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.



Forms of the vulnerability







Incorrectly filtered escape characters

->This form of SQL injection occurs when user input is not filtered for escape characters and

is then passed into an SQL statement. This results in the potential manipulation of the statements

performed on the database by the end-user of the application.



The following line of code illustrates this vulnerability:



statement = "SELECT * FROM `users` WHERE `name` = '" + userName + "';"





This SQL code is designed to pull up the records of the specified username

from its table of users. However, if the "userName" variable iscrafted

in a specific way by a malicious user, the SQL statement may do more than

the code author intended.

example, setting the "userName" variable as



' or '1'='1



Or using comments to even block the rest of the query (there are three types of sql comments):



' or '1'='1' -- '

' or '1'='1' ({ '

' or '1'='1' /* '

If this code were to be used in an authentication procedure then this example could be used to force the selection of a valid username because the evaluation of '1'='1' is always true.

The following value of "userName" in the statement below would cause the deletion of the "users" table as well as the selection of all data from the "userinfo" table (in essence revealing the information of every user), using an API that allows multiple statements:



a';DROP TABLE `users`; SELECT * FROM `userinfo` WHERE 't' = 't



This input renders the final SQL statement as follows:



SELECT * FROM `users` WHERE `name` = 'a';DROP TABLE `users`;

SELECT * FROM `userinfo` WHERE 't' = 't';



While most SQL server implementations allow multiple statements to be executed with one call in this way, some SQL APIs such as PHP's mysql_query(); function do not allow this for security reasons. This prevents attackers from injecting entirely separate queries, but doesn't stop them from modifying queries.