Technical note
XST Strikes Back
(or perhaps "Return from the Proxy"...)
Amit Klein, January 2006
Introduction
============
About three years ago, the concept of "Cross Site Tracing" [1]
was introduced to the web application security community. In
essence, the classic XST is about amplifying an existing XSS
vulnerability such that HttpOnly cookies and HTTP authentication
credentials can be compromised. This is done using a client side
XmlHttpRequest object that sends a TRACE request back to the
server, receives the request echoed back by the server's TRACE
function, and extracts the information from the echoed back
request.
The recommendation in [1] is to turn off TRACE support in the web
server, which indeed takes care of the attack as described.
However, let us now consider a situation wherein there is a proxy
server somewhere between the client (browser) and the server. In
such case, it is possible to force the proxy server (at least, in
theory) to respond to the TRACE request, rather than the origin
server itself. Thus, HTTP TRACE can still be used to compromise
the credentials of the user, even if the server does not support
the TRACE request.
The technique
=============
Forcing the first proxy server in the chain to respond to the
TRACE request (rather than forward it) is as simple as including
an HTTP request header "Max-Forwards: 0" ([2], section 14.31).
So, for IE (up to and including 6.0 SP1) and for Mozilla/Firefox
(up to and including Firefox 1.0.6), the XSS payload should be
(IE code, Mozilla/Firefox modifications commented):
var x = new ActiveXObject("Microsoft.XMLHTTP");
// var x = new XMLHttpRequest();
x.open("TRACE","/",false);
x.setRequestHeader("Max-Forwards","0");
x.send();
// x.send("");
alert(x.responseText);
In IE 6.0 SP2, it seems that Microsoft silently removed support
for TRACE in the XmlHttpRequest object. That is, no method
starting with "TRACE" is allowed. However, a simple trick,
involving a technique similar to the one used in [3] and [4] can
be used to bypass this protection. Instead of using "TRACE" for
the method, one can simply use "\r\nTRACE". To quote from [2]
(section 4.1):
"In the interest of robustness, servers SHOULD ignore any
empty line(s) received where a Request-Line is expected. In
other words, if the server is reading the protocol stream
at the beginning of a message and receives a CRLF first, it
should ignore the CRLF."
So the XSS payload for IE 6.0 SP2 would be:
var x = new ActiveXObject("Microsoft.XMLHTTP");
x.open("\r\nTRACE","/",false);
x.setRequestHeader("Max-Forwards","0");
x.send();
alert(x.responseText);
Squid (2.5stable10/NT) ,Apache (2.0.54 mod_proxy) and other
popular proxy servers were found to support TRACE and Max-
Forwards.
Recommendations
===============
Proxy server vendors
--------------------
1. Ship proxy servers with default secure configuration, namely
no TRACE support disabled.
2. In the least, enable turning off support for TRACE via a
configuration option.
Proxy server owners/maintainers
-------------------------------
Disable support for TRACE.
1. For Squid, add the following to the Squid configuration file
(squid.conf):
acl TRACE method TRACE
...
http_access deny TRACE
2. For Apache, use mod_rewrite to prevent support for TRACE (see
[1]). Make sure to place the directive in the section of
the httpd.conf file. Also, It would be a good idea to append the
"[nocase]" flag to the RewriteCond directive, to ensure case
insensitive comparison (though it seems that Apache will only
serve fully uppercase HTTP methods).
Browser vendors
--------------
Disable support for TRACE in the XmlHttpRequest object. Make sure
you do it right though.
Web site owners
---------------
As a workaround (perhaps not too practical), enable SSL traffic
only to your site.
Summary
=======
This is yet another example of peripheral web security issue,
such as the ones discussed in [5]. A web application may be
compromised through issues that are beyond the control of the web
site owner - in this case, support for TRACE in browsers and
proxy servers. In fact, in many cases the site owner has no way
of even knowing that the attack took place, because the TRACE
request is answered at the proxy server, and never arrives at the
web server (of course, if the first proxy server is the site's
reverse proxy server, or if no proxy server at all is present,
then the site owner may find out).
It seems that the TRACE method should be disabled across the
board - not just in web servers, but also in proxy servers and in
browsers (and possibly in other web devices).
References
==========
[1] "Cross-Site Tracing (XST)", Jeremiah Grossman, January 20th,
2003
http://www.cgisecurity.com/whitehat-mirror/WhitePaper_screen.pdf
[2] "Hypertext Transfer Protocol -- HTTP/1.1" RFC 2616
http://www.ietf.org/rfc/rfc2616.txt
[3] "XS(T) attack variants which can, in some cases, eliminate
the need for TRACE", Amit Klein, WebAppSec mailing list submission,
January 26th, 2003
http://www.securityfocus.com/archive/107/308433
[4] "Exploiting the XmlHttpRequest object in IE - Referrer
spoofing, and a lot more...", Amit Klein, BugTraq mailing list
submission, September 24th, 2005
http://www.securityfocus.com/archive/1/411585
[5] "Meanwhile, at the other side of the web server", Amit Klein,
BugTraq mailing list submission, June 9th, 2005
http://www.securityfocus.com/archive/1/401866