Do PDO Prepared Statements Always Prevent SQL Injection?

Are PDO Prepared Statements Sufficient to Prevent SQL Injection?

PDO (PHP Data Objects) prepared statements are a powerful tool for protecting against SQL injection. However, it's important to understand their limitations to truly safeguard against this vulnerability.

The Problem with Emulated Prepares:

By default, PDO emulates prepared statements for MySQL. However, this emulation has a security flaw that skilled attackers can exploit. In this attack scenario:

1. The server's character set is set to one that supports multibyte characters encoded with an escaped ' (backslash).
2. A payload is crafted that contains a combination of invalid multibyte characters.
3. PDO internally calls the MySQL C API function mysql_real_escape_string() to escape the payload, which it does according to the connection character set.
4. The invalid multibyte characters in the payload are treated as valid single bytes, resulting in an unescaped ' character.
5. This escaped ' character can be used to craft an SQL injection attack.

Preventing the Attack:

To prevent this attack, you can disable emulated prepared statements by setting the PDO attribute PDO::ATTR_EMULATE_PREPARES to false. This forces PDO to use true prepared statements, which are more secure.

You can also mitigate the attack by:

• Using a non-vulnerable character set for connection encoding, such as UTF-8.
• Enabling the NO_BACKSLASH_ESCAPES SQL mode.

Safe Examples:

The following examples illustrate safe usage of PDO prepared statements:

// Disable emulated prepares and use non-vulnerable character set
$pdo->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);
$pdo->setAttribute(PDO::ATTR_DEFAULT_FETCH_MODE, PDO::FETCH_ASSOC);
$query = 'SELECT * FROM users WHERE username = ?';
$stmt = $dbh->prepare($query);
$stmt->execute( array(':username' => $_REQUEST['username']) );

// Use DSN charset parameter in PHP ≥ 5.3.6
$pdo = new PDO('mysql:host=localhost;dbname=testdb;charset=gbk', $user, $password);
$query = 'SELECT * FROM users WHERE username = ?';
$stmt = $dbh->prepare($query);
$stmt->execute( array(':username' => $_REQUEST['username']) );

In conclusion, using PDO prepares is generally sufficient to prevent SQL injection, but it's crucial to be aware of and mitigate potential vulnerabilities. By understanding the limitations of emulated prepared statements and ensuring compatible configurations, you can effectively protect your applications from malicious injections.

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