What is Kerberos Constrained Delegation (KCD)?

Updated on August 29, 2025

Kerberos Constrained Delegation (KCD) is a crucial security feature that allows a service to impersonate a user to access a specific set of other services on their behalf. Unlike the older, unconstrained delegation model, KCD adheres to the principle of least privilege, providing the required functionality while significantly limiting the security risk.

This article provides a technical deep dive into how KCD works and why it is the preferred method for secure delegation in modern Active Directory environments. Understanding KCD is essential for IT professionals managing multi-tier applications and implementing secure authentication workflows.

The delegation process involves careful orchestration between multiple components in the Kerberos infrastructure. Each step is designed to maintain security while enabling the necessary functionality for complex application architectures.

Definition and Core Concepts

Kerberos Constrained Delegation (KCD) is a security feature that restricts a service’s ability to obtain service tickets on a user’s behalf to a pre-defined, limited list of target services. The configuration is managed on the account of the delegating service within Active Directory.

The core components of KCD include:

• Delegating Service: The “front-end” service (such as a web application) that needs to access a back-end resource on behalf of authenticated users.
• Target Service: The “back-end” service (such as a database or file server) that the delegating service needs to access using the user’s credentials.
• Service Principal Name (SPN): A unique identifier for a service instance within the Active Directory domain. Proper SPN registration is essential for both the delegating and target services to function correctly.

The fundamental difference between KCD and unconstrained delegation lies in the explicit configuration requirement. With KCD, administrators must specifically define which services can be accessed through delegation. This creates a controlled environment that prevents unauthorized access to domain resources.

How It Works

The KCD process is a secure, controlled workflow that occurs during the Kerberos Ticket Granting Service (TGS) exchange. This process ensures that delegation occurs only under predetermined conditions.

• User Authentication: A user first authenticates to the delegating service using their domain credentials. The user receives a service ticket for the front-end application through standard Kerberos authentication.
• Request to the TGS: When the delegating service needs to access a back-end resource on behalf of the user, it sends a request to the Kerberos TGS to obtain a new service ticket for the target service. This request includes the user’s authentication context.
• Validation: The TGS receives the request and examines the KCD configuration on the delegating service’s Active Directory account. The TGS verifies that the requested target SPN appears on the explicit list of allowed delegation targets.
• Service Ticket Issuance: If the request passes validation, the TGS issues a new service ticket for the user, encrypted with the target service’s secret key. This ticket maintains the user’s identity and permissions.
• Access: The delegating service uses this new ticket to securely access the back-end service on behalf of the user, maintaining the original security context.

A key security advantage of KCD is that the user’s Ticket-Granting Ticket (TGT) is never forwarded to the delegating service. This prevents the delegating service from impersonating the user to access any other services in the domain beyond those explicitly configured.

Key Features and Types

KCD includes several important features that extend its functionality and security model:

• Protocol Transition: KCD can perform “protocol transition,” allowing a service to obtain a Kerberos service ticket for a user who authenticated via a non-Kerberos protocol. This includes authentication methods such as NTLM, forms-based authentication, or certificate-based authentication. Protocol transition enables organizations to integrate legacy authentication systems with modern Kerberos-based delegation.
• Resource-Based Constrained Delegation (RBCD): This newer, more granular form of KCD shifts the configuration paradigm. Instead of configuring delegation on the delegating service’s account, RBCD is configured on the target resource’s account. This approach gives the owner of the target resource direct control over which front-end services can delegate to it.

RBCD provides several administrative advantages. It simplifies delegation management in multi-forest or multi-domain environments where traditional KCD configuration becomes complex. Resource owners can independently manage their delegation policies without requiring domain-wide administrative privileges.

The RBCD model also supports more dynamic environments where services may need to establish delegation relationships without extensive cross-domain coordination. This flexibility makes RBCD particularly valuable in cloud and hybrid infrastructure scenarios.

Advantages and Trade-offs

KCD provides significant security improvements over unconstrained delegation while maintaining the functionality required for complex application architectures.

• Advantages: KCD enforces the principle of least privilege by limiting delegation to only the services that are explicitly necessary. This significantly reduces the attack surface compared to unconstrained delegation. The explicit configuration requirement prevents accidental over-privileging and provides clear audit trails for delegation relationships.
  • The security model prevents credential theft attacks that exploit unconstrained delegation. Since user TGTs are not forwarded, compromised delegating services cannot use stolen credentials to access arbitrary domain resources. This containment significantly improves the overall security posture.
• Trade-offs: KCD configuration requires careful planning and precise implementation. Service Principal Names (SPNs) must be registered correctly for all involved services. Incorrect SPN registration leads to authentication failures and service disruptions.
  • The administrative overhead increases with the number of delegation relationships. Each delegation path must be explicitly configured and maintained. This complexity can lead to configuration errors that affect application functionality.

Network connectivity requirements become more stringent. All services involved in the delegation chain must maintain reliable communication with domain controllers. Network partitions or connectivity issues can disrupt the entire delegation process.

Troubleshooting and Considerations

Common issues with KCD implementation typically involve configuration errors and infrastructure problems that affect the Kerberos authentication process.

• Common Issues: The most frequent problems include incorrect SPN registration, where services cannot be properly identified within the domain. Misconfigurations in Active Directory delegation settings prevent proper authorization of delegation requests. Network connectivity problems that affect communication between services and domain controllers can cause intermittent authentication failures.
  • Time synchronization issues between domain members can cause Kerberos ticket validation failures. Clock skew beyond acceptable limits results in rejected authentication attempts. DNS resolution problems prevent proper service location and SPN resolution.
• Security Posture: Organizations should conduct regular audits of their Active Directory environment to identify any remaining instances of unconstrained delegation. Migration to KCD or RBCD should be prioritized to improve overall security posture. The audit process should include reviewing service accounts, identifying delegation relationships, and assessing the necessity of each delegation configuration.
  • Monitoring delegation usage helps identify potential security issues and optimization opportunities. Event log analysis can reveal authentication patterns and highlight potential misuse of delegation privileges.

Key Terms Appendix

• Kerberos Constrained Delegation (KCD): A secure form of delegation that restricts the target services a delegating service can access on behalf of users through explicit configuration.
• Unconstrained Delegation: The legacy delegation model that allows a service to impersonate users to access any service in the domain, presenting significant security risks.
• Resource-Based Constrained Delegation (RBCD): The modern delegation model that places configuration control on the target resource rather than the delegating service.
• Service Principal Name (SPN): The unique identifier for a service instance within a Kerberos realm, essential for proper service authentication and delegation.
• Protocol Transition: A feature enabling services to obtain Kerberos tickets for users who authenticated using non-Kerberos protocols.
• Ticket-Granting Service (TGS): The Kerberos component responsible for issuing service tickets, including those used in delegation scenarios.