

Step-by-Step Guide for Creating Additional Rules

Commonly Overlooked Rules

Scenarios

Deployment Considerations

Troubleshooting Software Restriction Policies

Appendix

Summary

Related Links

Introduction

Software restriction policies are part of Microsoft’s security and management strategy to assist enterprises in increasing the reliability, integrity, and manageability of their machines. Software restriction policies are one of many important management features in Windows Vista and earlier operating systems (Windows XP and Windows Server 2003). In Windows Vista and Windows Longhorn Server, software restriction policies include some new features to meet current security challenges.

This article provides an in-depth look at the new Software Restriction Policy features.

This article also provides an in-depth look at how software restriction policies can be used to:

Expanded Management Capabilities

Microsoft Windows 2000 brought significant management capabilities to the Microsoft Windows platform. In Windows 2000, you could manage the software for your machines in the following ways

Windows XP and Windows Server 2003 expand the management capabilities of Windows 2000 by adding the following features

Software restriction policies integrate with the operating system and common scripting runtimes to control the running of software at execution. In Windows 2000 you could hide access to applications by removing them from the Start menu or hiding the Run command. New software restriction policies go beyond this by simply removing the common access points for software.

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Software Restriction Policies—An Overview

This section discusses the behavior of hostile code and problems associated with unknown code.

Hostile Code Can Infiltrate Easier

With the increased use of networks and the Internet in daily business computing, the potential for encountering hostile code is higher than ever before. People collaborate in more sophisticated ways by using e-mail, instant messaging, and peer-to-peer applications. As these collaboration opportunities increase, so does the risk of viruses, worms, and other hostile code invading your systems. It is important to note that e-mail and instant messaging can transport unsolicited hostile code. Hostile code can take many forms—it can range from native Windows executables (.exe), to macros in word processing documents (.doc), to scripts (.vbs).

Viruses and worms often use social engineering to trick users into activating them. With the sheer number and variety of forms that code can take, it can be difficult for users to know what is safe to run. When activated, hostile code can damage content on a hard disk, flood a network with a denial-of-service attack, send confidential information to the Internet, or compromise the security of a machine.

The Problem with Unknown Code

Hostile code is not the only threat—many non-malicious software applications also cause problems. Any software not known and supported by an organization can conflict with other applications or change crucial configuration information. Software restriction policies were designed to help organizations control not just hostile code, but any unknown code—malicious or otherwise.

Responding to Unknown Code

Software restriction policies help a business respond to unknown code by:

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Software Restriction Policy Architecture

Figure 1 shows the three components of a software restriction policy

Default Rule or Default Security Level

A software restriction policy is created using the MMC Group Policy snap-in. A policy consists of a default rule about whether programs are allowed to run, and exceptions to that rule. The default rule can be set to Unrestricted, Disallowed, or Basic User—run, do not run, or run as a normal user.

Setting the default rule to Unrestricted allows an administrator to define exceptions, for example, the set of programs that are not allowed to run. A more secure approach is to set the default rule to Disallowed, and specify only the programs that are known and trusted to run. An intermediate secure approach is to set the default rule to Basic User. This means that when no exception rule match is found, the application will be run as a normal user.

Note Normal user has the following security identifiers (SIDs) disabled: Admins, cert admins, schema admins, enterprise admins, policy admins, and all privileges.

Warning Some applications that require admin privileges might not run as a normal user.

Security Level

This is the security level for exception (additional) rules defined with reference to the default rule. Software restriction policies can be used in the following three ways.

Four Rules Identify Software (Additional Rules)

Four types of additional rules can be defined in a software restriction policy. The purpose of an additional rule is to identify one or more software applications, and specify whether they are allowed to run. Creating additional rules largely consists of identifying software that is an exception to the default rule. Each rule can include descriptive text to help communicate why the additional rule was created.

