SQL injection is a code injection technique that exploits a security vulnerability occurring in the database layer of an application. The vulnerability is present when user input is either incorrectly filtered for string literal escape characters embedded in SQL statements or user input is not strongly typed and thereby unexpectedly executed. It is an instance of a more general class of vulnerabilities that can occur whenever one programming or scripting language is embedded inside another. SQL injection attacks are also known as SQL insertion attacks.
"An attack technique used to exploit web sites by altering backend SQL statements through manipulating application input." - Web Application Security Consortium Glossary
SQL Injection happens when a developer accepts user input that is directly placed into a SQL Statement and doesn't properly filter out dangerous characters. This can allow an attacker to not only steal data from your database, but also modify and delete it. Certain SQL Servers such as Microsoft SQL Server contain Stored and Extended Procedures (database server functions). If an attacker can obtain access to these Procedures it may be possible to compromise the entire machine. Attackers commonly insert single qoutes into a URL's query string, or into a forms input field to test for SQL Injection. If an attacker receives an error message like the one below there is a good chance that the application is vulnerable to SQL Injection.
