RE: how to trick most of cms avatar upload filter [exemple for : RunCms (PoC)]

["Richard Stanway" <please-reply-to-list () mailinator com>]

> -----Original Message-----
> From: Taneli Leppä [mailto:taneli () crasman fi]
> Sent: Thursday, November 02, 2006 3:06 PM
> To: securfrog () gmail com
> Cc: bugtraq () securityfocus com
> Subject: Re: how to trick most of cms avatar upload filter [exemple for
> : RunCms (PoC)]
>
>
> Taneli Leppä kirjoitti:
> > > then you need to take a good file editor , like: notepad++ (you can
> > > take whatever picture , and edit it without destroying it .)
> > > we need to put some php code AFTER the picture code . when  it's done
> > > , try the picture if it still work , if yes , we are ok :).
> > This actually seems to work. A quick workaround is to disable PHP in the
> > directory where the avatar images are stored (or any user-uploaded files
> > for that matter) in Apache:
> >
> > <Directory "/var/www/html/forum/avatardir">
> >   php_admin_flag engine off
> > </Directory>
>
> Actually, your exploit only works if the avatar system strips the
> .jpg extension for some reason. Unless you have configured .jpg
> as a valid extension to be executed through PHP. But the above
> still stands as a valid precaution for user uploaded material.

This isn't entirely true - the exploit will work most of the time due to how
most Apache setups are done, PHP is setup as a mime handler, and mod_mime
will scan every extension, so .php.jpg will actually get executed with the
PHP handler. See http://shsc.info/FileUploadSecurity for more information on
file upload attacks and more details on how Apache works in this regard, as
well as a patch against Apache 1.3 to remove the "double extension"
behaviour.

Rich.