RE: Malicious file upload in .JPG or GIF format

["Brett Moore" <brett.moore () insomniasec com>]

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: listbounce () securityfocus com [mailto:listbounce () securityfocus com] On Behalf Of Erin Carroll
Sent: Thursday, 21 February 2008 10:39 a.m.
To: 'H D Moore'; pen-test () securityfocus 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: listbounce () securityfocus com [mailto:listbounce () securityfocus com] On Behalf Of H D Moore
Sent: Wednesday, February 20, 2008 1:17 PM
To: pen-test () securityfocus 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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