Configuring Applications for Least Privilege

Next to reducing its surface area, the most effective strategy to reduce the risk of a successful attack on your Web server is to configure your applications to run with the least privilege possible. Doing this minimizes the amount of damage that results if an attacker successfully exploits any known or future vulnerability. Similar to reducing the surface area, this technique is not limited to blocking specific threats—it works well for any threat that may be present in your application today or that may be found in the future.

The key to reducing the privilege of the application code in the IIS environment is to understand the identity under which the code executes, select the identity with the minimal number of privileges required, and limit the rights of the identity to access server resources. To help achieve least privilege, we will review these techniques:

  • Use a low privilege application pool identity
  • Set NTFS file system (NTFS) permissions to grant minimal access
  • Reduce trust of ASP.NET applications
  • Isolate applications

These techniques are discussed next in this section.

Use a Low Privilege Application Pool Identity

The majority of code executed as part of a Web application is executed in the context of the IIS worker process and typically runs under the identity configured for the application pool. Therefore, using a least privilege application pool identity is the primary way to constrain the privileges and rights granted to the application code.

By default, IIS application pools are configured to run using the built-in Network Service account, which has limited rights on the Web server. When each IIS worker process is started, it also automatically receives membership in the IIS_IUSRS group. This group replaces the IIS_WPG group used in IIS 6.0 as the required group identifying all IIS worker processes on the computer. IIS setup may still create the IIS_WPG group for backward-compatibility reasons, in which case IIS_IUSRS will be made a member of this group.

In addition, certain code in your application may execute with the identity of the authenticated user associated with each request. Table 14-3 summarizes the identities that may be used in your application.

Table 14-3 Application Identities


When using authentication schemes that produce Windows tokens, such as Windows Authentication or Basic Authentication, be aware that when highly privileged users access your application, it will execute with higher privileges than intended. Therefore, it is recommended that you do not allow users that have administrative privileges on the server to access your application. For more information on what identity each application framework executes under, see Chapter 11.

Caution When using authentication schemes that produce Windows identities, your applications may execute with the identity of the authenticated user.

Also, when using anonymous authentication, you may opt for configuring the anonymous user to be the application pool identity, to ensure that all code always executes under the application pool identity. This makes it significantly easier to manage the access rights of the worker process. You can learn about configuring this in the section titled "Anonymous Authentication" later in this chapter.

Note To simplify access management, configure the anonymous authentication user to be the application pool identity.

When selecting the application pool identity for your applications, use the following guidelines to maintain or improve the security of your Web server:

  • Ensure that the application pool identity is not granted sensitive privileges or unnecessary rights to access resources. Often, in the face of "access denied" errors, administrators tend to grant the application pool identity full or otherwise unnecessary access to resources. This increases the privilege of the worker process and increases the risk of a serious compromise if the code in the worker process is compromised. Only grant the worker process the minimal access needed for the application to work. If this minimal access involves privileges or rights typically associated with administrative users, you need to re-evaluate your application's design.
  • Do not use highly privileged or administrative identities for IIS application pools. Never use LocalSystem, Administrator, or any other highly privileged account as an application pool identity. Just say no!
  • Consider using a lower privilege identity. If your application allows it, consider using a custom low privilege account for the IIS worker process. Unlike IIS 6.0, IIS 7.0 automatically injects the new IIS_IUSRS group into the worker process, eliminating the need for you to make the new identity a member of any group.
  • Separate code with different privilege requirements into different application pools. If your server has multiple applications that require different levels of privilege (for example, one requires the privilege to write to the Web application, and the other one doesn't), separate them into two different application pools.
  • When using anonymous authentication, configure the anonymous user to be the application pool identity. This significantly simplifies configuring access rights for your application by making the application code always execute with the application pool identity.
  • Grant minimal access. When granting access to the application pool identity, grant the minimal access necessary. You can use this in conjunction with separating applications into different application pools to maintain least privilege for your applications. To grant access to a resource for all IIS application pools, grant it to the IIS_IUSRS group. To grant access to a resource for a specific application pool, use the unique application pool identity. Alternatively, use the automatic Application Pool SID that is named IIS APPPOOL\<ApppoolName> (the latter does not work for UNC content, only local content). Do not grant access rights to Network Service because it grants access to all services running on the server under the Network Service identity.

