Active Directory: An Introduction

8 min readSep 17, 2024

Active Directory (AD) is a directory service developed by Microsoft that is pivotal in managing an organization’s IT infrastructure. Introduced with Windows 2000, AD is responsible for the centralized administration of network resources such as users, computers, printers, and applications across Windows domain networks.

The key purpose of AD is to streamline how organizations manage permissions and access to resources, ensuring that the right individuals and systems have access to the correct data. AD is the backbone of security and productivity for businesses of all sizes, facilitating smooth operations across multiple departments, offices, and geographical boundaries.

AD’s capability to store sensitive data such as usernames, passwords, and account permissions in a structured hierarchy makes AD vital. This hierarchy makes management more accessible and provides fine-grained control over who can access which resources. For example, AD allows administrators to set different access permissions based on employees’ roles, ensuring only authorized personnel can access specific information or applications.

However, because AD centralizes such valuable data, it becomes a prime target for cyberattacks. Attackers who compromise AD can access an organization’s network and resources. As a result, maintaining and securing AD is one of the highest priorities for IT departments. Weaknesses or misconfigurations in AD security can lead to unauthorized access, privilege escalation, and lateral movement, which can have devastating consequences for the organization.

Active Directory Structure

Active Directory operates on a hierarchical structure, which allows for scalable and organized management of an enterprise’s IT resources. The architecture facilitates central control while maintaining flexibility across different departments or locations. At its core, AD’s structure comprises several components, each serving a specific function in organizing and controlling resources.

The top level of this hierarchy is known as the forest. A forest consists of one or more domains containing various objects like users, groups, and computers. This hierarchical structure ensures that resources are managed efficiently and security boundaries, known as trusts, are clearly defined.

AD’s hierarchical architecture has several layers, each serving a different purpose:

Forest: The forest is the highest level of the AD structure. It represents an entire instance of Active Directory, consisting of one or more domains with a standard schema and global catalog. A forest acts as a security boundary, meaning that objects within one forest are isolated from objects in another unless explicit trust relationships are established between forests.
Tree: A tree is a collection of one or more domains that share a contiguous namespace and are connected by trust relationships. Domains within a tree inherit trust relationships, meaning users and resources can be accessed across domains within the same tree without establishing separate trust configurations.
Domain: A domain is a logical grouping of objects (such as users, computers, and groups) that share the same AD database. Domains are the fundamental unit for organizing resources and applying security policies. Each domain has its own security policies and trust relationships, making it possible to define distinct boundaries for resource access. For example, an organization might create separate domains for different departments or geographic locations.
Organizational Units (OUs): OUs are subdivisions within a domain that allow administrators to organize resources logically. OUs help with the delegation of administrative tasks by enabling role-based access. For instance, you can have an OU for departments like HR, IT, or Sales, where each department’s resources can be managed independently within the same domain. OUs also facilitate the application of Group Policy Objects (GPOs) to specific sets of users or computers within the domain.
Trusts: Trusts define the relationships between different domains or forests. A trust enables users in one domain to access resources in another domain or forest without requiring separate credentials. Trusts can be one-way or two-way and can be established automatically or manually, depending on the organization’s requirements. This is especially useful in large organizations or scenarios where mergers or acquisitions bring together multiple AD environments.

AD’s architecture provides flexibility, scalability, and control, allowing organizations to manage resources efficiently while maintaining strict security boundaries.

Example Active Directory Structure: CONTOSO.LOCAL

To illustrate how AD’s hierarchical structure works, consider the CONTOSO.LOCAL forest that consists of three domains: ADMIN, OPERATIONS, and DEVELOPMENT. Each domain has its own set of objects, including users, groups, computers, and organizational units (OUs). Trust relationships allow users in one domain to access resources in another when necessary.

This example showcases a typical AD structure with three domains. The ADMIN.CONTOSO.LOCAL domain contains an EMPLOYEES OU subdivided into COMPUTERS, GROUPS, and USERS, along with a SERVICE ACCOUNTS OU for critical service-related accounts.

While structurally similar, the OPERATIONS and DEVELOPMENT domains are omitted here for simplicity but could manage operational and development-related resources, respectively.

Active Directory structure showing forest, domains, OUs, and trust relationships

Contoso is a fictional company name commonly used by Microsoft in their documentation, tutorials, and demo environments. It’s often used as a placeholder to represent a typical organization in examples related to Microsoft products like Windows, Azure, and Active Directory. (https://learn.microsoft.com/en-us/microsoft-365/enterprise/contoso-overview?view=o365-worldwide)

Active Directory Objects

In Active Directory, every information or resource is stored as an object. These objects are essential components that represent individual entities such as users, computers, groups, and more. Each object is defined by attributes, which provide detailed information about the object, such as its name, permissions, group memberships, and other properties. The flexibility of AD’s object-based model allows administrators to manage various resources in a centralized and structured manner.

AD objects are categorized into different types, each serving a specific role within the directory. Understanding these object types is crucial for managing resources and controlling access within the domain.

