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12.2. Filenames

It's fairly easy to construct a filename that refers to something other than what you intended. For example, say you have a $username variable that contains the name the user wants to be called, which the user has specified through a form field. Now let's say you want to store a welcome message for each user in the directory /usr/local/lib/greetings, so that you can output the message any time the user logs into your application. The code to print the current user's greeting is:

<?php include("/usr/local/lib/greetings/$username") ?>

This seems harmless enough, but what if the user chose the username "../../../../etc/passwd"? The code to include the greeting now includes /etc/passwd instead. Relative paths are a common trick used by hackers against unsuspecting scripts.

Another trap for the unwary programmer lies in the way that, by default, PHP can open remote files with the same functions that open local files. The fopen( ) function and anything that uses it (e.g., include( ) and require( )) can be passed an HTTP or FTP URL as a filename, and the document identified by the URL will be opened. Here's some exploitable code:

<?php
  chdir("/usr/local/lib/greetings");
  $fp = fopen($username, "r");
?>

If $username is set to "http://www.example.com/myfile", a remote file is opened, not a local one.

The situation is even more dire if you let the user tell you which file to include( ):

<?php
  $file = $_REQUEST['theme'];
  include($file);
?>

If the user passes a theme parameter of "http://www.example.com/badcode.inc" and your variables_order includes GET or POST, your PHP script will happily load and run the remote code. Never use parameters as filenames like this.

There are several solutions to the problem of checking filenames. You can disable remote file access, check filenames with realpath( ) and basename( ), and use the open_basedir option to restrict filesystem access.



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