Targeting cross-forest trusts in Active Directory (AD) settings is one of the most pernicious attack vectors in cybersecurity. These trusts, which are intended to simplify authentication and resource sharing across various AD forests, can allow attackers to escalate privileges and cause havoc across entire networks. This playbook looks into how attackers exploit cross-forest trusts, the tools they employ, and what organisations can do to fight against these dangers.

What Are Cross-Forest Trusts

Cross-forest trusts are relationships established between two AD forests that allow users in one forest to access resources in another. These trusts can be configured in several ways:

• Bidirectional Trusts: Both forests trust each other.
• Inbound Trusts: One forest trusts another but not vice versa.
• Outbound Trusts: The reverse of inbound trusts.

While these configurations are essential for enabling collaboration between domains, they also create potential attack surfaces. If improperly configured or monitored, attackers can exploit these relationships to move laterally and escalate their privileges.

Common Attack Techniques

1. Kerberoasting Across Forests

Kerberoasting is a well-known attack technique that targets Kerberos authentication. When extended to cross-forest environments, this attack becomes even more dangerous.

How It Works:

1. Attackers enumerate Service Principal Names (SPNs) in the target domain using tools like PowerView:

Get-DomainUser -SPN -Domain <target_domain>

2. They request Kerberos Ticket Granting Service (TGS) tickets for these SPNs using tools like Rubeus:

.\Rubeus.exe kerberoast /domain:<target_domain> /user:<SPN_user>

3. The TGS hash is then cracked offline using tools like Hashcat or JohnTheRipper.

Real-World Implication: If the attacker successfully cracks the hash, they gain access to service accounts, which often hold administrative privileges.

2. Admin Password Reuse

Password reuse remains a perennial security issue. In environments with bidirectional forest trusts, attackers can exploit this weakness to compromise multiple domains.

Example Scenario:

1. An attacker compromises adm_bob.smith in Domain A.
2. They discover that bsmith_admin in Domain B uses the same password.
3. With this reused credential, the attacker gains administrative access to Domain B.

3. Foreign Group Membership Abuse

Attackers often look for users from one domain who have privileged group memberships in another domain.

Steps:

1. Enumerate foreign group memberships using PowerView:

Get-DomainForeignGroupMember -Domain <target_domain>

2. Identify accounts with elevated privileges in the target domain.
3. Exploit these memberships to escalate privileges.

4. SID History Abuse

Security Identifier (SID) history is a feature meant to facilitate user migrations between domains or forests. However, if SID filtering is not enabled, attackers can manipulate SID history attributes to gain unauthorised access.

How It Works:

1. An attacker adds an administrative SID from Forest A to a user account in Forest B.
2. When authenticating across the trust, this user inherits administrative privileges from Forest A.

Tools of the Trade

These tools are widely available and frequently used by penetration testers and attackers alike.

Mitigation Strategies

Organisations can take several steps to mitigate the risks associated with cross-forest trust abuse:

1. Secure Trust Configurations

• Use Selective Authentication instead of Forest-wide or Domain-wide authentication.
• Enable SID filtering on all inter-forest trusts to prevent SID history abuse.

2. Strengthen Password Policies

• Enforce unique and complex passwords for privileged accounts.
• Regularly rotate passwords and avoid reusing them across domains.

3. Monitor and Detect Suspicious Activity

• Deploy a Security Information and Event Management (SIEM) solution to monitor authentication patterns.
• Use tools like Microsoft Defender for Identity or Azure Sentinel to detect unusual TGS requests.

4. Apply the Principle of Least Privilege

• Restrict privileged account usage across domains.
• Limit unconstrained delegation settings wherever possible.

Why This Matters

Abusing cross-forest trust is a genuine threat, not just a hypothetical one. As organisations continue to expand their networks and integrate multiple AD forests, understanding these attack vectors becomes critical for maintaining security.

Further Reading

For those looking to dive deeper into this topic, here are some excellent resources:

1. The Hacker Recipes: Trusts – A comprehensive guide on exploiting AD trusts.
2. CrowdStrike: Kerberoasting – An overview of Kerberoasting attacks and defenses.
3. CRTE Notes on Cross-Forest Attacks – Practical examples of cross-forest attacks.

By understanding how attackers exploit cross-forest trusts and implementing robust defences, organisations can significantly reduce their exposure to these advanced threats. As always, vigilance and pre-emptive security measures are key to staying ahead of adversaries in today’s developing threat landscape.

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