How to prevent file inclusion attacks using PHP

With the rapid development of the Internet, security issues have become an important issue that cannot be ignored. File inclusion attacks are a very common and dangerous attack method. The key is that attackers can use this vulnerability to obtain sensitive information on the server. Therefore, how to use PHP to prevent file inclusion attacks has become a problem that many developers must solve.

1. Understand file inclusion attacks

File inclusion attacks are a common web attack and are listed as one of the top ten web security vulnerabilities of OWASP (Open Web Application Security Project). It can be divided into local file inclusion (LFI) and remote file inclusion (RFI).

When using unfiltered external data to generate file paths, you are vulnerable to LFI attacks. For example, the following code concatenates a user-submitted file name with a path name:

<?php
$file = '/home/user/'. $_GET['file'];
include($file);
?>

LFI attacks occur when users provide data using relative paths such as ../.

RFI attack means that the attacker can execute his own remote code in the server. For example, the following code includes the user-submitted URL directly through the file_get_contents() function:

<?php
$url = $_GET['url'];
$content = file_get_contents($url);
echo $content;
?>

RFI attacks occur when an attacker provides his or her own malicious URL.

2. Prevent LFI attacks

In order to prevent LFI attacks, we must filter the file names and paths provided by the user. Using absolute paths is a good way to prevent LFI attacks.

The following is an example of possible filtering, using a whitelist approach including file names and paths that are allowed to be included:

<?php
$allowed_files = array("file1.php", "file2.php");
$allowed_paths = array("/path1/", "/path2/");

$file = $_GET['file'];
$path = $_GET['path'];

if (!in_array($file, $allowed_files) || !in_array($path, $allowed_paths)) {
    die("Access Denied");
}

include("/home/user/" . $path . $file);
?>

Using a whitelist approach can reduce the risk of LFI attacks. If you don't want to use the whitelist method, you can also use the method of limiting file types:

<?php
$file = $_GET['file'];

// 判断$file是否是php文件
if (!preg_match("/.php$/", $file)) {
    die("Access Denied");
}

include("/home/user/" . $file);
?>

Restricting file types can prevent other file types from being included.

3. Prevent RFI attacks

In order to prevent RFI attacks, we must filter the URL provided by the user. When using a whitelist, access should only be allowed to the remote servers that you specify. For example, you can set the allow_url_fopen option in the php.ini file, or use the curl function.

The following is an example of preventing RFI attacks:

<?php
$url = $_GET['url'];

// 验证是否是信任的主机
if (!preg_match("/^http://(192.168.0.16|www.example.com)/", $url)) {
    die("Access Denied");
}

$content = file_get_contents($url);
echo $content;
?>

In this example, we limit the verification process to specified hosts.

4. Other security measures for using PHP

1. Disable dangerous functions

Some functions can access system files, such as eval(), exec( ), etc., so they can be easily contaminated or misused. For improved security, these functions should be disabled.

2. PHP version

In older PHP versions, there is more risk of file inclusion vulnerabilities. Therefore, keeping your PHP version updated is one way to reduce vulnerabilities and improve security.

3. Permission Control

To ensure that files are only available to the scripts or users intended to execute them, appropriate permission controls (such as ownership and access rights of the file) should be used .

Therefore, when writing web applications using PHP, security must be the first priority. Taking appropriate security measures, such as using whitelisting methods, restricting file types, disabling dangerous functions, updating PHP versions, and using permission controls, can effectively reduce the risk of file inclusion attacks.

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