Hackers have a wide arsenal of attack mechanisms, from which they choose the one most suited to a particular vulnerability. SQL Injection is perhaps one of the most common application layer attack techniques used by hackers.

SQL injection is the hacking technique which attempts to pass SQL commands through a web application for execution by a backend database.

The technologies vulnerable to this attack are dynamic script languages including ASP, ASP.NET, PHP, JSP, and CGI. All an attacker needs to perform an SQL Injection hacking attack is a web browser and creative guess work to important table and field names. The sheer simplicity of SQL Injection has fuelled its popularity.

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.

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")

The call 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:

http://www.mydomain.com/products/products.asp?productid=123

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 ProductName and 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 Products table.

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.


Example of a SQL Injection attack

Here is a sample basic HTML form with two inputs, login and password.

The easiest way for the login.php to work is by building a database query that looks like this:

"SELECT id
FROM logins
WHERE username = '$username'
AND password = '$password'";

If the variables $username and $password are requested directly from the user's input, this can easily be compromised. Suppose that we gave "Joe" as a username and that the following string was provided as a password: anything' OR 'x'='x

"SELECT id
FROM logins
WHERE username = 'Vinod'
AND password = 'anything' OR 'x'='x'";

Because the application is not really thinking about the query, but just constructing a string, the use of the single quotes has turned the WHERE into a two-component clause. The 'x'='x' part guarantees to be true regardless of what the first part contains.

This will allow the attacker to bypass the login form without actually knowing a valid username / password combination!

Labels: Technology

This entry was posted on Thursday, September 28, 2006 at 7:17 PM. You can skip to the end and leave a response.