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.
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.
SELECT ProductName, ProductDescription FROM Products WHERE ProductNumber = ProductNumber
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.
sql_query= "SELECT ProductName, ProductDescription FROM Products WHERE ProductNumber = " & Request.QueryString("ProductID")
Request.QueryString("ProductID") extracts the value of the Web form variable
ProductID so that it can be appended as the SELECT condition.
When a user enters the following URL:
The corresponding SQL query is executed:
SELECT ProductName, ProductDescription FROM Products WHERE ProductNumber = 123
An attacker may abuse the fact that the
ProductID 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 "
123 OR 1=1" to the
ProductID variable results in the following URL:
http://www.mydomain.com/products/products.asp?productid=123 or 1=1
The corresponding SQL Statement is:
SELECT ProductName, Product Description From Products WHERE ProductNumber = 123 OR 1=1
This condition would always be true and all
ProductDescription pairs are returned. The attacker can manipulate the application even further by inserting malicious commands. For example, an attacker can request the following URL:
http://www.mydomain.com/products/products.asp?productid=123;DROP TABLE Products
In this example the semicolon is used to pass the database server multiple statements in a single execution. The second statement is "
DROP TABLE Products" which causes SQL Server to delete the entire
An attacker may use SQL injection to retrieve data from other tables as well. This can be done using the SQL
UNION SELECT statement. The
UNION SELECT statement allows the chaining of two separate SQL SELECT queries that have nothing in common. For example, consider the following SQL query:
SELECT ProductName, ProductDescription FROM Products WHERE ProductID = '123' UNION SELECT Username, Password FROM Users;
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:
http://www.mydomain.com/products/products.asp?productid=123 UNION SELECT user-name, password FROM USERS
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 of
ProductID to be "
123;EXEC master..xp_cmdshell dir--", which returns the list of files in the current directory of the SQL Server process.