Thursday, October 4, 2012

How to prevent mysql injection in Php before it is submitted

SQL injection refers to the act of someone inserting a MySQL statement to be run on your database without your knowledge. Injection usually occurs when you ask a user for input, like their name, and instead of a name they give you a MySQL statement that you will unknowingly run on your database.

 This is the important bit, we take the $username and $password variables that we just filled with data, and we use the function mysql_real_escape_string
on it, what this does is remove and characters that should not be in there, it actually does the same job as stripslashes() apart from this is the correct
method used for MYSQL Databases.

The rest of the code is pretty much self explanatory, after we have checked all the data and made it A-Z characters, we then perform the mysql
query on the database and then we just check the returned information with a series of IF statement, these are all very self explanatory.

This concludes this article, for more information on this subject, checkout this website is filled with information on everything php.

Use prepared statements and parameterized queries. These are SQL statements that are sent to and parsed by the database server separately from any parameters. This way it is impossible for an attacker to inject malicious SQL.
You basically have two options to achieve this:
  1. Using PDO:
    $stmt = $pdo->prepare('SELECT * FROM employees WHERE name = :name');
    $stmt->execute(array(':name' => $name));
    foreach ($stmt as $row) {
        // do something with $row
  2. Using mysqli:
    $stmt = $dbConnection->prepare('SELECT * FROM employees WHERE name = ?');
    $stmt->bind_param('s', $name);
    $result = $stmt->get_result();
    while ($row = $result->fetch_assoc()) {
        // do something with $row


Note that when using PDO to access a MySQL database real prepared statements are not used by default. To fix this you have to disable the emulation of prepared statements. An example of creating a connection using PDO is:
$dbConnection = new PDO('mysql:dbname=dbtest;host=;charset=utf8', 'user', 'pass');

$dbConnection->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);
$dbConnection->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
In the above example the error mode isn't strictly necessary, but it is advised to add it. This way the script will not stop with a Fatal Error when something goes wrong. And gives the developer the chance to catch any error(s) (which are throwed as PDOExceptions.
What is mandatory however is the setAttribute() line, which tells PDO to disable emulated prepared statements and use real prepared statements. This makes sure the statement and the values aren't parsed by PHP before sending it the the MySQL server (giving a possible attacker no chance to inject malicious SQL).
Although you can set the charset in the options of the constructor it's important to note that 'older' versions of PHP (< 5.3.6) silently ignored the charset parameter in the DSN.


What happens is that the SQL statement you pass to prepare is parsed and compiled by the database server. By specifying parameters (either a ? or a named parameter like :name in the example above) you tell the database engine where you want to filter on. Then when you call execute the prepared statement is combined with the parameter values you specify.
The important thing here is that the parameter values are combined with the compiled statement, not a SQL string. SQL injection works by tricking the script into including malicious strings when it creates SQL to send to the database. So by sending the actual SQL separately from the parameters you limit the risk of ending up with something you didn't intend. Any parameters you send when using a prepared statement will just be treated as strings (although the database engine may do some optimization so parameters may end up as numbers too, of course). In the example above, if the $name variable contains 'Sarah'; DELETE * FROM employees the result would simply be a search for the string "'Sarah'; DELETE * FROM employees", and you will not end up with an empty table.
Another benefit with using prepared statements is that if you execute the same statement many times in the same session it will only be parsed and compiled once, giving you some speed gains.
Oh, and since you asked about how to do it for an insert, here's an example (using PDO):
$preparedStatement = $db->prepare('INSERT INTO table (column) VALUES (:column)');

$preparedStatement->execute(array(':column' => $unsafeValue));


MySQL & PHP Code:

// a good user's name
$name = "timmy"; 
$query = "SELECT * FROM customers WHERE username = '$name'";
echo "Normal: " . $query . "<br />";

// user input that uses SQL Injection
$name_bad = "' OR 1'"; 

// our MySQL query builder, however, not a very safe one
$query_bad = "SELECT * FROM customers WHERE username = '$name_bad'";

// display what the new query will look like, with injection
echo "Injection: " . $query_bad; 


Normal: SELECT * FROM customers WHERE username = 'timmy'
Injection: SELECT * FROM customers WHERE username = '' OR 1'' 

MySQL & PHP Code:

//NOTE: you must be connected to the database to use this function!
// connect to MySQL

$name_bad = "' OR 1'"; 

$name_bad = mysql_real_escape_string($name_bad);

$query_bad = "SELECT * FROM customers WHERE username = '$name_bad'";
echo "Escaped Bad Injection: <br />" . $query_bad . "<br />";

$name_evil = "'; DELETE FROM customers WHERE 1 or username = '"; 

$name_evil = mysql_real_escape_string($name_evil);

$query_evil = "SELECT * FROM customers WHERE username = '$name_evil'";
echo "Escaped Evil Injection: <br />" . $query_evil;


Escaped Bad Injection:
SELECT * FROM customers WHERE username = '\' OR 1\''
Escaped Evil Injection:
SELECT * FROM customers WHERE username = '\'; DELETE FROM customers WHERE 1 or username = \''

No comments:

Post a Comment