require()

The require() function in PHP is used to include and execute the contents of a specified file during the execution of a script. It's very similar to the include() function, but with a key difference in how it handles errors.

If require() fails to find or open the specified file, it generates a fatal error and halts the execution of the script. This is in contrast to include(), which only emits a warning (E_WARNING) and the script will continue to execute. require() is commonly used when the file is essential for the application to run. For example, files containing necessary functions, classes, or configurations are typically included using require().

If the same file is require()d multiple times within a script, the file's contents will be executed each time it is called. To prevent this, require_once() can be used, which ensures that the file is included only once.

The syntax is:

require 'path/to/file.php';

A more realistic example may be:

<?php
// Include a critical configuration file
require 'config.php';

// Rest of the script that depends on the configuration settings
?>

The require() function in PHP can be vulnerable to security risks, similar to include() and include_once(). The primary vulnerability associated with require() is Local File Inclusion (LFI), which can occur if the function is used with improperly validated or sanitized user input.

An example of a vulnerable use:

<?php
// A vulnerable script where the file name is taken from user input
$page = $_GET['page'];
require($page . '.php');
?>

Danger

In this example, if a user can control the value of $_GET['page'], they might be able to include files like config.php by manipulating the URL, such as http://example.com/?page=../config.

A more secure implementation may look like:

<?php
// Define a whitelist of allowed files
$allowedPages = [
    'home' => 'home.php',
    'contact' => 'contact.php',
    'about' => 'about.php'
];

// Get the user input
$page = $_GET['page'] ?? 'default'; // Default page if none is specified

// Validate the user input against the whitelist
if (array_key_exists($page, $allowedPages)) {
    // Use the file from the whitelist, ensuring the input cannot affect the file path
    require $allowedPages[$page];
} else {
    // Handle the error or redirect to a default page
    echo 'Page not found.';
    // Or you can redirect to a default page like this:
    // header('Location: default.php');
    // exit();
}
?>
  1. Whitelisting: Only files listed in $allowedPages are allowed to be included. This prevents attackers from including arbitrary files.
  2. Validation: The user input ($page) is checked against the whitelist. This ensures that only predefined paths can be used in require().
  3. Default Values: The use of $_GET['page'] ?? 'default' ensures that there is always a valid value for $page. If the user does not provide input, it defaults to 'default'.
  4. Error Handling: If the user input does not match any key in the whitelist, the script handles this gracefully by displaying an error message or redirecting to a default page.