How to Prevent SQL injection in PHP 2017

SQL injection happens when you interpolate some content into a SQL query string, and the result modifies the syntax of your query in ways you didn’t intend.

It doesn’t have to be malicious, it can be an accident. But accidental SQL injection is more likely to result in an error than in a vulnerability.

The harmful content doesn’t have to come from a user, it could be content that your application gets from any source, or even generates itself in code.

It can lead to vulnerabilities because attackers can send values to an application that they know will be interpolated into a SQL string. By being very clever, they can manipulate the result of queries, reading data or even changing data that they shouldn’t be allowed to do.

Vulnerable SQL injection PHP Code Example:

$password = $_POST['password'];
$id = $_POST['id'];
$sql = "UPDATE Accounts SET PASSWORD = '$password' WHERE account_id = $id";

Now suppose the attacker sets the POST request parameters to “password=xyzzy” and “id=account_id” resulting in the following SQL:

UPDATE Accounts SET PASSWORD = ‘xyzzy’ WHERE account_id = account_id
Although I expected $id to be an integer, the attacker chose a string that is the name of the column. Of course now the condition is true on every row, so the attacker has just set the password for every account. Now the attacker can log in to anyone’s account — including privileged users.

It isn’t SQL that’s injected, it’s content that’s interpolated (“injected”) into a SQL string, resulting in a different kind of query than I intended. I trusted the dynamic content without verifying it, and executed the resulting SQL query blindly. That’s where the trouble starts.

How to prevent a SQl injection

1) Filter Input, Stop believing your users: The biggest threat to the application is from its users. Users need not be well mannered and obedient as you are expecting. Some users have really bad intentions and some simply try to test their hacking skills. Whatever code you are going to write, write it using the best practices and consider the security aspects of it. Validate every field in the form

2) Use database wrapper classes or PDO – Database wrappers or PDO (in PHP) can reduce the risk of direct access of the input values to the database.

3) Verify that the content is in a format you expect, instead of assuming. For example, apply a regular expression, or use a data type coercion like the intval() function.

4) Escape Output, in this case “output” is where the content is combined with the SQL string. When interpolating strings, avoid imbalanced quotes by using a function that escapes literal quote characters and any other characters that may be string boundaries.

5) 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:

Using PDO (for any supported database driver):

$stmt = $pdo->prepare('SELECT * FROM employees WHERE name = :name');

$stmt->execute(array('name' => $name));

foreach ($stmt as $row) {
// do something with $row
}
Using MySQLi (for MySQL):
$stmt = $dbConnection->prepare('SELECT * FROM employees WHERE name = ?');
$stmt->bind_param('s', $name);

$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 (e.g. 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=127.0.0.1;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 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 (< 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 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 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.

here’s an example (using PDO):

 $preparedStatement = $db->prepare('INSERT INTO table (column) VALUES (:column)');

$preparedStatement->execute(array('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';
}

The point where SQL is injected is any point that your application accepts input from the user.

Whether this becomes a dangerous vulnerability for your web application depends on whether this input is later used as part of an SQL query without properly checking its type and escaping it if necessary.

Without proper escaping, some SQL code ‘injected’ by the user could be executed by the SQL engine as SQL code, rather than a simple string or value.

Refference:

https://stackoverflow.com/questions/6678245/php-sql-injection

https://stackoverflow.com/questions/601300/what-is-sql-injection

https://stackoverflow.com/questions/910465/avoiding-sql-injection-without-parameters

https://stackoverflow.com/questions/60174/how-can-i-prevent-sql-injection-in-php

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