WHAT IS AN SQL INJECTION AND ITS TYPES

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SQL injection

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SQL injection is a technique often used to attack databases through a website. This is done by including portions of SQL statements in a web form entry field in an attempt to get the website to pass a newly formed rogue SQL command to the database (e.g. dump the database contents to the attacker). SQL injection is a code injection technique that exploits a security vulnerability in a website's software. The vulnerability happens 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 commands are thus injected from the web form into the database of an application (like queries) to change the database content or dump the database information like credit card or passwords to the attacker. SQL injection is mostly known as an attack vector for websites but can be used to attack any type of SQL database.

In operational environments, it has been noted that applications experience, on average, 71 attempts an hour.^[1] When under direct attack, some applications occasionally came under aggressive attacks and at their peak, were attacked 800–1300 times per hour.^[1]

Forms and Validity

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.^[2] 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
• DBMS specific SQLIA
• Compounded SQLIA

Some security researchers propose that Classic SQLIA is outdated^[3] though many web applications are not hardened against them. Inference SQLIA is still a threat, because of its dynamic and flexible deployment as an attacking scenario. The DBMS specific SQLIA should be considered as supportive regardless of the utilization of Classic or Inference SQLIA. Compounded SQLIA is a new term derived from research on SQL Injection Attacking Vector in combination with other different web application attacks as:

• SQL Injection + Insufficient authentication^[4]
• SQL Injection + DDos attacks^[5]
• SQL Injection + DNS hijacking^[6]
• SQL Injection +XSS^[7]

A complete overview of the SQL Injection classification is presented in the next figure. The Storm Worm is one representation of Compounded SQLIA.^[8]

A Classification of SQL Injection Attacking Vector till 2010

This classification represents the state of SQLIA, respecting its evolution until 2010—further refinement is underway.^[9]

Technical Implementations

 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 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'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.

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 userinfo 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 drop (delete) the "users" table from the database, since the SQL would be rendered as follows:

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

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.^[11]

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.^[12]

SELECT 1/0 FROM users WHERE username='ooo';

Another type of blind SQL injection uses a conditional timing delay on which the attacker can learn whether the SQL statement resulted in a true or in a false condition ^[13]

Mitigation

Parameterized statements

With most development platforms, parameterized statements can be used that work with parameters (sometimes called placeholders or bind variables) instead of embedding user input in the statement. A placeholder can only store the value of the given type and not the arbitrary SQL fragment. Hence the SQL injection would simply be treated as a strange (an probably invalid) parameter value.

In many cases, the SQL statement is fixed, and each parameter is a scalar, not a table. The user input is then assigned (bound) to a parameter. ^[14]

Enforcement at the coding level

Using object-relational mapping libraries avoids the need to write SQL code. The ORM library in effect will generate parameterized SQL statements from object-oriented code.

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 mysql_real_escape_string(); before sending the SQL query:

$query = sprintf("SELECT * FROM `Users` WHERE UserName='%s' AND Password='%s'",

                  mysql_real_escape_string($Username),

                  mysql_real_escape_string($Password));

mysql_query($query);

This function, i.e. mysql_real_escape_string(), calls MySQL's library function mysql_real_escape_string, which prepends backslashes to the following characters: \x00, \n, \r, \, ', " and \x1a. This function must always (with few exceptions) be used to make data safe before sending a query to MySQL.^[15]
There are other functions for many database types in PHP such as pg_escape_string() for PostgreSQL. There is, however, one function that works for escaping characters, and is used especially for querying on databases that do not have escaping functions in PHP. This function is: addslashes(string $str ). 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).^[16]
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.^[17]

Pattern check

Integer, float or boolean parameters can be checked if they 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 SQL server, a database logon could be restricted from selecting on some of the system tables which would limit exploits that try into 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;