A software restriction policy supports the following four ways to identify software

Hash Rules

A hash rule is a cryptographic fingerprint that uniquely identifies a file regardless of where it is accessed or what it is named. An administrator may not want users to run a particular version of a program. This may be the case if the program has security or privacy bugs, or compromises system stability. With a hash rule, software can be renamed or moved into another location on a disk, but it will still match the hash rule because the rule is based on a cryptographic calculation involving file contents.

In Windows Vista a new hash rule will contain two hashes (MD5 or SHA-1) and SHA-256.

In earlier versions of Windows (Windows XP and Windows Server 2003), the default hash was MD5. The software restriction policy hash was upgraded to support a new algorithm (SHA-256) as part of a Windows-wide upgrade in cryptographic algorithms. Computers running Windows Vista will always try to read the SHA256 hash first.

Why were two hashes created (MD5 or SHA-1 hash along with SHA-256 hash)?

Software restriction policy server and clients might run on different operating systems. It is important to ensure that an Software restriction policy Group Policy created on a Windows Longhorn Server could be correctly applied on a down-level client. Conversely, the Windows XP or Windows Server 2003 software restriction policy could be applied on a Windows Vista client.

Changes to Safer Application Programming Interfaces (APIs)

To facilitate this hash algorithm upgrade, the safer APIs will contain two new data structures.

SAFER_CODE_PROPERIES

SAFER_HASH_IDENTIFICATION2

For more information about safer API usage, see winsafer.h in platform SDK

Certificate Rules

A certificate rule specifies a code-signing, software publisher certificate. For example, a company can require that all scripts and ActiveX controls be signed with a particular set of publisher certificates. Certificates used in a certificate rule can be issued from a commercial certificate authority (CA) such as VeriSign, a Windows Longhorn Server/Windows Server 2003 Public Key Infrastructure (PKI), or a self-signed certificate.

A certificate rule is a strong way to identify software because it uses signed hashes contained in the signature of the signed file to match files, regardless of name or location. If you want to make exceptions to a certificate rule, you can use a hash rule to identify the exceptions.

Note Certificate rules are not enabled unless they are enabled in the Enforcement Property dialog box. The Basic User security level is not supported for certificate rules.

Path Rules

A path rule can specify a folder or fully qualified path to a program. When a path rule specifies a folder, it matches any program contained in that folder and any programs contained in subfolders. Both local and UNC paths are supported.

Using Environment Variables in Path Rules. A path rule can use environment variables. Since path rules are evaluated in the client environment, the ability to use environment variables (for example, %WINDIR%) allows a rule to adapt to a particular user's environment.

Important Environment variables are not protected by access control lists (ACLs). If users can start a command prompt, they can redefine an environment variable to a path of their choosing.

Using Wildcards in Path Rules. A path rule can incorporate the question mark (?) and asterisk (*) wildcards, allowing rules such as "*.vbs" to match all Microsoft Visual Basic Script files.Example

Registry Path Rules. Many applications store paths to their installation folders or application directories in the Windows registry. You can create a path rule that looks up these registry keys. For example, some applications can be installed anywhere on the file system. These locations may not be easily identifiable by using specific folder paths, such as C:\Program Files\Microsoft Platform SDK, or environment variables, such as %ProgramFiles%\Microsoft Platform SDK. If the program stores its application directories in the registry, you can create a path rule that will use the value stored in the registry, such as %HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\PlatformSDK\Directories\Install Dir%.

This type of path rule is called a registry path rule. The registry path is formatted as follows:

%[Registry Hive]\[Registry Key Name]\[Value Name]%

Note Any registry path rule suffix should not contain a backslash (\) character immediately after the last percent (%) sign in the rule.

Important When you set a path rule, you should check the ACL entries on the path. If users have write access to a path, they can modify its contents. For example, if you allow C:\Program Files, any power user on the machine can copy software to the Program Files folder.

Path Rule Precedence. When there are multiple matching path rules, the most specific matching rule takes precedence.

The following is a set of paths, from highest precedence (more specific match) to lowest precedence (more general match).

Network Zone Rules

A rule can identify software from the network zone from which it is downloaded. These zones are:

Currently, this applies only to Microsoft Windows Installer (*.MSI) packages. It does not apply to software downloaded in Microsoft Internet Explorer.

