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 25-1. Poor variable checking leads to....
<?php
// remove a file from the user's home directory
$username = $_POST['user_submitted_name'];
$homedir = "/home/$username";
$file_to_delete = "$userfile";
unlink ("$homedir/$userfile");
echo "$file_to_delete has been deleted!";
?>
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Since the username is postable from a user form, they can submit
a username and file belonging to someone else, and delete files.
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 25-2. ... 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 = "../etc/";
$homedir = "/home/../etc/";
$file_to_delete = "passwd";
unlink ("/home/../etc/passwd");
echo "/home/../etc/passwd has been deleted!";
?>
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There are two important measures you should take to prevent these
issues.
Here is an improved script:
Example 25-3. 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
$homedir = "/home/$username";
$file_to_delete = basename("$userfile"); // strip paths
unlink ($homedir/$file_to_delete);
$fp = fopen("/home/logging/filedelete.log","+a"); //log the deletion
$logstring = "$username $homedir $file_to_delete";
fwrite ($fp, $logstring);
fclose($fp);
echo "$file_to_delete has been deleted!";
?>
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However, even this is not without it's 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 25-4. More secure file name checking
<?php
$username = $_SERVER['REMOTE_USER']; // using an authentication mechanisim
$homedir = "/home/$username";
if (!ereg('^[^./][^/]*$', $userfile))
die('bad filename'); //die, do not process
if (!ereg('^[^./][^/]*$', $username))
die('bad username'); //die, do not process
//etc...
?>
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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.