Microsoft Internet Explorer Content-Disposition HTML File Handling Flaw
April 10, 2006

Content-Disposition (defined in RFC 2183) is often used by web
application developers as a mechanism to instruct the web browser on
how it should handle a file download. This is commonly used to help
prevent access to the application scope when handling file attachments
and mitigates the ability to leverage client-side attacks, such as
XSS, through file downloads.

While Internet Explorer does handle downloading most file types
correctly with Content-Disposition, it mishandles HTML files and
instead opens them inline, exposing the application scope. As such, it
is strongly advisable that web-based software vendors use alternative
methods to mitigate this class of attack.

A simple PoC is available at the following URL:
http://xs.vc/content-disposition/
Feel free to compare the results of Firefox and IE.

Vulnerable Versions:
All versions up to and including Internet Explorer 7 Beta 2.

References:
http://www.faqs.org/rfcs/rfc2183.html
http://support.microsoft.com/kb/182315/
http://msdn.microsoft.com/library/default.asp?url=/workshop/networking/moniker/overview/mime_handling.asp

I felt it was necessary to make this flaw public now because while the
weakness results from IEs flawed support of RFC 2183, the exposure is
with the 3rd party applications which support it.

Due to the simplicity of exploitation, it is not unlikely this is
being used in the wild.


Thank you,

Darren Bounds