When to Use Each Rule

Note Each rule has a globally unique identifier (GUID) associated with it. An example GUID is {f8c2c158-e1af-4695-bc93-07cbefbdc594}. Two identical rules will have two different GUIDs. GUIDs help you troubleshoot to determine the specific rule in the specific policy that is being used. For more information, see the Troubleshooting section

Rule Precedence

Rules are evaluated in a specific order. The rules that more specifically match a program take precedence over rules that more generally match a program.

Table 2 and the following examples illustrate how rules are processed when a program is started.

Program being started: C:\WINDOWS\SYSTEM32\EventQuery.vbs

This program matches the following rules

Rule 3 is the most specific match for this program. Because Rule 3 has a security level of Unrestricted, the program is allowed to run.

Program being started: C:\WINDOWS\SYSTEM32\pagefileconfig.vbs

This program matches the following rules

Rule 1 is the most specific match for this program. Because Rule 1 has a security level of Disallowed, the program is disallowed.

Program being started: \\LOGIN_SRV\Scripts\CustomerScript1.vbs

This program matches the following rules

Rule 2 is the most specific match for this program. Because Rule 2 has a security level of Unrestricted, the program is allowed to run.

Program being started: C:\Documents and Settings\user1\LOVE-LETTER-FOR-YOU.TXT.VBS

This program matches Rule 4 because it has a .vbs extension.

Rule 4 is the most specific match for this program. Because Rule 4 has a security level of Disallowed, the program is disallowed.

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Software Restriction Policy Options

This section discusses the various options that influence the behavior of a software restriction policy. These options alter the scope of enforcement behavior or the Authenticode trust settings for digitally signed files.

Enforcement Options

The two enforcement options are dynamic-link library (DLL) checking and Skip Administrators.

DLL Checking

A program, such as Internet Explorer, consists of an executable file, iexplore.exe, and many supporting DLLs. By default, software restriction policy rules are not enforced against DLLs. This is the recommended option for most customers for the following three reasons.

DLL checking is provided as an option if you want the highest assurance possible when running programs in your environment. While viruses primarily target executables for infection, some target DLLs. To ensure that a program has not been infected by a virus, you can use a set of hash rules that identify the executable and all its required DLLs.

To turn on DLL checking

Skip Administrators

An administrator may want to disallow the running of programs for most users, but allow administrators to run all programs. For example, a customer may have a shared machine that multiple users connect to using Terminal Server. The administrator may want users to be able to run only specific applications on the machine, but allow members of the local administrators group to run any program. To do this, use the Skip Administrators option.

If the software restriction policy is created in a GPO attached to an object in Active Directory, the preferred way to use this option is to deny the Apply Group Policy permission on the GPO to a group containing the administrators. This way less network traffic is consumed downloading GPO settings that do not apply to administrators. However, software restriction policies defined in Local Security Policy objects have no way to filter, based on users. In this case, the Skip Administrators option should be used.

To turn on Skip Administrators

Enforce Certificate Rule

By default, software restriction policy rules are not enforced against certificate rules because enabling this option results in performance degradation.

To Enforce Certificate Rules

Defining Executables

The Designated File Types dialog box (shown in Figure 3) lists the file types to which the software restriction policy applies. The designated file types are file types that are considered executable. For example, a screen saver file (SCR) is considered executable because when it is double-clicked in Microsoft Windows Explorer, it is loaded as a program.

The rules in a software restriction policy only apply to the file types listed in the Designated File Types dialog box. If your environment uses a file type that you want to be able to apply rules to, add it to the list. For example, if you use Perl scripting files, you may choose to add .pl and other file types associated with the Perl engine to the Designated File Types list.

Trusted Publishers

The Trusted Publishers options (shown in Figure 4) allow you to configure settings related to ActiveX controls and other signed content.

Table 3 shows Trusted Publisher options related to the use of ActiveX controls and other signed content.

