Saturday, July 23, 2011

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 SQLstatements or user input is not strongly typed and thereby unexpectedly executed. It happens from using poorly designed query language interpreters.

SQL Injection Attack, abbreviated SQLIA, is a web attacking vector. SQLIA is considered one of the top 10 web application vulnerabilities of 2010 by the Open Web Application Security Project. The attacking vector contains four main sub-classes depending on the technical aspects of the attack's deployment:
  • Classic SQLIA
  • Inference SQL Injection
  • DBMS specific SQLIA
  • Compounded SQLIA
Some security researchers propose that Classic SQLIA is outdated though many web applications are not hardened against them.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
  • SQL Injection + DDos attacks
  • SQL Injection + DNS Hijacking
  • SQL Injection + XSS
The Storm Worm is one representation of Compounded SQLIA. A complete overview of the SQL Injection classification is presented in the next figure, Krassen Deltchev in 2010.
This Classification represents the state of SQLIA, respecting its evolution till 2010; further refinement is underway.

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):
' 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'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.

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.

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';


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. 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. This is an example using Java and the JDBC API:
java.sql.PreparedStatement prep = connection.prepareStatement(
                "SELECT * FROM `users` WHERE USERNAME = ? AND PASSWORD = ?");
prep.setString(1, username);
prep.setString(2, password);

Enforcement at the database level

The H2 Database Engine supports the ability to enforce query parameterization. However, query by example may not be possible or practical because it is difficult to implement query by example using parameterized queries.

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.


A straightforward, though error-prone, way to prevent injections is to escape characters that have a special meaning in SQL. This technique is called HTML sanitization 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 functionmysql_real_escape_string(); before sending the SQL query:
$query = sprintf("SELECT * FROM `Users` WHERE UserName='%s' AND Password='%s'",
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.

Source: Wikipedia


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