Mitigating SQL Injection

Input Sanitization

<SNIP>
  $username = $_POST['username'];
  $password = $_POST['password'];

  $query = "SELECT * FROM logins WHERE username='". $username. "' AND password = '" . $password . "';" ;
  echo "Executing query: " . $query . "<br /><br />";

  if (!mysqli_query($conn ,$query))
  {
          die('Error: ' . mysqli_error($conn));
  }

  $result = mysqli_query($conn, $query);
  $row = mysqli_fetch_array($result);
<SNIP>

Sanitiye this by using mysqli_real_escape_string:

<SNIP>
$username = mysqli_real_escape_string($conn, $_POST['username']);
$password = mysqli_real_escape_string($conn, $_POST['password']);

$query = "SELECT * FROM logins WHERE username='". $username. "' AND password = '" . $password . "';" ;
echo "Executing query: " . $query . "<br /><br />";
<SNIP>

As expected, the injection no longer works due to escaping the single quotes. A similar example is the pg_escape_string() which used to escape PostgreSQL queries.

Input Validation

User input can also be validated based on the data used to query to ensure that it matches the expected input. For example, when taking an email as input, we can validate that the input is in the form of [e-mail], and so on.

<?php
if (isset($_GET["port_code"])) {
	$q = "Select * from ports where port_code ilike '%" . $_GET["port_code"] . "%'";
	$result = pg_query($conn,$q);
    
	if (!$result)
	{
   		die("</table></div><p style='font-size: 15px;'>" . pg_last_error($conn). "</p>");
	}
<SNIP>
?>

Validate "port_code":

<SNIP>
$pattern = "/^[A-Za-z\s]+$/";
$code = $_GET["port_code"];

if(!preg_match($pattern, $code)) {
  die("</table></div><p style='font-size: 15px;'>Invalid input! Please try again.</p>");
}

$q = "Select * from ports where port_code ilike '%" . $code . "%'";
<SNIP>

User Privileges

As discussed initially, DBMS software allows the creation of users with fine-grained permissions. We should ensure that the user querying the database only has minimum permissions.

CREATE USER 'reader'@'localhost';
GRANT SELECT ON ilfreight.ports TO 'reader'@'localhost' IDENTIFIED BY 'p@ssw0Rd!!';

The commands above add a new MariaDB user named reader who is granted only SELECT privileges on the ports table.

Web Application Firewall

Web Application Firewalls (WAF) are used to detect malicious input and reject any HTTP requests containing them. This helps in preventing SQL Injection even when the application logic is flawed. WAFs can be open-source (ModSecurity) or premium (Cloudflare). Most of them have default rules configured based on common web attacks. For example, any request containing the string INFORMATION_SCHEMA would be rejected, as it's commonly used while exploiting SQL injection.

Parameterized Queries

Another way to ensure that the input is safely sanitized is by using parameterized queries. Parameterized queries contain placeholders for the input data, which is then escaped and passed on by the drivers. Instead of directly passing the data into the SQL query, we use placeholders and then fill them with PHP functions.

<SNIP>
  $username = $_POST['username'];
  $password = $_POST['password'];

  $query = "SELECT * FROM logins WHERE username=? AND password = ?" ;
  $stmt = mysqli_prepare($conn, $query);
  mysqli_stmt_bind_param($stmt, 'ss', $username, $password);
  mysqli_stmt_execute($stmt);
  $result = mysqli_stmt_get_result($stmt);

  $row = mysqli_fetch_array($result);
  mysqli_stmt_close($stmt);
<SNIP>