MSSQL Error Based

Context

This article focuses on exploiting MSSQL error-based SQL injection to extract sensitive information from a database. It presumes a foundation in SQL syntax, understanding of database error messages, and comprehension of HTTP requests.

Theory

Error-Based SQL Injection in MSSQL

Error-based SQL injection in MSSQL is a method that exploits verbose database error messages to gather information about the database's underlying structure and data. By forcing the database to return errors through crafted SQL queries, attackers can infer details such as table names, column names, and data types.

Vulnerability Model

The success of this technique depends heavily on the application's error-handling settings. Applications configured to display detailed database error messages will inadvertently reveal critical database schema details, providing attackers with the necessary information to construct subsequent, more precise attacks.

Key Functions and Operators

CONVERT Function

The CONVERT function in SQL transforms one data type into another. By deliberately mismatching data types, attackers can prompt meaningful error messages that expose the structure and constraints of the database.

CAST Function

Similar to CONVERT, the CAST function is also used for data type conversion. Crafting queries that invoke these functions with incompatible data can produce errors that aid attackers in deducing database architecture or logic flaws.

IN Operator

The IN operator checks if a value exists within a specified set. It can be manipulated to generate errors by using subqueries that violate expected column types or data cardinality assumptions.

EQUAL Operator

The EQUAL operator is used for comparison within SQL statements. By forcing comparisons between incompatible types, attackers can cause errors that reveal insights into database content or structure.

Practice

Exploiting Error-Based SQL Injection

To effectively utilize MSSQL error-based SQL injection, follow these steps:

• Trigger a Division by Zero Error
  Run the following command to test for error verbosity:

  SELECT 1/0;
  

  If error details are disclosed, it confirms the potential for exploiting error messages.

• Force a Conversion Error
  Issue a query that attempts inappropriate data type conversion:

  SELECT CONVERT(int, 'text');
  

  This will raise a descriptive conversion error, aiding in inferring the database's data interpretations.

• Use the IN Operator inappropriately
  Induce an error by using the IN operator with a failing subquery:

  SELECT 1 WHERE 1 IN (SELECT name FROM sysobjects);
  

  Incorrect usage in the subquery can prompt detailed error feedback.

• Manipulate the EQUAL Operator
  Cause an error by comparing values of different types:

  SELECT 1 WHERE 1 = (SELECT TOP 1 name FROM sysobjects);
  

  Such type mismatches induce errors, unveiling further data about the structure.

By successfully executing these steps, attackers can glean significant schema and data insights.

Tools

• sqlmap
• Burp Suite

This guide outlines the steps and tools necessary to exploit error-based SQL injections within MSSQL environments, enabling attackers to extract valuable information from vulnerable applications.