Introductio to MYSQL

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).[1]SQL injection must exploit a security vulnerability in an application’s software, for example, when user input is either incorrectly filtered forstring 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.

SQL injection (SQLI) is considered one of the top 10 web application vulnerabilities of 2007 and 2010 by the Open Web Application Security Project.[5] In 2013, SQLI was rated the number one attack on the OWASP top ten.[6] There are five main sub-classes of SQL injection:1

• Classic SQLI
• Blind or Inference SQL injection
• Database management system-specific SQLI
• Compounded SQLI

• SQL injection + insufficient authentication[7]
• SQL injection + DDoS attacks[8]
• SQL injection + DNS hijacking[9]
• SQL injection + XSS[10]

The Storm Worm is one representation of Compounded SQLI.[11]

This classification represents the state of SQLI, respecting its evolution until 2010—further refinement is underway.[12]

Incorrectly filtered escape characters

This form of SQL injection occurs when user input is not filtered forescape 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[13]). All three lines have a space at the end:

' 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

Preventing SQL injection

Escaping

A straightforward, though error-prone, way to prevent injections is to escape characters that have a special meaning in SQL. The manual for an SQL DBMS explains which characters have a special meaning, which allows creating a comprehensive blacklist of characters that need translation. For instance, every occurrence of a single quote (') in a parameter must be replaced by two single quotes ('') to form a valid SQL string literal. For example, in PHP it is usual to escape parameters using the function mysqli_real_escape_string(); before sending the SQL query:

$mysqli = new mySqli('hostname', 'db_username', 'db_password', 'db_name');
$query = sprintf("SELECT * FROM `Users` WHERE UserName='%s' AND Password='%s'",
                  $$mysqli->real_escape_string($Username),
                  $$mysqli->real_escape_string($Password));
$mysqli->query($query);

This function prepends backslashes to the following characters: x00, n, r, , ‘, ” and x1a. This function is normally used to make data safe before sending a query to MySQL.[18]
There are other functions for many database types in PHP such as pg_escape_string() for PostgreSQL. The functionaddslashes(string $str ) works for escaping characters, and is used especially for querying on databases that do not have escaping functions in PHP. It returns a string with backslashes before characters that need to be quoted in database queries, etc. These characters are single quote (‘), double quote (“), backslash () and NUL (the NULL byte).[19]
Routinely passing escaped strings to SQL is error prone because it is easy to forget to escape a given string. Creating a transparent layer to secure the input can reduce this error-proneness, if not entirely eliminate it.[20]

Pattern check

Integer, float or boolean parameters can be checked if their value is valid representation for the given type. Strings that must follow some strict pattern (date, UUID, alphanumeric only, etc.) can be checked if they match this pattern.

Database permissions

Limiting the permissions on the database logon used by the web application to only what is needed may help reduce the effectiveness of any SQL injection attacks that exploit any bugs in the web application.

For example, on Microsoft SQL Server, a database logon could be restricted from selecting on some of the system tables which would limit exploits that try to insert JavaScript into all the text columns in the database.

deny select on sys.sysobjects to webdatabaselogon;
deny select on sys.objects to webdatabaselogon;
deny select on sys.tables to webdatabaselogon;
deny select on sys.views to webdatabaselogon;
deny select on sys.packages to webdatabaselogon;