While there are a number of security risks in the world of electronic commerce, SQL injection is one of the most common Web site attack techniques used to steal customer data such as credit card numbers, hold customer data hostage by encrypting it or destroy data outright.

Where a Web server only understands and speaks the HTTP protocol, a database's native tongue is Structured Query Language (SQL), which is essentially a set of command statements that instruct a database to execute specific actions. Every database server has a similar series of commands to query its tables, narrow down results to a few specific entries, and combine information from one table to another.

Here is an example SQL query:

SELECT -- FROM users WHERE Email = '" + Email + "' AND Password = '" + Password + "';

The WHERE specifies a condition, that an e-mail address and password combination match data present in the "users" table. When this command is given to the database server, it returns true if a match is found and a false if there is no match.

When clients send data on the Web, they use URLs and forms to assemble the database query statements. The following URL represents an example login page for a Web application:

GET /shopping_cart/login.asp?Email=jdoe@example.com&Password=$ecret123 HTTP/1.1

This URL shows that the destination application is a Microsoft ASP page and it is accepting two parameters, one called "Email" and the other called "Password." If the user credentials are correct then the result of this query will provide response data that represents a successful authentication and will be used to allow the client to proceed to the corresponding Web page.

Developers of traditional application code generally trust user input. They believe that database queries are coming from a trusted source, namely the database server itself, rather than from an untrusted user's Web browser. SQL injection is an attack technique where an untrusted user inserts SQL query data into input fields sent to back-end databases in an attempt to trick the database into executing the commands.

The Web application firewall in the example was configured in a "detection only" mode where it was logging alerts and events but not blocking any inbound attacks or outbound data leakages. Due to this configuration, the inbound SQL injection attack from the previous section was allowed to continue on to the vulnerable Web application when it would have been blocked.

The Web page returned indicates that the SQL injection reconnaissance probe was successful, giving the hacker valuable information, including the exact version of the database and the database user. Armed with this information, the attacker can fine tune the attack and execute further reconnaissance probes to enumerate more information about the database itself, such as the table and column names. After a number of intermediary reconnaissance probes, the attacker has the information needed to send a complex SQL injection attack, attempting to extract customer record details. By targeting specific customer data such as credit card name, expiration data and security code, the attacker can extract a vast amount of sensitive customer data.