Windows Constrained Delegation

👉 Overview

👀 What ?

Windows Constrained Delegation (WCD) is a security feature in the Microsoft Windows operating system that allows an administrator to delegate specific permissions to a service. This service, in turn, can impersonate a user and perform actions on their behalf, but constrained to specific services on specific machines.

🧐 Why ?

Understanding WCD is crucial because it can be both a powerful tool and a potential security risk. As a tool, it helps in achieving a fine-grained access control in a distributed environment. However, if not configured properly, it can be exploited to escalate privileges and perform unauthorized actions.

⛏️ How ?

To use WCD, you should first identify the service you wish to delegate permissions to. Once identified, you can configure the delegation using the Active Directory Users and Computers (ADUC) tool. Simply select the service, go to its properties, and under the 'Delegation' tab, you can specify the services and the computers to which the delegation should be constrained.

⏳ When ?

The use of WCD started with the release of Windows Server 2003. It was introduced as a way to give administrators more control over what a service can do when impersonating a user.

⚙️ Technical Explanations

Windows Constrained Delegation (WCD) is a security feature designed to delegate specific permissions to a service, allowing it to impersonate a user and perform actions on their behalf within defined limits. The process begins when a service obtains a Kerberos ticket on behalf of a user. Kerberos is a network authentication protocol that uses tickets to allow nodes to prove their identity over a non-secure network, in a way that is designed to prevent eavesdropping or replay attacks.

The Kerberos ticket acquired by the service is used to authenticate to another service. The ticket, however, is 'constrained' to specific services on specific machines, thus shaping the scope of what actions can be performed. This constraint is enforced by the Key Distribution Center (KDC), which issues the ticket. The KDC is a part of the Kerberos protocol that supplies temporary session keys to users and services that communicate over a non-secure network.

The restrictions imposed by the KDC ensure that even if a malicious actor compromises a service with delegation permissions, they cannot perform actions beyond the constraints defined by the system administrator. This is an important aspect of WCD, as it helps to mitigate the damage of potential security breaches.

Despite its utility, WCD should be handled with care. If improperly configured, it can be exploited to escalate privileges and perform unauthorized actions. Therefore, understanding WCD is crucial for maintaining secure operations in a Windows environment.

For example, let's consider a situation where a web server is running a service that needs to access a database server on behalf of a user. This service needs to have delegated permissions to access the database server.

1. Configuring the Service for Delegation in Active Directory

   The first step is to configure the web server service for delegation in Active Directory. This is done using the Active Directory Users and Computers (ADUC) tool.

   Open ADUC > Navigate to the service account > Right-click and select Properties > Select the 'Delegation' tab
   
   

   On the 'Delegation' tab, select the option "Trust this user for delegation to specified services only". Then, select "Use any authentication protocol". Below this, you can add the services to which this service can present delegated credentials. In this case, it would be the database service on the database server.

2. Acquiring a Kerberos Ticket

   When a user authenticates to the web server, the service will obtain a Kerberos ticket on behalf of the user.

   kinit user@DOMAIN
   
   

   This command initiates the Kerberos authentication process for the user.

3. Presenting the Kerberos Ticket to the Database Server

   The web server service will then present the Kerberos ticket when connecting to the database server.

   klist
   
   

   This command will show the Kerberos tickets currently held by the user.

4. Access Control on the Database Server

   The database server checks the Kerberos ticket and validates if the request comes from an authorized service. If it does, it allows the connection and operations to proceed.

Remember, while WCD can be a powerful tool, it's also a potential security risk if not configured properly. The permissions should be as specific as possible to limit the potential for abuse.