Unconstrained Delegation – A Hidden Risk in Active Directory

Introduction

Unconstrained Delegation is a feature in 微软 Active Directory designed to simplify service authentication by allowing services to impersonate users across an entire domain. While convenient, this functionality introduces significant security risks. Attackers can exploit Unconstrained Delegation to extract Kerberos tickets, gain unauthorized access to privileged accounts, and escalate privileges within the network. Understanding this threat is critical for organizations relying on Active Directory to manage authentication and access control.

This article explains the risks associated with Unconstrained Delegation, demonstrates how attackers exploit it, and provides actionable strategies to mitigate these vulnerabilities.

Understanding Unconstrained Delegation

Unconstrained Delegation enables services to request and use Kerberos Ticket-Granting Tickets (TGTs), credentials issued by the Key Distribution Center (KDC). These tickets allow services to authenticate users and grant access to resources across the network, facilitating user impersonation. When a service is configured for Unconstrained Delegation, it can accept any user’s authentication request and forward it to other services, making it vulnerable to abuse. Unlike Constrained Delegation, which limits impersonation to specific services defined by an administrator to reduce the risk of misuse, Unconstrained Delegation imposes no such restrictions, making it inherently riskier.

The risk arises from the storage of TGTs in memory on systems configured for Unconstrained Delegation. Attackers who compromise these systems can extract the TGTs and use them to impersonate high-privilege users, such as domain administrators, to escalate their access and compromise the environment.

Demonstrating Exploitation

The first step in exploiting Unconstrained Delegation is identifying accounts or systems configured with this feature. This can be achieved using PowerShell to audit Active Directory. For example, the following command lists systems with Unconstrained Delegation enabled:

Get-ADComputer -Filter {TrustedForDelegation -eq $True} -Properties TrustedForDelegation        

Once a vulnerable system is identified, attackers often turn to Mimikatz, an open-source tool created by security researcher Benjamin Delpy. Delpy’s groundbreaking work in developing Mimikatz has provided both security professionals and attackers with deep insights into the vulnerabilities of Windows authentication systems. This tool is widely used for extracting authentication credentials from memory and demonstrating weaknesses in Windows authentication mechanisms. Originally designed as a proof-of-concept to demonstrate flaws in Windows authentication mechanisms, Mimikatz has become an indispensable tool for penetration testers and red teams. Its powerful capabilities, however, have also made it a favored tool for malicious actors, highlighting the importance of securing systems against its misuse.

Key Features of Mimikatz:

• Credential Dumping: Extracts Kerberos tickets, plaintext passwords, and NTLM hashes from memory.
• Pass-the-Hash and Pass-the-Ticket Attacks: Enables lateral movement and privilege escalation by reusing extracted credentials.
• Flexible Commands: Offers a wide range of modules for specific use cases, such as exporting certificates or simulating attacks.

How to Obtain Mimikatz:

Mimikatz is freely available on GitHub. To use it, download the source code and compile it, or obtain precompiled binaries from trusted sources. As with any powerful tool, ensure compliance with ethical guidelines and legal permissions before deploying Mimikatz.

This powerful tool is used to extract TGTs from the system’s memory. The extracted tickets can then be used to impersonate privileged accounts. For example, an attacker can use the following Mimikatz command to dump Kerberos tickets:

sekurlsa::tickets        

After extracting a TGT, attackers can use it to access resources or perform administrative actions as the impersonated user. This demonstrates how Unconstrained Delegation can enable lateral movement and privilege escalation in a compromised domain.

Interpreting the Results

To ensure the exploitation results lead to actionable security improvements, follow these detailed steps:

• Identify Vulnerable Systems:

Use the following PowerShell command to identify systems configured with Unconstrained Delegation:

Get-ADComputer -Filter {TrustedForDelegation -eq $True} -Properties TrustedForDelegation        

Cross-reference the list of systems with their access to sensitive accounts or critical resources. Prioritize systems for immediate remediation based on their potential impact.

• Assess Extracted Credentials:

Evaluate the privileges associated with the extracted Kerberos Ticket-Granting Tickets (TGTs). Determine which accounts these tickets allow attackers to impersonate and the scope of their access across the network. Use the following command to review active tickets on affected systems:

klist        

• Analyze Access Pathways:

BloodHound is an open-source tool designed to analyze and visualize Active Directory attack paths. Originally developed by Will Schroeder (@harmj0y) and the BloodHound development team, this tool has become a critical resource for both penetration testers and defenders. Their innovative approach to mapping relationships and permissions within Active Directory has reshaped how organizations address security gaps. To get started:

• Download BloodHound: Obtain BloodHound from its GitHub repository. Follow the setup instructions to configure the tool and its backend.
• Use SharpHound for Data Collection: Run SharpHound on a domain-connected machine to collect Active Directory data:

SharpHound.exe -c All        

• Upload and Analyze Data: Import the collected data into the BloodHound interface. Use built-in queries such as "Shortest Paths to Domain Admins" to identify high-value attack paths and chokepoints within your network.

BloodHound helps identify vulnerable systems and accounts, enabling you to focus on securing critical resources and minimizing attack surfaces.

• Audit for Related Activity:

Examine Windows Event Logs to detect unusual delegation activity. Focus on Event IDs related to Kerberos authentication, such as 4769 (Service Ticket Request) and 4770 (Ticket Granting Ticket Renewal). Configure your SIEM system to flag patterns like repeated ticket requests or abnormal resource access originating from flagged systems.

• Develop a Targeted Remediation Plan:

Based on the findings, implement specific fixes. These include disabling Unconstrained Delegation on high-risk systems, transitioning service accounts to use Constrained Delegation, and enhancing monitoring configurations. Ensure all remediation efforts are documented and integrated into ongoing security policies to prevent future vulnerabilities.

By following these steps, you can translate exploitation findings into actionable insights and tangible security improvements.

Mitigation Strategies

Organizations can reduce the risks associated with Unconstrained Delegation by implementing the following measures:

• Disable Unconstrained Delegation: Audit all systems and accounts for Unconstrained Delegation settings and disable it where it is unnecessary.
• Transition to Constrained Delegation: Use Constrained Delegation to limit the scope of impersonation to specific services.
• Enable Kerberos Armoring: Implement Kerberos Armoring (Flexible Authentication Secure Tunneling, or FAST), a security enhancement to the Kerberos protocol that protects against certain types of attacks by encrypting sensitive data during authentication exchanges. For example, FAST mitigates the risk of Kerberos ticket interception attacks by ensuring that sensitive authentication exchanges are encrypted, even on untrusted networks. This added layer of security is particularly effective in environments with high exposure to lateral movement risks. Learn more about FAST here.
• Harden High-Value Accounts: Add privileged accounts to the Protected Users group, a security feature in Active Directory that prevents high-value accounts from storing credentials locally, thereby reducing the risk of credential theft.
• Monitor Delegation Activity: Deploy monitoring tools to detect and alert on suspicious delegation-related activity.

These steps minimize the attack surface and reduce the likelihood of successful exploitation.

Conclusion

Unconstrained Delegation presents a significant risk in Active Directory environments when left misconfigured or improperly managed. By granting unrestricted impersonation capabilities, it creates opportunities for attackers to exploit vulnerable systems and escalate privileges. To address these risks, organizations must adopt a proactive approach that includes auditing delegation settings, transitioning to safer alternatives like Constrained Delegation, and implementing advanced monitoring solutions. These measures, combined with robust user account protections, can help organizations secure their infrastructure and prevent devastating breaches.

Call to Action

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