User Accounts

Definition: User accounts represent individual users who have access to the domain. Each user account is associated with attributes such as username, password, email address, and group memberships. These attributes define what resources the user can access and what actions they can perform.
Purpose: User accounts authenticate individuals and authorize their access to network resources, such as files, printers, and applications.
Attributes: Examples include a unique Security Identifier (SID), login credentials, and access permissions. User accounts can also be grouped for easier management.

Groups

Definition: Groups are collections of user accounts, computers, or other groups treated as a single entity for permission management. There are two main types of groups: Security Groups and Distribution Groups.
Security Groups: Used to assign permissions to shared resources like files, folders, or printers. Security group members inherit the group’s permissions, simplifying the management of large numbers of users.
Distribution Groups: Primarily used for email distribution lists but cannot be used to assign permissions.
Purpose: Groups help streamline permission management by allowing administrators to apply policies to a collection of users or computers rather than individually managing each account.

Computers

Definition: A computer object represents a machine part of the domain. Each computer object has attributes that define its identity within the domain and its permissions.
Purpose: Computer objects are used to manage and apply policies to machines, such as security configurations, software deployment, and auditing. Administrators can apply Group Policy Objects (GPOs) to computers to enforce standard configurations across the network.

GPOs (Group Policy Objects)

Definition: GPOs are a set of rules and settings that control the configuration of users and computers within an AD domain. GPOs can be applied at different levels within the hierarchy (such as at the domain or OU level) and can manage settings related to security, software, desktop environments, and more.
Purpose: GPOs allow administrators to enforce security policies, software installations, and configurations across multiple users and computers, ensuring consistency and compliance within the network.

Active Directory Security Challenges

Active Directory (AD) is crucial in managing access to an organization’s IT resources, making it a prime target for cyberattacks. Given its central role in managing permissions, identities, and access rights, compromising AD can give attackers control over large parts of the network. Once attackers gain unauthorized access to AD, they can escalate privileges, move laterally within the network, and potentially take over critical resources.

Ensuring the security of AD is a complex task, as attackers continually develop new methods to exploit misconfigurations, outdated protocols, and vulnerabilities.

Here are some of the most common security challenges that organizations face in securing AD:

Privilege Escalation

Explanation: Privilege escalation is a common tactic attackers use to gain higher network access levels. Once an attacker gains initial access to AD, often through a compromised user account or vulnerability, they exploit misconfigurations or vulnerabilities to elevate their privileges to those of a domain administrator or another high-privilege account.
Example: Attackers may exploit overly broad permissions, unpatched vulnerabilities, or weak account security to escalate from a regular user account to an administrator account.
Impact: If attackers succeed, they could gain complete control over AD, enabling them to change security settings, create new user accounts, or modify group memberships.

Lateral Movement

Explanation: After gaining access to one machine or user account, attackers attempt to move laterally across the network to compromise additional systems. This is often done by leveraging stolen credentials or exploiting network vulnerabilities. Attackers may use tools like Mimikatz (https://github.com/ParrotSec/mimikatz) to extract passwords from memory or use Kerberos tickets to impersonate legitimate users.
Example: An attacker compromises a low-privilege account and then uses the credentials of that account to access another machine where they attempt to gain access to higher-privilege accounts or sensitive data.
Impact: Lateral movement allows attackers to map the network, identify high-value targets, and expand their control, making it harder to detect the breach early on.

Credential Theft

Explanation: Credential theft is one of the most dangerous threats to AD security. Attackers use techniques such as pass-the-hash or pass-the-ticket to steal and reuse authentication tokens or hashes without knowing a user’s password. Attackers can impersonate legitimate users to access resources once they have the hashed credentials or tokens.
Pass-the-Hash: Attackers steal the hashed version of a user’s password and reuse it to authenticate themselves without knowing the plaintext password.
Pass-the-Ticket: Attackers steal Kerberos tickets, which are used for authentication, and replay them to access resources.
Impact: Credential theft allows attackers to bypass password protections and impersonate legitimate users, making it very difficult to detect unauthorized activity.

Legacy Protocols

Explanation: Many older organizations still rely on legacy authentication protocols, such as NTLM (NT LAN Manager), which are more vulnerable to attacks. These protocols often lack the robust security features of modern alternatives like Kerberos, making them prime targets for attackers.
Example: NTLM relay attacks allow attackers to intercept and relay NTLM authentication attempts, gaining unauthorized access to sensitive resources.
Impact: Using legacy protocols increases the attack surface, allowing attackers to exploit weak security configurations or outdated authentication mechanisms.

Conclusion

Active Directory remains the backbone of many organizations’ IT infrastructure, offering a structured approach to managing users, resources, and permissions. However, with this centralization comes increased responsibility to ensure AD security. Securing AD is more critical than ever as cyber threats evolve, requiring technical controls and vigilant monitoring. This is where tools like PingCastle come into play. In the next part (An Introduction to PingCastle: Securing Your Active Directory), we’ll explore how PingCastle helps assess and strengthen AD security by identifying vulnerabilities and providing actionable insights for maintaining a robust AD environment.