Scope of Software Restriction Policies

Software restriction policies do not apply to the following:

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Software Restriction Policy Design

This section covers how software restriction policies are administered using Group Policy snap-ins, things to be concerned about when editing a policy for the first time, and what is involved in applying a software restriction policy to a group of users.

Integration with Group Policy

Software restriction policies are administered using the following Group Policy snap-ins.

Domain Policy

To set up a domain policy

Local Security Policy

To set up a security policy

If editing a GPO, you can set User and Machine software restriction policies as shown in Figure 5.

Local Security Policy

In a domain, both user and machines policies can be set in one MMC snap-in, as explained previously. For local security policy, there is no single place where software restriction policies can be set for both user and machine. The following steps show how to set user and machine local security policy.

Local Security Policy (Machine)

To set up a security policy

If you are editing the local security policy, the software restriction policy settings are located as indicated in Figure 6.

Local Security Policy (User)

To set up a security policy

If you are editing the local user security policy, the software restriction policy settings are a indicated in Figures 7-12

First-time Considerations

The first time you edit a policy, you will see the message in the result pane as shown in Figure 13. The message is warning you that creating a policy will define default values. These default values can override settings from other software restriction policies.

To create a policy

Applying a Software Restriction Policy to a Group of Users

A software restriction policy is delivered through Group Policy to a site, domain, or OU. However, an administrator may want to apply a software restriction policy to a group of users within a domain. To do this, the administrator can use GPO filtering.

For more information about GPO filtering, see the Windows Server 2003 Group Policy article at http://www.microsoft.com/windowsserver2003/technologies/management/grouppolicy/default.mspx

Terminal Servers

Software restriction policies are an integral part of securing a Windows Server 2003 terminal server. Terminal server administrators can now thoroughly lock down software access on a terminal server. Software restriction policies are even more imperative on a terminal server because of the potentially vast number of users on a single machine. On a single-user Windows XP client, running a bad application inconveniences only one user, whereas running the same application on a terminal server could inconvenience more than 100 users. Software restriction policies prevent this problem. This service also removes the need for such applications as appsec.exe to govern software execution on a Windows Server 2003 terminal server.

In addition, Microsoft recommends that you view 278295 (How to Lock Down a Windows 2000 Terminal Server Session) to further lock down the client sessions on a terminal server.

Sometimes, several terminal servers have the same software installed on them, but their administrator wants to grant a certain group of users access to some software and a different group of users access to different software. Some software will be shared between the groups. For example, a law firm hosts its applications across a farm of terminal servers. The servers all have the same software installed. The access rules to the software are as follows:

To achieve this software access, the administrator creates five GPOs with customized software restriction policies. Each GPO is filtered so that only the users in the AllEmployees, AccountingEmployees, Lawyers, MailRoomEmployees, and Executives groups receive their intended GPO

Because only executives should be able to access any software on their local workstations, as well as on the terminal servers, the administrator uses the loopback feature of Group Policy. The loopback feature allows an administrator to apply policy to a user based on the computer the user is logging on to. In loopback replace mode, the computer GPO settings are reapplied when the user logs on, and the user GPO settings are ignored. For more information about how to configure loopback, see the Group Policy related white papers at http://technet2.microsoft.com/windowsserver/en/whitepapers.mspx.

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Step-by-Step Guide for Designing a Software Restriction Policy

This section outlines the steps to follow when designing a software restriction policy.

Items to Address

When designing a policy, decisions need to be made regarding the following items

Stepping Through the Process

Step 1. GPO or Local Security Policy

Should the policy apply to many machines or users in a domain or OU, or should it only apply to the local machine?

Step 2. User or Machine Policy

Should the policy apply to users regardless of where they log in, or to a machine regardless of who logs in?

Step 3. Default Security Level

Do you know all the software your users will be running, or can they install any software they choose?

Step 4. Additional Rules

Identify the applications you choose to allow or disallow using the four rule types outlined previously in the Software Restriction Policy Architecture section above.

