What is SQL Injection?

SQL injection is a technique used to take advantage of non-validated input vulnerabilities to pass SQL commands through a Web application for execution by a backend database. Attackers take advantage of the fact that programmers often chain together SQL commands with user-provided parameters, and can therefore embed SQL commands inside these parameters. The result is that the attacker can execute arbitrary SQL queries and/or commands on the backend database server through the Web application.

Detailed Description

Databases are fundamental components of Web applications. Databases enable Web applications to store data, preferences and content elements. Using SQL, Web applications interact with databases to dynamically build customized data views for each user. A common example is a Web application that manages products. In one of the Web application's dynamic pages (such as ASP), users are able to enter a product identifier and view the product name and description. The request sent to the database to retrieve the product's name and description is implemented by the following SQL statement.

Typically, Web applications use string queries, where the string contains both the query itself and its parameters. The string is built using server-side script languages such as ASP, JSP and CGI, and is then sent to the database server as a single SQL statement. The following example demonstrates an ASP code that generates a SQL query.

When a user enters the following URL:

The corresponding SQL query is executed:

An attacker may abuse the fact that the Product ID parameter is passed to the database without sufficient validation. The attacker can manipulate the parameter's value to build malicious SQL statements. For example, setting the value to the ProductID variable results in the following URL:

The corresponding SQL Statement is:

This condition would always be true and all Product Name and Product Description pairs are returned. The attacker can manipulate the application even further by inserting malicious commands. For example, an attacker can request the following URL:

In this example the semicolon is used to pass the database server multiple statements in a single execution. The second statement is which causes SQL Server to delete the entire Products table.

An attacker may use SQL injection to retrieve data from other tables as well. This can be done using the statement. The statement allows the chaining of two separate SQL queries that have nothing in common. For example, consider the following SQL query:

The result of this query is a table with two columns, containing the results of the first and second queries, respectively. An attacker may use this type of SQL injection by requesting the following URL:

The security model used by many Web applications assumes that an SQL query is a trusted command. This enables attackers to exploit SQL queries to circumvent access controls, authentication and authorization checks. In some instances, SQL queries may allow access to host operating system level commands. This can be done using stored procedures. Stored procedures are SQL procedures usually bundled with the database server. For example, the extended stored procedure xp _cmdshell executes operating system commands in the context of a Microsoft SQL Server. Using the same example, the attacker can set the value which returns the list of files in the current directory of the SQL Server process.