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mysql php security sql sql-injection

How can I prevent SQL injection in PHP?

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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?

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    9477

    +50

    The correct way to avoid SQL injection attacks, no matter which database you use, is to separate the data from SQL, so that data stays data and will never be interpreted as commands by the SQL parser. It is possible to create an SQL statement with correctly formatted data parts, but if you don’t fully understand the details, you should always 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'
      $stmt->execute();
      
      $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

    PDO

    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=127.0.0.1;charset=utf8mb4', '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 PDO will inform you of all MySQL errors by means of throwing the PDOException.

    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.

    Mysqli

    For mysqli we have to follow the same routine:

    mysqli_report(MYSQLI_REPORT_ERROR | MYSQLI_REPORT_STRICT); // error reporting
    $dbConnection = new mysqli('127.0.0.1', 'username', 'password', 'test');
    $dbConnection->set_charset('utf8mb4'); // charset
    

    Explanation

    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";
    }
    

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    • 54

      Also, the official documentation of mysql_query only allows to execute one query, so any other query besides ; is ignored. Even if this is already deprecated there are a lot of systems under PHP 5.5.0 and that may use this function. php.net/manual/en/function.mysql-query.php

      Jan 19, 2016 at 17:40

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      This is a bad habit but is a post-problem solution : Not only for SQL injection but for any type of injections (for example there was a view template injection hole in F3 framework v2) if you have a ready old website or app is suffering from injection defects , one solution is to reassign the values of your supperglobal predefined vars like $_POST with escaped values at bootstrap. By PDO, still it is possible to escape (also for today frameworks) : substr($pdo->quote($str, \PDO::PARAM_STR), 1, -1)

      – Alix

      Jan 24, 2016 at 15:08

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      This answer lacks the explanation of what is a prepared statement – one thing – it’s a performance hit if you use a lot of prepared statements during your request and sometimes it accounts for 10x performance hit. Better case would be use PDO with parameter binding off, but statement preparation off.

      – donis

      Nov 18, 2016 at 8:54

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      Using PDO is better, in case you are using direct query make sure you use mysqli::escape_string

      Nov 6, 2017 at 8:17

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      @Alix this sounds like a good idea in theory, but sometimes the values need a different kind of escapes, for example for SQL and HTML

      – p0358

      Nov 18, 2018 at 23:24

    1718

    To use the parameterized query, you need to use either Mysqli or PDO. To rewrite your example with mysqli, we would need something like the following.

    <?php
    mysqli_report(MYSQLI_REPORT_ERROR | MYSQLI_REPORT_STRICT);
    $mysqli = new mysqli("server", "username", "password", "database_name");
    
    $variable = $_POST["user-input"];
    $stmt = $mysqli->prepare("INSERT INTO table (column) VALUES (?)");
    // "s" means the database expects a string
    $stmt->bind_param("s", $variable);
    $stmt->execute();
    

    The key function you’ll want to read up on there would be mysqli::prepare.

    Also, as others have suggested, you may find it useful/easier to step up a layer of abstraction with something like PDO.

    Please note that the case you asked about is a fairly simple one and that more complex cases may require more complex approaches. In particular:

    • If you want to alter the structure of the SQL based on user input, parameterized queries are not going to help, and the escaping required is not covered by mysql_real_escape_string. In this kind of case, you would be better off passing the user’s input through a whitelist to ensure only ‘safe’ values are allowed through.

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      using mysql_real_escape_string is enough or i must use parameterized too?

      – peiman F.

      Mar 8, 2018 at 21:29

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      @peimanF. keep a good practice of using parametrized queries, even on a local project. With parametrized queries you are guaranteed that there will not be SQL injection. But keep in mind you should sanitize the data to avoid bogus retrieval (i.e. XSS injection, such as putting HTML code in a text) with htmlentities for example

      – Goufalite

      Mar 9, 2018 at 8:02


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      @peimanF. Good practise to parametrized queries and bind values, but real escape string is good for now

      – Richard

      Apr 4, 2018 at 18:03

    • I understand the inclusion of mysql_real_escape_string() for completeness, but am not a fan of listing the most error-prone approach first. The reader might just quickly grab the first example. Good thing it’s deprecated now 🙂

      Dec 5, 2018 at 0:13

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      @SteenSchütt – All the mysql_* functions are deprecated. They were replaced by similar mysqli_* functions, such as mysqli_real_escape_string.

