SQL Injection
SQL injection is a code injection technique that exploits a security vulnerability occurring in the database layer of an application (like queries). 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.
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 (there are three types of SQL comments):[10]
‘ or ‘1’=’1′ — ‘
‘ or ‘1’=’1′ ({ ‘
‘ or ‘1’=’1′ /* ‘
renders one of the following SQL statements by the parent language:
SELECT * FROM `users` WHERE `name` = ” OR ‘1’=’1′;
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(); function do not allow this for security reasons. This prevents attackers from injecting entirely separate queries, but doesn’t stop them from modifying queries.
Blind SQL Injection
Blind SQL Injection is used when a web application is vulnerable to an SQL injection but the results of the injection are not visible to the attacker. The page with the vulnerability may not be one that displays data but will display differently depending on the results of a logical statement injected into the legitimate SQL statement called for that page. This type of attack can become time-intensive because a new statement must be crafted for each bit recovered. There are several tools that can automate these attacks once the location of the vulnerability and the target information has been established.
Conditional responses
One type of blind SQL injection forces the database to evaluate a logical statement on an ordinary application screen.
SELECT `booktitle` FROM `booklist` WHERE `bookId` = ‘OOk14cd’ AND ‘1’=’1′;
will result in a normal page while
SELECT `booktitle` FROM `booklist` WHERE `bookId` = ‘OOk14cd’ AND ‘1’=’2′;
will likely give a different result if the page is vulnerable to a SQL injection. An injection like this may suggest to the attacker that a blind SQL injection is possible, leaving the attacker to devise statements that evaluate to true or false depending on the contents of another column or table outside of the SELECT statement’s column list.
SELECT 1/0 FROM `users` WHERE `username`=’ooo’;
mimmoo 