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.

Forms of SQL injection vulnerabilities

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 a 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

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 some SQL servers such as 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 (in essence revealing the information of every user):

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 '%';

Other SQL implementations won't execute multiple commands in the same SQL query as a security measure. This prevents hackers from injecting entirely separate queries, but doesn't stop them from modifying queries.

Incorrect type handling

This form of SQL injection occurs when a user supplied field is not strongly typed or is not checked for type constraints. This could take place when a numeric field is to be used in a SQL statement, but the programmer makes no checks to validate that the user supplied input is numeric. For example:

statement := "SELECT * FROM data WHERE id = " + a_variable + ";"

It is clear from this statement that the author intended a_variable to be a number correlating to the "id" field. However, if it is in fact a string then the end user may manipulate the statement as they choose, thereby bypassing the need for escape characters. For example, setting a_variable to

1;DROP TABLE users

will delete the "users" table from the database as the rendered SQL would be rendered as follows:

SELECT * FROM data WHERE id = 1;DROP TABLE users;

Vulnerabilities inside the database server

Sometimes vulnerabilities can exist within the database server software itself, as was the case with the MySQL server's real_escape_chars() functions

http://dev.mysql.com/doc/refman/5.0/en/news-5-0-22.html for more details .

Real-world examples

• On October 31, 2004, After being linked from Slashdot, the Dremel site was changed to a Goatse pumpkin
• On October 26, 2005, Unknown Heise readers replaced a page by the German TV station ARD which advertised a pro-RIAA sitcom with Goatse using SQL injection
• On November 01, 2005, A high school student used a SQL injection to break into the site of a Taiwanese information security mazagine from the Tech Target group and steal customer's information.
• On January 13, 2006, Russian hackers broke into a Rhode Island government web site and allegedly stole credit card data from individuals who have done business online with state agencies.
• On March 29, 2006, Susam Pal discovered a SQL injection flaw in www.incredibleindia.org, an official Indian government tourism site.
• On January 1, 2007, Dr.Jr7 SQL injected Nokia's website in a rather tame and civil way, but then Digg users proceeded to change it to Goatse and bukkake
• On March 2, 2007, Sebastian Bauer discovered a SQL injection flaw in knorr.de login page.
• On June 29, 2007, Hacker Defaces Microsoft U.K. Web Page using SQL injection.
• On August 12, 2007, The United Nations web site was defaced using SQL injection.

In popular culture

• The webcomic xkcd illustrated the vulnerabilities that are encountered with SQL Injection.

See also

• Code injection
• Application security

External links

• "IIS 6 SQL Injection Prevention ISAPI (GNU License)" by Rodney Viana at Codeplex
• "SQLrand Preventing SQL Injection Attacks" by Stephen W. Boyd and Angelos D. Keromytis
• "What is SQL Injection?" By CGISecurity.com
• "SQL Injection WASC Threat Classification Entry" By The Web Application Security Consortium
• Avoid SQL injection
• PHP and SQL Injections
• "Ben Forta's ColdFusion Blog: SQL Injection Attacks, Easy To Prevent, But Apparently Still Ignored" By Ben Forta
• "Imperva Glossary: SQL Injection"