      Dec 1, 2019 at 1:21

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    Every answer here covers only part of the problem.
    In fact, there are four different query parts which we can add to SQL dynamically: –

    • a string
    • a number
    • an identifier
    • a syntax keyword

    And prepared statements cover only two of them.

    But sometimes we have to make our query even more dynamic, adding operators or identifiers as well.
    So, we will need different protection techniques.

    In general, such a protection approach is based on whitelisting.

    In this case, every dynamic parameter should be hardcoded in your script and chosen from that set.
    For example, to do dynamic ordering:

    $orders  = array("name", "price", "qty"); // Field names
    $key = array_search($_GET['sort'], $orders)); // if we have such a name
    $orderby = $orders[$key]; // If not, first one will be set automatically. 
    $query = "SELECT * FROM `table` ORDER BY $orderby"; // Value is safe
    

    To ease the process I wrote a whitelist helper function that does all the job in one line:

    $orderby = white_list($_GET['orderby'], "name", ["name","price","qty"], "Invalid field name");
    $query  = "SELECT * FROM `table` ORDER BY `$orderby`"; // sound and safe
    

    There is another way to secure identifiers – escaping but I rather stick to whitelisting as a more robust and explicit approach. Yet as long as you have an identifier quoted, you can escape the quote character to make it safe. For example, by default for mysql you have to double the quote character to escape it. For other other DBMS escaping rules would be different.

    Still, there is an issue with SQL syntax keywords (such as AND, DESC and such), but white-listing seems the only approach in this case.

    So, a general recommendation may be phrased as

    • Any variable that represents an SQL data literal, (or, to put it simply – an SQL string, or a number) must be added through a prepared statement. No Exceptions.
    • Any other query part, such as an SQL keyword, a table or a field name, or an operator – must be filtered through a white list.

    Update

    Although there is a general agreement on the best practices regarding SQL injection protection, there are still many bad practices as well. And some of them too deeply rooted in the minds of PHP users. For instance, on this very page there are (although invisible to most visitors) more than 80 deleted answers – all removed by the community due to bad quality or promoting bad and outdated practices. Worse yet, some of the bad answers aren’t deleted, but rather prospering.

    For example, there(1) are(2) still(3) many(4) answers(5), including the second most upvoted answer suggesting you manual string escaping – an outdated approach that is proven to be insecure.

    Or there is a slightly better answer that suggests just another method of string formatting and even boasts it as the ultimate panacea. While of course, it is not. This method is no better than regular string formatting, yet it keeps all its drawbacks: it is applicable to strings only and, like any other manual formatting, it’s essentially optional, non-obligatory measure, prone to human error of any sort.

    I think that all this because of one very old superstition, supported by such authorities like OWASP or the PHP manual, which proclaims equality between whatever “escaping” and protection from SQL injections.

    Regardless of what PHP manual said for ages, *_escape_string by no means makes data safe and never has been intended to. Besides being useless for any SQL part other than string, manual escaping is wrong, because it is manual as opposite to automated.

    And OWASP makes it even worse, stressing on escaping user input which is an utter nonsense: there should be no such words in the context of injection protection. Every variable is potentially dangerous – no matter the source! Or, in other words – every variable has to be properly formatted to be put into a query – no matter the source again. It’s the destination that matters. The moment a developer starts to separate the sheep from the goats (thinking whether some particular variable is “safe” or not) he/she takes his/her first step towards disaster. Not to mention that even the wording suggests bulk escaping at the entry point, resembling the very magic quotes feature – already despised, deprecated and removed.

    So, unlike whatever “escaping”, prepared statements is the measure that indeed protects from SQL injection (when applicable).

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