Set NTFS Permissions to Grant Minimal Access

By default, all files required for IIS worker processes to function grant access to the IIS_IUSRS group, which ensures that IIS worker processes can function regardless of the selected application pool identity. However, it is up to you to grant access to the application content so that the Web server and the application can successfully access its resources. Additionally, it is up to you to grant access to the additional resources the IIS worker process uses, such as ISAPI extensions, CGI programs, or custom directories configured for logging or failed request tracing.

Table 14-4 indicates the level of access the Web server typically requires for different kinds of resources.

Table 14-4 Access Levels for Web Server Resources


When granting access to content directories, you can use one of the following techniques:

  • Grant access to IIS_IUSRS. This enables all IIS worker processes to access the content when using the application pool identity, or when using anonymous authentication. However, this does not enable you to isolate multiple application pools. If using a -Windows-based authentication scheme, you also will need to grant access to all of the authenticated users that use your application.
  • Grant access to the identity of the application's application pool. This will enable only the IIS worker processes running in the application pool with the configured identity to access the content. If using anonymous authentication, you additionally need to set the anonymous user to be the application pool identity. If using a Windows-based authentication scheme, you also will need to grant access to all of the authenticated users that use your application. This approach is the basis for application pool isolation. For more information, see the section titled "Isolating Applications" later in this chapter.
  • Configure fixed credentials for the application's virtual directory and grant access to these credentials. This will prompt the IIS worker process to access the content by using the fixed credentials, regardless of the authenticated user identity. This option is often used when granting access to remote UNC shares to avoid the difficulties of ensuring that authenticated user identities can be delegated to access the remote network share. It can also be an efficient way to manage access to the content for a single identity regardless of the authenticated user (which can be set to the application pool identity when using custom application pool identities). Finally, it can be used to control access to the application when you host multiple applications inside the same application pool.

Note If you are using IIS Manager to administer the application remotely, you will also need to grant Read access to the NT Service\WMSvc account. For more information, see Chapter 8, "Remote Administration."

If you are using an authentication scheme (other than anonymous authentication) that produces Windows identities for authentication users, such as most of the IIS authentication schemes, you will also need to make sure that all authenticated users that require the use of your application have access to its content. This is because the Web server will use the authenticated user identity to access application content. Also, many application frameworks will by default impersonate the authenticated user when executing application code and accessing application resources.

When you need to allow multiple Windows users to use the application, you should add all of these users to a specific group and grant this group access to the application content. Alternatively, when using the fixed credentials model, you do not have to grant access to the authenticated users. Instead, IIS and application code will always impersonate the fixed virtual directory credentials. For more information on setting up the fixed credentials model, see the section titled "Managing Remote Content" in Chapter 9, "Managing Web Sites."

When your content is on a UNC share, you will likely need to use the fixed credentials model because most IIS authentication schemes do not produce Windows tokens that can be used for remote network shares (with the exception of basic authentication and IIS client certificate mapping authentication). Alternatively, you can configure your Web server for Constrained Delegation and Protocol Transition to allow the authenticated user tokens to be used against the remote share. However, using the fixed credentials for virtual directories on UNC shares is significantly easier to configure, so it is recommended over setting up delegation. For more information, see the section titled "UNC Authentication" later in this chapter.

Note Unlike in IIS 6.0, in which the authenticated user having access to the content directory is typically sufficient (except for ASP.NET applications), IIS 7.0 also requires the IIS worker process identity to have access to the content directories before they can read the web.config configuration files. This happens before IIS determines the authenticated user.

Reduce Trust of ASP.NET Applications

In addition to constraining the execution rights of the application by using a low privilege application pool identity, you can further sandbox the .NET parts of your application by using the ASP.NET trust levels. The ASP.NET parts of the application include the ASP.NET applications themselves, as well as managed modules that provide services for any application in ASP.NET Integrated mode applications.