Step 5. Policy Options

Several policy options are available

Step 6. Linking the Policy to a Site, Domain, or Organizational Unit

To link a GPO to a site

To link a GPO to a domain or OU

Filtering

GPO filtering can be done at this stage. You can have a portion of an OU receive a GPO by filtering based on group membership. You can also filter based on a WMI query.

Testing a Policy

If you want to test your policy immediately, instead of waiting for the next Group Policy refresh interval, run gpupdate.exe /force, and then log on again to test your policy. You need to reboot the machine to enforce software restriction policies when Software Restriction Policies are created for the first time.

Important You need to reboot the machine to enforce software restriction policies when Software Restriction Policies are created for the first time to test the policy. If you have only one user, logging off would be sufficient to test the policy. However, if you have many users using multiple terminal server sessions. Software restriction policies will not be applied to those active sessions if you just log off. For this purpose, it is recommended that you reboot the machine for the first time when Software Restriction Policies are created, and from then on, gpupdate/force will update the policy.

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Step-by-Step Guide for Creating Additional Rules

The following steps are helpful when creating additional rules. To illustrate the principles behind the steps, each one illustrates an example of creating rules for Office XP.

Step 1. List the Software Applications

List the software you are trying to identify. For the Office XP example, the software consists of Microsoft Word, Microsoft Excel, Microsoft PowerPoint, and Outlook

Step 2. Decide Rule Type

See Table 1 to decide which rule type to use. Also, determine the security level for your rule. For the example, path rules set to the Unrestricted security level are used.

Step 3. Record the Folders Where the Software Is Installed

List the paths where the software is installed. Three ways to do this include:

For the example, you will see the following tasks running.

Step 4. Identify Dependent Programs

Some programs launch other programs to perform tasks. Your software application may depend on one or more supporting programs. For example, Word launches Microsoft Clip Organizer to manage clip art. Clip Organizer uses the following programs.

Office also uses files in the C:\Program Files\Common Files folder.

Step 5. Generalize the Rules

In this step, you should group related rules to create a more general rule. Consider using environment variables, wild cards, and registry path rules.

Continuing the example, each program is stored in C:\Program Files\Microsoft Office\Office10, so it is sufficient to use one path rule for that folder instead of four separate path rules. Also, if Office is always installed in the Program Files folder on your machines, use an environment variable instead of an explicit path. Thus, our proposed rules are:

Step 6. Have You Allowed Too Much?

In this step, you look at what else is allowed by the rules you have proposed. Creating a rule that is too general may allow programs to run that you did not intend. The Office10 folder in the example also contains:

Because these programs are acceptable to run, the rules do not have to be changed

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Commonly Overlooked Rules

When designing a policy, consider the following areas when creating rules.

Login Scripts

Login scripts are stored on a central server. Often, this central server can change each time a user logs on. If your default rule is Disallowed, be sure to create rules that identify the locations of your login scripts. Consider using wildcards to identify these locations if the login servers have similar names.

System File Protection

System file protection contains backup copies of many system programs in a folder named dllcache. These programs can be started by a user who knows the full path to the backup copy. If you want to disallow users running programs contained in the backup folder, you may want to create the following rule. %WINDIR%\system32\dllcache, Disallowed

Common Startup Locations

Windows has many locations that contain links to programs that run at start up. If you do not make provisions for these programs, users will receive error messages when they log on.Common startup locations include:

Common startup locations include:

Virus Scanning Programs

Most anti-virus software has a real-time scanner program that starts when the user logs on and scans all files accessed by the user, looking for possible virus contamination. Make sure your rules allow your virus scanning programs to run.

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Scenarios

This section examines some typical problems and how software restriction policies can be used to solve them.

Block Malicious Scripts

An organization wants to be protected from script-based viruses. The LoveLetter virus, technically called a worm, was estimated to have caused between 6 and 10 billion dollars in damage. This worm, which has more than 80 variants, continues to be encountered frequently.

The LoveLetter worm, written in Visual Basic Script (VBScript) language, is encountered as LOVE-LETTER-FOR-YOU.TXT.VBS. A software restriction policy blocks this worm simply by disallowing any .vbs file from running.

