OSEC

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RE: Malicious file upload in .JPG or GIF format

From: Brett Moore (brett.mooreinsomniasec.com)
Date: Wed Feb 20 2008 - 16:39:20 CST


The null byte trick is the most common used. Complement that with
some ../ or ..\ (or which ever encoding you wish) to get the file
placed into a folder with execute script permissions.

* Black listed file types.
Example would be a .net site that checks that a file does not
end with .aspx, .asp, htr, .idc etc..
But they forget extensions like .asa and .cer which are both
mapped to asp.dll

* Filename must include .jpg
Some checks are so bad that as long as the string includes .jpg
then it passes.. filename.jpg.aspx or folder\.jpg\realname

* Filename must end with .jpg
Null bytes already discussed, what about a very long filename
that is extended with spaces, and then gets truncated in code.

* Alternate data streams
Read this post about abusing alternate data streams
http://ha.ckers.org/blog/20070606/additional-image-bypass-on-windows/

* Content upload to database
SQL injection through filename or other parameters passed.

* Usual encoding problems
Supplied filename is checked, then decoded.

Secure coding doesn't rely on the supplied filename, and should
have at least some sort of file validation.

* Header checking
Graphic header checking is common, but can be circumvented in
most script file as the header can be treated as plain text.

* Format checking
Third party utils to check file formats, or scan for viruses, or
decrypt archives, can all be vulnerable to exploitation.

And best practise says not to upload content into the webroot or
other accessible areas. Again that’s where traversal comes into
play.

Something I haven't seen, and which would be very poor programming,
would be a race condition. If the file is saved, then checked, then
deleted. By sending enough file upload posts it may be possible to
swamp the app/server and get in a request to the uploaded file.

Brett

-----Original Message-----
From: listbouncesecurityfocus.com [mailto:listbouncesecurityfocus.com] On Behalf Of Erin Carroll
Sent: Thursday, 21 February 2008 10:39 a.m.
To: 'H D Moore'; pen-testsecurityfocus.com
Subject: RE: Malicious file upload in .JPG or GIF format

I used this exact method (NULL byte) just last night actually when testing some servers. However, I did run into permissions issues with the uploaded malicious .asp. While the ASP upload script was vulnerable to NULL byte name truncation, the server treated the submitted file with different permissions which didn't allow for http://foo.com/malicious.asp to be invoked.

OffTopic: Btw HD, I like the new mfsWeb console. Been a while since I used anything other than the CLI and I see the GUI has come a long way since I last played with it. Any hints on what's in your roadmap or when the next release is coming?

-----Original Message-----
From: listbouncesecurityfocus.com [mailto:listbouncesecurityfocus.com] On Behalf Of H D Moore
Sent: Wednesday, February 20, 2008 1:17 PM
To: pen-testsecurityfocus.com
Subject: Re: Malicious file upload in .JPG or GIF format

The usual trick is to upload an ASP, ASPX, PHP, JSP, or other dynamic web
page to the server. If the applications allows you to set the extension
and the upload directory supports that scripting language, your job is
done.

If the server changes the extension to .JPG/.GIF (or only allows those
extensions), then you need to be more creative. On Apache, you can name a
file something.php.jpg, and Apache will still treat it as PHP.

Another option you can try is by sending an upload request (with a tool or
a HTTP request editor) that embeds a NULL byte before the .JPG extension.
ASP scripts tend to be vulnerable to this -- the script will see the
entire file name, but the underlying file operation will truncate the
name of the file after the NULL byte. So something.asp%00.jpg would
become something.asp.

Finally, one trick that might help, is to upload a HTML document, with a
JPG extension, and see whether the browser treats it as HTML or an image
when you browse to it. Some browsers handle this different, sometimes
ignoring the mime type in favor of the file magic (not sure if this works
with images in IE 7).

What this allows you to do is upload arbitrary HTML content to the server,
which can contain javascript, which in turn can read the domain-specific
credentials of users visiting that page. This still requires the ability
to send users to your not-really-a-jpeg HTML page (for example, by
emailing them a link).

Good luck,

-HD

On Wednesday 20 February 2008, whitehat wrote:
> I'm doing Web Application Pen-Testing. In one of the pages there is an
> option to upload an image(.JPG or .GIF).
> How a hacker can exploit it and what are the chances of uploading a
> malicious .exe file (virus kind of stuff) in .JPG or .GIF format by
> changing its extension.

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