SQL Injection :- What, Why & How
1) What is SQL Injection.
2) Why It is happening (Forms of vulnerability)
3) How to prevent it?
1) What Is SQL Injection
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.
2) Forms of 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 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
' or '1'='1
Or using comments to even block the rest of the query:
' or '1'='1';/*'
renders this SQL statement by the parent language:
SELECT * FROM users WHERE name = '' 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() do not allow this for security reasons. This prevents attackers from injecting entirely separate queries, but doesn’t stop them from modifying queries.