0 SQL INJECTION

SQL injection is a code injection technique, used to attack data driven applications, in which malicious SQL statements are inserted into an entry field for execution (e.g. to dump the database contents to the attacker).SQL injection must exploit a security vulnerability in an application's software, for example, when user input is either incorrectly filtered for string literal escape characters embedded in SQL statements or user input is not strongly typed and unexpectedly executed. SQL injection is mostly known as an attack vector for websites but can be used to attack any type of SQL database.

In a 2012 study, security company Imperva observed that the average web application received 4 attack campaigns per month, and retailers received 2 times as many attacks as other industries.

SQL injection attack (SQLIA) is considered one of the top 10 web application vulnerabilities of 2007 and 2010 by the Open Web Application Security Project.In 2013, SQLIA was rated the number one attack on the OWASP top ten.The attacking vector contains five main sub-classes depending on the technical aspects of the attack's deployment:
Classic SQLIA
Inference SQL injection
Interacting with SQL injection
Database management system-specific SQLIA
Compounded SQLIA
SQL injection + insufficient authentication
SQL injection + DDoS attacks
SQL injection + DNS hijacking
SQL injection + XSS
SQL injection + Filter bypass + Havij + Backtrack R6

A complete overview of the SQL Injection classification is presented in the next figure. The Storm Worm is one representation of Compounded SQLIA.

This classification represents the state of SQLIA, respecting its evolution until 2010—further refinement is underway.

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 + "';"

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):

' 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 and specified:

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'smysql_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. As an example, a book review website uses a query string to determine which book review to display. So the URL http://books.example.com/showReview.php?ID=5 would cause the server to run the query

SELECT * FROM bookreviews WHERE ID = 'Value(ID)';

from which it would populate the review page with data from the review with ID 5, stored in the table bookreviews. The query happens completely on the server; the user does not know the names of the database, table, or fields, nor does the user know the query string. The user only sees that the above URL returns a book review. A hacker can load the URLs http://books.example.com/showReview.php?ID=5 AND 1=1 andhttp://books.example.com/showReview.php?ID=5 AND 1=2, which may result in queries

SELECT * FROM bookreviews WHERE ID = '5' AND '1'='1';

SELECT * FROM bookreviews WHERE ID = '5' AND '1'='2';

respectively. If the original review loads with the "1=1" URL and a blank or error page is returned from the "1=2" URL, the site is likely vulnerable to a SQL injection attack. The hacker may proceed with this query string designed to reveal the version number of MySQL running on the server:http://books.example.com/showReview.php?ID=5 AND substring(@@version,1,1)=4, which would show the book review on a server running MySQL 4 and a blank or error page otherwise. The hacker can continue to use code within query strings to glean more information from the server until another avenue of attack is discovered or his or her goals are achieved.