XSS Vulnerability in Internet Explorer HTML Attachment Download

Update: MS fixed this issue in the IE8 6/9/09 security update.  Now IE8 behaves like Firefox (unclear on whether ‘X-Download-Options: noopen’ still exists at all).

I have noticed a Cross-Site Scripting vulnerability in the way Internet Explorer handles the downloading and opening of HTML files when they are downloaded as an attachment, rather than opened normally. This vulnerability exists in all versions of Internet Explorer, including the latest patch level of IE7 as of 12/23/08.

This vulnerability related to sites serving user-submitted HTML files with “Content-Disposition: attachment”.

When directly opening a downloaded HTML file, Internet Explorer violates the Same Origin Policy by allowing any script inside the downloaded file to access the cookies of the site the file was downloaded from. This script should be restricted to running in a local context, not a domain context.

Firefox exhibits better behavior by first downloading the HTML attachment and then opening it with a file:// URL. When scripts in the downloaded HTML file are executed, they are treated as if run from a local file, not as if run from the domain the file was downloaded from, and they cannot access the source domain’s cookies.

This vulnerability would allow an attacker to execute a Cross-Site Scripting attack on a site that allowed uploading file attachments. An HTML file could be uploaded containing malicious script that could steal user credentials or forge user actions on the site (if downloaded and opened by an IE user).

I have created a screencast reproducing the incorrect behavior in Internet Explorer as well as the correct behavior in Firefox. Additionally, I’ve set up a downloadable HTML file that you can use to reproduce the issue yourself!

The screencast and example are available at: http://www.awgh.org/iebug

Microsoft has addressed this issue in IE8, but their solution leaves me with some questions.  The write-up in their development blog is here, skip down to the section titled “MIME-Handling: Force Save”.  This section acknowledges that this is a potential vector for script injection in IE and describes the solution as implemented in IE8.

The solution in IE8:

The web server can set the response header “X-Download-Options” to the value “noopen”.  This will tell Internet Explorer to only offer the option to save the file or cancel.  It simply removes the “Open” option when this option is set.

I see two problems with this solution.  First, call me a pessimist, but I can see someone actually disabling “noopen” simply to bring back the “Open” dialog option.

Second, I doubt that most web server admins are going to be worried enough about this issue to remember to set this header, or even know they should do it.  It’s a bit of an esoteric bug – it only affects sites that serve untrusted HTML with “Content-Disposition: attachment”.  Even if Microsoft web servers set “noopen” by default, I doubt that most LAMP admins will bother adding this to their server options.

Why force the server to fix this problem?  Why not treat the “Open” option the same way Firefox does, by first downloading the file, then opening it with only a local script context?

This method of script injection will continue to work in Internet Explorer 8, as long as the site has not set the “X-Download-Options” header to “noopen”.

So the moral of the story:  If you are the admin of a site that serves untrusted HTML files with “Content-Disposition: attachment” set, please make sure the “X-Download-Options” header is set to “noopen”.

3 comments… add one
  • Minh

    Yeah,

    I totally agree – firefox handles this far better.

    Do you know of any way of handling this issue for older versions of IE 6 & 7?

  • Stephen

    I don’t know why you say “Now IE8 behaves like Firefox”. Your example page still shows the problem in Internet Explorer 8 when I test it.

    It’s fixed in IE9, though. IE9 behaves like Firefox.

    For IE8, the “noopen” header is needed for MOST files, not just HTML, due to IE’s mime-type sniffing algorithm. Serving a file as application/octet-stream? Need to disallow “Open” because it may get sniffed as HTML!

    And “nosniff” doesn’t seem to have any effect in combination with with “attachment”.

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