Viewed   1.2k times

If user input is inserted without modification into an SQL query, then the application becomes vulnerable to SQL injection, like in the following example:

$unsafe_variable = $_POST['user_input']; 

mysql_query("INSERT INTO `table` (`column`) VALUES ('$unsafe_variable')");

That's because the user can input something like value'); DROP TABLE table;--, and the query becomes:

INSERT INTO `table` (`column`) VALUES('value'); DROP TABLE table;--')

What can be done to prevent this from happening?



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 (for any supported database driver):

    $stmt = $pdo->prepare('SELECT * FROM employees WHERE name = :name');
    $stmt->execute([ 'name' => $name ]);
    foreach ($stmt as $row) {
        // Do something with $row
  2. Using MySQLi (for MySQL):

    $stmt = $dbConnection->prepare('SELECT * FROM employees WHERE name = ?');
    $stmt->bind_param('s', $name); // 's' specifies the variable type => 'string'
    $result = $stmt->get_result();
    while ($row = $result->fetch_assoc()) {
        // Do something with $row

If you're connecting to a database other than MySQL, there is a driver-specific second option that you can refer to (for example, pg_prepare() and pg_execute() for PostgreSQL). PDO is the universal option.

Correctly setting up the connection

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', 'password');

$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 it gives the developer the chance to catch any error(s) which are thrown as PDOExceptions.

What is mandatory, however, is the first 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 to 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 (before 5.3.6) silently ignored the charset parameter in the DSN.


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 an 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 of 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([ 'column' => $unsafeValue ]);

Can prepared statements be used for dynamic queries?

While you can still use prepared statements for the query parameters, the structure of the dynamic query itself cannot be parametrized and certain query features cannot be parametrized.

For these specific scenarios, the best thing to do is use a whitelist filter that restricts the possible values.

// Value whitelist
// $dir can only be 'DESC', otherwise it will be 'ASC'
if (empty($dir) || $dir !== 'DESC') {
   $dir = 'ASC';
Wednesday, August 3, 2022

Your advice is indeed incorrect.

mysql_real_escape_string() will not work for dynamic table names; it is designed to escape string data, delimited by quotes, only. It will not escape the backtick character. It's a small but crucial distinction.

So I could insert a SQL injection in this, I would just have to use a closing backtick.

PDO does not provide sanitation for dynamic table names, either.

This is why it is good not to use dynamic table names, or if one has to, comparing them against a list of valid values, like a list of tables from a SHOW TABLES command.

I wasn't really fully aware of this either, and probably guilty of repeating the same bad advice, until it was pointed out to me here on SO, also by Col. Shrapnel.

Tuesday, October 18, 2022

As long as you're using Prepare or Query, you're safe.

// this is safe
db.Query("SELECT name FROM users WHERE age=?", req.FormValue("age"))
// this allows sql injection.
db.Query("SELECT name FROM users WHERE age=" + req.FormValue("age"))
Thursday, September 22, 2022

This answer depends on the driver you are using.

Erlang ODBC has a function param_query that binds a set of parameters to the query and it might also escape all the SQL special characters.

erlang-mysql-driver has prepared statements:

%% Register a prepared statement
              <<"UPDATE developer SET country=? where name like ?">>),

%% Execute the prepared statement
mysql:execute(p1, update_developer_country, [<<"Sweden">>,<<"%Wiger">>]),

(code from Yariv's blog)

As a last resort you can always escape the characters

 NUL (0x00) -->  
 BS  (0x08) --> b
 TAB (0x09) --> t
 LF  (0x0a) --> n
 CR  (0x0d) --> r
 SUB (0x1a) --> z
 "   (0x22) --> "
 %   (0x25) --> %
 '   (0x27) --> '
    (0x5c) --> \
 _   (0x5f) --> _ 
Friday, November 18, 2022

Yes, I would say it's good practice to have users connect using accounts that only allow the least privileges they need to use the site. If your web users should only be reading data from the database then I would definitely create an account that only has read access and have them hit the DB through that.

The more important thing would be to secure your web application. You can still be victim of a devastating SQL Injection attack even if a user does not write to your database (think stolen credit card numbers or passwords).

Monday, August 29, 2022
Only authorized users can answer the search term. Please sign in first, or register a free account.
Not the answer you're looking for? Browse other questions tagged :