Re: A Study In Scarlet - Exploiting Common Vulnerabilities in PHP Applications

From: Julian Hall (jules@acris.co.uk)
Date: 07/27/01


Message-ID: <3B619F54.4DAACC0E@acris.co.uk>
Date: Fri, 27 Jul 2001 18:05:25 +0100
From: Julian Hall <jules@acris.co.uk>
To: Shaun Clowes <shaun@securereality.com.au>
Subject: Re: A Study In Scarlet - Exploiting Common Vulnerabilities in PHP  Applications

Sorry for the delay in responding here, but I've not had a chance to catch up
with the list for a while, but:

Shaun Clowes wrote:

> As an alternate attack assisted by file upload consider the following
> example PHP code:
>
> <?php
> if (file_exists($theme)) // Checks the file exists on the local system (no
> remote files)
> include("$theme");
> ?>

Is anyone really that naive? I, and I'm sure most other PHP uses, would
automatically write:

<?php
    $themefile = "themes/$theme.inc";
    include ($themefile);
?>

If I was even remotely thinking about security I would check for the presence
of directory seperator characters in $theme (as it stands obviously the code
would allow the inclusion of any file with the '.inc' suffix). You never
include code from a filename specified directly by the user. That's a primary
rule, and applies to server applications written in any language, not just PHP
and other similar systems.