However, many organizations use VBScript files for systems management and login scripts. Blocking all VBScript files from running protects an organization, but a VBScript can no longer be used for legitimate purposes. A software restriction policy overcomes this handicap by blocking the undesirable VBScript, while allowing legitimate ones to run.

This policy can be created using the rules in Table 4.

This policy prevents all scripting files associated with the Microsoft Windows Script Host from running, except those that are digitally signed by the IT Department certificate. For information about how to obtain a certificate and digitally sign files, see the Appendix.

Blocking Mapped Drive

If you set a disallowed path rule policy to block a drive that is mapped to a file server (\\server\share), software restriction policy does not work. The reason is that when you map a drive to a share, the drive is linked to that share path (\\server\share). Software restriction policy will always resolve the link to the actual share path (in this case \\server\share) and search for any rules that apply to that path (in thhis case \\server\share) not to the mapped drive.

Manage Software Installation

You can configure your organization's machines so that only approved software can be installed. For software that uses Windows Installer technology, this can be accomplished by the policy shown in Table 5.

This policy prevents all Windows Installer packages from installing. It allows MSI files digitally signed by the IT Department certificate and the OWC10.MSI package, located at \\products\install, to be installed. For information about how to obtain a certificate and digitally sign files, see the Appendix.

This policy also shows how you can use the precedence of the path and certificate rules to allow only the software you want. For any other package that your organization cannot or does not want to digitally sign, you can create hash rules, or fully qualified path rules, to make exceptions for them.

Line-of-Business PC

In some cases, an administrator may want to manage all the software that runs on a machine. This is because even when users have insufficient rights to replace system files or files in shared folders such as Program Files, they can copy a program to a place on the file system that they can write to, and then start that program.

Viruses contracted this way can damage the system by modifying operating system settings and files; they can also cause great damage by misusing the user's privileges. For example, mass-mailer worms can be spread by accessing the user's address book and sending e-mail. Even normal users on a system are vulnerable to this kind of attack.

As long as users are not administrators on their local machines, the policy in Table 6 protects them from accidentally running malicious code. Because users cannot modify the contents of the Program Files or Windows folders, they can only run software installed by an administrator.

This policy disallows all software on the user's machine, except that installed in the Windows directory, Program Files directory, or the respective subfolders. It does not apply to administrators.

If a user receives a virus attachment in an e-mail, for example, WORM.vbs, the mail program will copy it to the profile directory (%USERPROFILE%) and launch it from there. Because the profile directory is not a subfolder of the Windows folder or the Program Files folder, programs launched from there will not run.

If all the programs a user needs are not installed in %WINDIR% or %PROGRAMFILES%, or there are programs in those folders that the administrator does not want the user running, the administrator can make additional exceptions as shown in Table 7.

The effects of these exceptions are:

Different Policies for Different Users

In this scenario, there are machines that are shared by many users. The machines have the same software installed on them, but the administrator wants to grant a certain group of users access to some software, and a different group of users access to other software. There also will be software that is shared between the groups.

Example

A computer lab at a university runs 15 machines with identical software. It runs Office, computer-aided design (CAD) software, and the Microsoft Visual C++ compiler. For licensing reasons, the administrators of the computer lab want to ensure the following:

To achieve the objectives of this scenario, the administrator creates three GPOs with customized software restriction policies. Each GPO is filtered so that only the users in AllStudents, EngStudents, and CSStudents receive the GPO intended for them.

Because the students need to receive the policy when they log on to the lab computers, but not when they log on to their personal computers, the administrator uses the Group Policy loopback feature. The loopback feature allows an administrator to apply policy to a user based on the computer the user is logging on to. In loopback replace mode, the machine GPOs are reapplied when the user logs on, skipping the normal user policies.

See Tables 8, 9, and 10, and Figure 8.

For more information about how to configure loopback, see the Windows Server 2003 Group Policy article at http://www.microsoft.com/windowsserver2003/technologies/management/grouppolicy/default.mspx.