What is SQL Injection? and How to Prevent it?

SQL injection is a code injection 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 an instance of a more general class of vulnerabilities that can occur whenever one programming or scripting language is embedded inside another. SQL injection attacks are also known as SQL insertion attacks.

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

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.

While most SQL server implementations allow multiple statements to be executed with one call, some SQL APIs such as php's mysql_query do not allow this for security reasons. This prevents hackers from injecting entirely separate queries, but doesn't stop them from modifying queries. 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), using an API that allows multiple statements:

a';DROP TABLE users; SELECT * FROM data WHERE 't' = 't

This input renders the final SQL statement as follows:

SELECT * FROM users WHERE name = 'a';DROP TABLE users; SELECT * FROM DATA WHERE 't' = 't';

Preventing SQL injection

To protect against SQL injection, user input must not directly be embedded in SQL statements. Instead, parameterized statements must be used (preferred), or user input must be carefully escaped or filtered.

[edit]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. The user input is then assigned (bound) to a parameter. This is an example using Java and the JDBC API:

PreparedStatement prep = conn.prepareStatement("SELECT * FROM USERS WHERE USERNAME=? AND PASSWORD=?");
prep.setString(1, username);
prep.setString(2, password);

Similarly, in C#:

using (SqlCommand myCommand = new SqlCommand("SELECT * FROM USERS WHERE USERNAME=@username AND PASSWORD=HASHBYTES('SHA1',
 @password)", myConnection))
    {                    
        myCommand.Parameters.AddWithValue("@username", user);
        myCommand.Parameters.AddWithValue("@password", pass);
 
        myConnection.Open();
        SqlDataReader myReader = myCommand.ExecuteReader())
        ...................
    }

In PHP version 5 and above, there are multiple choices for using parameterized statements. The PDO[5] database layer is one of them:

$db = new PDO('pgsql:dbname=database');
$stmt = $db->prepare("SELECT priv FROM testUsers WHERE username=:username AND password=:password");
$stmt->bindParam(':username', $user);
$stmt->bindParam(':password', $pass);
$stmt->execute();

There are also vendor-specific methods. For example in MySQL 4.1 and above with the mysqli[6] extension. Example[7]:

$db = new mysqli("localhost", "user", "pass", "database");
$stmt = $db -> prepare("SELECT priv FROM testUsers WHERE username=? AND password=?");
$stmt -> bind_param("ss", $user, $pass);
$stmt -> execute();

In ColdFusion, the CFQUERYPARAM statement is useful in conjunction with the CFQUERY statement to nullify the effect of SQL code passed within the CFQUERYPARAM value as part of the SQL clause.[8][9]. An example is below.

<cfquery name="Recordset1" datasource="cafetownsend">
SELECT *
FROM COMMENTS
WHERE COMMENT_ID =<cfqueryparam value="#URL.COMMENT_ID#" cfsqltype="cf_sql_numeric">
</cfquery>

[edit]Enforcement at the database level

Currently only the H2 Database Engine supports the ability to enforce query parameterization.[citation needed]

[edit]Enforcement at the coding level

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

[edit]Escaping

A straight-forward, though error-prone way to prevent injections is to escape dangerous characters. One of the reasons for it being error prone is that it is a type of blacklist which is less robust than a whitelist. 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. In PHP, for example, it is usual to escape parameters using the functionmysql_real_escape_string before sending the SQL query:

$query = sprintf("SELECT * FROM Users where UserName='%s' and Password='%s'", 
                  mysql_real_escape_string($Username), 
                  mysql_real_escape_string($Password));
mysql_query($query);

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