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Updated on September 10, 2025
Kerberos Unconstrained Delegation represents one of the most dangerous legacy features in Active Directory environments. This authentication mechanism allows a service to impersonate any user for access to any other service on the network—a capability that creates catastrophic security vulnerabilities.
Originally designed to simplify authentication in multi-tier applications, unconstrained delegation has become a primary attack vector for threat actors seeking domain compromise. The feature fundamentally breaks the principle of least privilege by granting services unlimited impersonation capabilities.
This technical analysis examines how unconstrained delegation operates, why it poses severe security risks, and the modern alternatives that have replaced it. Understanding these mechanisms is critical for Active Directory administrators and security professionals working to harden enterprise environments.
Definition and Core Concepts
Kerberos Unconstrained Delegation is a configuration setting applied to service accounts within Active Directory. When enabled, this setting grants the service account the ability to receive and cache copies of user Ticket-Granting Tickets (TGTs) during authentication events.
The delegating service acts as the front-end component—typically a web server or application server—that receives the user’s TGT. This service can then use the cached TGT to authenticate as the user to any other service within the domain.
The Ticket-Granting Ticket (TGT) serves as the master ticket that a client obtains from the Kerberos Key Distribution Center (KDC) during initial authentication. The TGT provides domain-wide authentication authority and represents the core vulnerability in unconstrained delegation.
Key characteristics of unconstrained delegation include unlimited scope of impersonation, persistent TGT caching in service memory, and no restrictions on target services. These features make it fundamentally insecure for modern enterprise environments.
How It Works: The TGT Forwarding Mechanism
The unconstrained delegation workflow relies on TGT forwarding—a mechanism that creates its primary security weakness. Understanding this process is essential for recognizing why the feature poses such significant risks.
User Authentication Process
A user initiates authentication to a service configured for unconstrained delegation. The client requests a service ticket from the KDC for the target service. The KDC validates the user’s credentials and prepares the service ticket response.
TGT Forwarding and Embedding
The KDC embeds a copy of the user’s TGT within the service ticket before transmission. This embedded TGT contains full domain authentication authority for the user. The service ticket with embedded TGT is transmitted to the delegating service.
TGT Caching and Storage
The delegating service extracts the TGT from the received service ticket. The service caches this TGT in its process memory for future use. Multiple user TGTs accumulate in the service’s memory over time.
Impersonation and Access
The service can present any cached TGT to the KDC to request new service tickets. This allows the service to impersonate users for access to any domain resource. The impersonation capability persists until the TGT expires—typically 10 hours by default.
Security Risks and Attack Vectors
Unconstrained delegation creates multiple attack vectors that can lead to complete domain compromise. These risks make the feature incompatible with modern security requirements.
TGT Theft and Memory Extraction
The primary vulnerability stems from TGT storage in service memory. If an attacker compromises the delegating service’s host, they can dump process memory to extract cached TGTs. Tools like Mimikatz can efficiently extract these tickets from Local Security Authority Subsystem Service (LSASS) memory.
Memory dumps contain TGTs from all users who have authenticated to the service. This creates a persistent collection of high-value authentication tokens. The extraction process requires local administrator privileges on the compromised host.
Privilege Escalation Through High-Value Targets
When privileged users authenticate to services with unconstrained delegation, their TGTs become available for theft. Domain administrators, enterprise administrators, and service accounts with elevated privileges represent high-value targets.
A single compromised service with unconstrained delegation can provide access to the most privileged accounts in the domain. This creates a path to complete domain takeover through lateral movement and privilege escalation.
Pass-the-Ticket (PtT) Attacks
Stolen TGTs enable Pass-the-Ticket attacks that bypass traditional authentication mechanisms. Attackers can inject stolen TGTs into their own sessions without requiring password hashes or plaintext credentials.
PtT attacks using stolen TGTs maintain the original user’s access rights and permissions. The attacks remain effective until the TGT expires, providing extended windows for malicious activity. Detection becomes challenging since the authentication appears legitimate to monitoring systems.
Persistent Domain Compromise
Unconstrained delegation can enable persistent access to domain resources. Attackers who steal TGTs from service accounts or highly privileged users can maintain domain access across multiple sessions.
The persistence mechanism operates independently of password changes or account lockouts. Only TGT expiration or explicit ticket revocation can terminate the unauthorized access.
Detection and Identification Methods
Organizations must identify unconstrained delegation configurations to assess their security posture. Several methods can detect these dangerous settings within Active Directory environments.
Active Directory Attribute Queries
The TrustedForDelegation attribute on computer accounts indicates unconstrained delegation configuration. PowerShell and LDAP queries can identify accounts with this attribute set to true. Regular auditing scripts should check for this configuration across all computer accounts.
User accounts with the TrustedToAuthForDelegation attribute also represent unconstrained delegation risks. These accounts can perform protocol transition and represent users without possessing their credentials.
Administrative Tools and Scripts
Built-in Active Directory administrative tools can identify delegation settings through the account properties interface. The “Trust this computer for delegation to any service” checkbox indicates unconstrained delegation configuration.
PowerShell cmdlets like Get-ADComputer with specific filters can automate the detection process. Custom scripts can generate reports of all accounts configured for unconstrained delegation across the domain.
Migration and Security Alternatives
Modern Active Directory implementations should eliminate unconstrained delegation in favor of more secure alternatives. These alternatives provide necessary functionality while maintaining security boundaries.
Kerberos Constrained Delegation (KCD)
Kerberos Constrained Delegation restricts services to specific target services rather than allowing unlimited access. KCD uses Service for User (S4U) extensions to enable controlled impersonation without TGT forwarding.
S4U2Self allows services to obtain service tickets for users without possessing their TGTs. S4U2Proxy enables services to request service tickets for specific backend services on behalf of users. These mechanisms eliminate the primary attack vector while maintaining multi-tier authentication capabilities.
Resource-Based Constrained Delegation (RBCD)
Resource-Based Constrained Delegation moves delegation control to the target resource rather than the delegating service. RBCD provides more granular access control and reduces administrative overhead.
The msDS-AllowedToActOnBehalfOfOtherIdentity attribute on target services controls which accounts can perform delegation. This approach enables distributed delegation management while maintaining security boundaries.
Implementation Considerations
Migration from unconstrained delegation requires careful planning and testing. Applications using unconstrained delegation must be reconfigured to use constrained delegation alternatives. Testing should verify that multi-tier authentication continues to function correctly after migration.
Some legacy applications may require additional configuration changes to support constrained delegation. Service Principal Name (SPN) configuration becomes more critical in constrained delegation scenarios.
Key Terms and Definitions
- Kerberos Unconstrained Delegation: A legacy delegation method that allows services to impersonate users for access to any domain service by forwarding and caching TGTs.
- Ticket-Granting Ticket (TGT): The master authentication ticket issued by the KDC that provides domain-wide authentication authority for a specific user.
- Pass-the-Ticket (PtT): An attack technique that uses stolen authentication tickets to impersonate users without requiring passwords or password hashes.
- Delegating Service: The front-end service configured for unconstrained delegation that receives and caches user TGTs during authentication.
- Kerberos Constrained Delegation (KCD): A secure alternative to unconstrained delegation that restricts services to specific target services using S4U protocol extensions.
- Resource-Based Constrained Delegation (RBCD): A delegation method where target resources control which services can perform delegation on behalf of users.
