Sql Injection...

SQL injection is a 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 in fact an instance of a more general class of vulnerabilities that can occur whenever one programming or scripting language is embedded inside another



Incorrectly filtered escape characters
This form of SQL injection occurs when user input is not filtered for escape characters and is then passed into a 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 + "';"
If the "userName" variable is crafted in a specific way by a malicious user, the SQL statement may do more than the code author intended. For example, setting the "userName" variable as

a' or 't'='t
renders this SQL statement by the parent language:

SELECT * FROM users WHERE name = 'a' or 't'='t';
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 't'='t' is always true.

On MS SQL Server any valid SQL command may be injected via this method, including the execution of multiple statements. 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 "data" table:

a';DROP TABLE users; SELECT * FROM data WHERE name LIKE '%
This input renders the final SQL statement as follows:

SELECT * FROM users WHERE name = 'a';DROP TABLE users; SELECT * FROM data WHERE name LIKE '%';