Chapter 26. Filesystem Security

Table of Contents
Null bytes related issues

PHP is subject to the security built into most server systems with respect to permissions on a file and directory basis. This allows you to control which files in the filesystem may be read. Care should be taken with any files which are world readable to ensure that they are safe for reading by all users who have access to that filesystem.

Since PHP was designed to allow user level access to the filesystem, it's entirely possible to write a PHP script that will allow you to read system files such as /etc/passwd, modify your ethernet connections, send massive printer jobs out, etc. This has some obvious implications, in that you need to ensure that the files that you read from and write to are the appropriate ones.

Consider the following script, where a user indicates that they'd like to delete a file in their home directory. This assumes a situation where a PHP web interface is regularly used for file management, so the Apache user is allowed to delete files in the user home directories.

Example 26-1. Poor variable checking leads to....


<?php
// remove a file from the user's home directory
$username = $_POST['user_submitted_name'];
$userfile = $_POST['user_submitted_filename'];
$homedir  = "/home/$username";

unlink("$homedir/$userfile");

echo "The file has been deleted!";
?>

Since the username and the filename are postable from a user form, they can submit a username and a filename belonging to someone else, and delete it even if they're not supposed to be allowed to do so. In this case, you'd want to use some other form of authentication. Consider what could happen if the variables submitted were "../etc/" and "passwd". The code would then effectively read:

Example 26-2. ... A filesystem attack


<?php
// removes a file from anywhere on the hard drive that
// the PHP user has access to. If PHP has root access:
$username = $_POST['user_submitted_name']; // "../etc"
$userfile = $_POST['user_submitted_filename']; // "passwd"
$homedir  = "/home/$username"; // "/home/../etc"

unlink("$homedir/$userfile"); // "/home/../etc/passwd"

echo "The file has been deleted!";
?>

There are two important measures you should take to prevent these issues.

Only allow limited permissions to the PHP web user binary.
Check all variables which are submitted.

Here is an improved script:

Example 26-3. More secure file name checking


<?php
// removes a file from the hard drive that
// the PHP user has access to.
$username = $_SERVER['REMOTE_USER']; // using an authentication mechanisim
$userfile = basename($_POST['user_submitted_filename']);
$homedir  = "/home/$username";

$filepath = "$homedir/$userfile";

if (file_exists($filepath) && unlink($filepath)) {
    $logstring = "Deleted $filepath\n";
} else {
    $logstring = "Failed to delete $filepath\n";
}
$fp = fopen("/home/logging/filedelete.log", "a");
fwrite($fp, $lo gstring);
fclose($fp);

echo htmlentities($logstring, ENT_QUOTES);

?>

However, even this is not without its flaws. If your authentication system allowed users to create their own user logins, and a user chose the login "../etc/", the system is once again exposed. For this reason, you may prefer to write a more customized check:

Example 26-4. More secure file name checking


<?php
$username     = $_SERVER['REMOTE_USER']; // using an authentication mechanisim
$userfile     = $_POST['user_submitted_filename'];
$homedir      = "/home/$username";

$filepath     = "$homedir/$userfile";

if (!ctype_alnum($username) || !preg_match('/^(?:[a-z0-9_-]|\.(?!\.))+$/iD', $userfile)) {
    die("Bad username/filename");
}

//etc...
?>

Depending on your operating system, there are a wide variety of files which you should be concerned about, including device entries (/dev/ or COM1), configuration files (/etc/ files and the .ini files), well known file storage areas (/home/, My Documents), etc. For this reason, it's usually easier to create a policy where you forbid everything except for what you explicitly allow.

Null bytes related issues

As PHP uses the underlying C functions for filesystem related operations, it may handle null bytes in a quite unexpected way. As null bytes denote the end of a string in C, strings containing them won't be considered entirely but rather only until a null byte occurs. The following example shows a vulnerable code that demonstrates this problem:

Example 26-5. Script vulnerable to null bytes


<?php
$file = $_GET['file']; // "../../etc/passwd\0"
if (file_exists('/home/wwwrun/'.$file.'.php')) {
    // file_exists will return true as the file /home/wwwrun/../../etc/passwd exists
    include '/home/wwwrun/'.$file.'.php';
    // the file /etc/passwd will be included
}
?>

Therefore, any tainted string that is used in a filesystem operation should always be validated properly. Here is a better version of the previous example:

Example 26-6. Correctly validating the input


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

// Whitelisting possible values
switch ($file) {
    case 'main':
    case 'foo':
    case 'bar':
        include '/home/wwwrun/include/'.$file.'.php';
        break;
    default:
        include '/home/wwwrun/include/main.php';
}
?>