XSS, Extensions and Content-Types

In this post I'll look at which Content-Types and Extensions can actually be used for XSS in modern browsers.


Why does Content-Type and Extension matter?

Applications come in many different shapes and sizes and XSS can occur all over the place. It's easy to assume that just because an application returns you a response with unfiltered/unencoded output you've found an exploitable XSS issue but this is often not the case. Browsers render content based on a number of factors including content-type returned by the server, page content and page extension, without the correct combination XSS won't be possible.

To try and get a definitive answer of what works and what doesn't I thought I'd do some testing. I used multiple file types each containing their legitimate content however I also embedded a JavaScript alert payload in each. Each test case was ran against Chrome 43, IE 11 and Firefox 39. In the tables below "yes" means the payload rendered and "no" means it didn't.

Test #1 - Following Best Practice

In the first test I forced the server to return the correct extension and content-type for each test page.

Content-Type	Extension	Chrome	IE	Firefox
None	None	yes	yes	yes
text/plain	txt	no	no	no
text/html	html	yes	yes	yes
application/javascript	js	no	no	no
application/json	json	no	no	no
application/xml	xml	yes	yes	yes
text/css	css	no	no	no
image/jpeg	jpeg	no	no	no

Following best practice modern browsers appeared to be pretty secure. The one exception appears to be XML, it seems odd that the browser would allow the rendering of JavaScript in an XML file.


Test #2 - Modifying Extension

In the second test I made the server return the correct content-type but forced a .html extension.

Content-Type	Extension	Chrome	IE	Firefox
None	html	yes	yes	yes
text/plain	html	no	no	no
text/html	html	yes	yes	yes
application/javascript	html	no	yes	no
application/json	html	no	no	no
application/xml	html	yes	yes	yes
text/css	html	no	no	no
image/jpeg	html	no	no	no

This test seemed to show that browsers will prioritize the use of the content-type over the extension.


Test #3 - Using a text/html Content-Type

In the third test I used the correct extension for each file but made the server return a text/html content-type.

Content-Type	Extension	Chrome	IE	Firefox
text/html	None	yes	yes	yes
text/html	txt	yes	yes	yes
text/html	html	yes	yes	yes
text/html	js	yes	yes	yes
text/html	json	yes	yes	yes
text/html	xml	yes	yes	yes
text/html	css	yes	yes	yes
text/html	jpeg	yes	no	yes

It looks like browsers rely heavily on the content-type returned by the server. It doesn't matter about the extension or contents of the file, the browser will blindly follow the content-type returned.


Test #4 - Using a text/plain Content-Type

Next I used the correct extension for each file but made the server return a text/plain content-type.

Content-Type	Extension	Chrome	IE	Firefox
text/plain	txt	no	no	no
text/plain	html	no	no	no
text/plain	js	no	no	no
text/plain	json	no	no	no
text/plain	xml	no	no	no
text/plain	css	no	no	no
text/plain	jpeg	no	no	no

When using a content-type of text/plain none of the payloads executed. I can't remember for sure, but I believe in the past text/plain used to allow XSS? Looks like this is no longer possible.


Test #5 - MIME sniffing

In this last test case I accessed a page that had no extension and received no content-type header from the server. This should have caused the browser to fall back on mime sniffing to render the page.

MIME	Content-Type	Extension	Chrome	IE	Firefox
text/plain	None	None	no	yes	no
text/html	None	None	yes	yes	yes
application/javascript	None	None	yes	yes	yes
application/json	None	None	no	yes	no
application/xml	None	None	no	yes	no
text/css	None	None	no	yes	no
image/jpeg	None	None	no	no	no

Both Chrome and Firefox refused to render pages that contained additional non-html content. IE however didn't seem to care what the page content was, if it contained HTML it rendered!

Conclusion

Performing application testing on a regular basis I find content-type and extension XSS issues arise quite a lot. This test showed that modern browsers are relatively secure as long as the correct content-type is returned. All browsers also seemed to implement the same analysis techniques aside from IE which seemed slightly more permissive.

Hopefully you guys have found this post useful, if anyone has any suggestions for bypasses to achieve XSS in any of the above, or if I've listed any findings incorrectly (quite possible) drop me a message below :)

Pwndizzle out.