Active Directory Trust Security: How to Secure AD Trusts Against Exploits

Active Directory (AD) trusts are the invisible highways that connect domains and forests, enabling seamless authentication and resource sharing. But just like highways, if they aren’t well-patrolled, attackers can use them to move fast—and far.

Poorly secured AD trusts is a fast way for a single breach to cascade into an enterprise-wide compromise. Whether you manage a sprawling enterprise forest or support a multi-domain environment, understanding these trust relationships—and their risks—is critical to protecting your infrastructure.

Types of Active Directory Trusts (and Why They Matter)

An Active Directory (AD) trust is a secure relationship established between two AD domains or forests that allows users in one domain to access resources in another. These trusts are essential for authentication and resource sharing in multi-domain or multi-forest environments.

There are several types of AD trusts:

• One-way vs. Two-way Trusts

• Transitive vs. Non-transitive Trusts

• Forest Trusts, External Trusts, Shortcut Trusts, and Realm Trusts

Trusts use Kerberos and NTLM protocols for authentication. How a trust is set up affects both how easy it is to share resources and how well the environment is protected from threats. This article explores the security implications of Active Directory (AD) trust relationships. Whether you're managing a complex enterprise forest or supporting a multi-domain environment, this guide will help you better understand and secure AD trusts.

Poorly secured AD trusts is a fast way for attackers to turn a single breach into an enterprise-wide compromise.

Security Risks That Hide in Active Directory Trusts

While AD trusts are necessary for enterprise collaboration, they significantly expand the attack surface of your Active Directory environment. Here are the most common security concerns:

• Transitive Trust Vulnerabilities. Transitive trusts allow access between domains indirectly. A compromise in a less secure domain can cascade into others, including those with higher privilege levels.

• Unmonitored Foreign Security Principals (FSPs). Users from a trusted domain (FSPs) can be added to groups in the trusting domain. Without proper auditing, this creates silent privilege escalation paths.

• Limited Visibility Across Trust Boundaries. Security teams often lack visibility into the security posture of external domains, especially during mergers, acquisitions, or external partnerships.

• Unrestricted Kerberos Delegation. Improper delegation settings allow attackers to impersonate users across trust boundaries.

• Legacy or Forgotten Trusts. Old trust relationships from deprecated domains may remain active, providing unnecessary and unmonitored access paths.

How Attackers Turn Trusts into Attack Paths

Compromising one domain can lead to a full forest compromise through trust exploitation. Here’s how attackers do it:

• Abuse of Foreign Security Principals. If group memberships aren’t restricted, attackers can add accounts from one domain into privileged groups in another, gaining elevated access.

• Cross-Domain Kerberos Attacks. Pass-the-Ticket and Pass-the-Hash attacks can extend through trust boundaries. SIDHistory Injection can allow impersonation of users in trusted domains.

• ACL Abuse via Tools Like BloodHound. Attackers use enumeration tools to find and exploit overly permissive Access Control Lists across domains.

• Golden Ticket Attacks with Compromised KRBTGT. A stolen KRBTGT account allows attackers to forge Kerberos Ticket Granting Tickets (TGTs) valid across trusted domains.

• Exploiting Forest Trust SID Filtering Gaps. Without proper SID filtering, attackers can inject SIDs to impersonate privileged users across forests.

Best Practices to Lock Down AD Trusts

The following best practices are designed to reduce the risk of lateral movement, privilege escalation, and data breaches across AD environments. By implementing these measures, organizations can strengthen their overall identity security posture, limit unnecessary exposure, and better detect suspicious activity.

• Enforce SID Filtering. Enable SID filtering to prevent SIDHistory injection across forest and external trusts.

• Use Selective Authentication. Apply Selective Authentication to limit which users from a trusted domain can access resources.

• Audit AD Trusts Regularly. Use tools like Get-ADTrust, nltest /domain_trusts, and security-focused scripts to map and analyze all trust relationships. Decommission unused or legacy trusts.

• Monitor Cross-Domain Logons. Set up SIEM alerts for:

  • Event ID 4769: Kerberos service ticket request

  • Logon Type 3/10: Network and remote interactive logons

• Protect Privileged Groups. Regularly review group memberships, especially Domain Admins and Enterprise Admins. Prevent foreign users from being added unintentionally.

• Rotate KRBTGT Passwords Regularly. Protect against Golden Ticket attacks by rotating the KRBTGT account password twice in succession.

• Reduce Transitive Trust Use. Favor non-transitive, explicit trusts for limited, well-audited access. Avoid creating broad trust paths that attackers can traverse.

Conclusion: Trust but Verify, Monitor, and Harden

Active Directory trust relationships are a prime target for attackers seeking lateral movement and privilege escalation.

By understanding how trusts work, being aware of their vulnerabilities, and following the best practices above, organizations can secure their AD infrastructure and minimize risk.

Remember: A compromise in one trusted domain can cascade into a full forest compromise. Trust—but verify, monitor, and harden.