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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