ASP.NET trust levels use the Code Access Security (CAS) infrastructure in the .NET Framework to limit the execution of the application code, by defining a set of code permissions that control what application code can and cannot do. By default, ASP.NET applications and managed modules execute using the Full trust level, which does not limit their execution. In this trust level, the application can perform any action that is allowed by the Windows privileges and resource access rights.

You can reduce the trust level of ASP.NET applications to limit their execution further. This can be an effective way to achieve lower privilege for your application. By default, you have options described in Table 14-5, which are defined by the ASP.NET trust policy files.

Table 14-5 Default Trust Level Options


It is recommended that you run ASP.NET applications by using the Medium trust level. In this trust level, the application is not able to access resources outside of itself and cannot perform operations that can compromise the security of the Web server overall. However, if you do this, you should test the application to make sure that it does not experience any security exceptions due to the lack of required permissions. You may also want to performance test the application to make sure that using the reduced trust level does not negatively impact your application's performance.

Note The Medium trust level is the recommended trust level to constrain the execution of ASP.NET applications and managed modules, and to host multiple applications on a shared Web server.

You can use IIS Manager to configure the trust level used for ASP.NET applications and managed modules by double-clicking .NET Trust Levels. You can do this for the entire Web server—or for a specific application—by selecting that application node prior to using the .NET Trust Levels feature. You can also set the trust level directly by changing the level attribute in the system.web/trust configuration section.

Caution The system.web/trust configuration section is not locked by default, which means that any application can configure its own trust level. If you don't want this, lock the configuration section at the server level.

Isolating Applications

Application pools provide a great way to isolate multiple applications running on the same machine, both in terms of availability and security. This provides the following benefits:

  • Failures, instability, and performance degradation experienced in one application do not affect the applications in a different application pool.
  • Applications running in a different application pool can restrict access to their resources to that application pool only, preventing other applications from being able to access their resources.

The recommended way to configure applications for isolation is to place each application into a separate application pool. When you do this, IIS 7.0 makes it easy to isolate applications by automatically injecting a unique application pool security identifier, called the application pool SID, into the IIS worker process of each application pool. Each application pool SID is generated based on the application pool name and has the name IIS APPPOOL\ <ApppoolName>. The application pool SID makes it possible to quickly isolate applications by placing NTFS permissions on their content to grant access only to the application pool SID of the application's application pool.

Note You can quickly isolate applications by setting permissions on their content to allow only the Application Pool SID of the corresponding application pool.

To make Application Pool SID–based isolation effective, you need to do the following:

  1. Configure anonymous authentication to use the application pool identity.
  2. Grant access to the application content for the IIS APPPOOL\ <ApppoolName> SID.
  3. Do not grant access to the application content to IIS_IUSRS, IIS_WPG or any other application pool identity that may be used by another application pool.
  4. Configure separate locations for all temporary and utility directories that IIS and the application use for each application or application pool, and set permissions on them to allow access only for the IIS APPPOOL\ <ApppoolName> SID.

Table 14-6 shows some of the common directories that IIS and ASP.NET applications use. The directories must be configured for isolation for each Web site or application and receive the appropriate permissions to enable access only by the associated application pool.

Table 14-6 Common Directories Used by IIS and ASP.NET Applications


Note The Application Pool SIDs can be used only for isolating local content. If you are using content located on a UNC share, you need to either use a custom application pool identity or configure fixed credentials for each virtual directory. Then you should use that identity to grant access to the network share.

IIS 7.0 provides automatic isolation of the server-level configuration by using configuration isolation. No action is necessary to enable this, because it is done by default. For more information about configuration isolation, see the section titled "Understanding Configuration Isolation" later in this chapter.

Note The server-level configuration in applicationHost.config is isolated automatically using configuration isolation.

However, .NET Framework configuration in the machine.config and root web.config files—as well as the configuration in the distributed web.config files that are part of the Web site's directory structure—are not isolated. To properly isolate the distributed web.config files, set the appropriate permissions on the content directories, as described earlier in this section.

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