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TUCoPS :: Web :: General :: headerx.txt

Header Based Exploitation




                                   Author: Zenomorph
                                 admin@cgisecurity.com
               Header Based Exploitation: Web Statistical Software Threats




I.   Introduction 
II.  Type of Threats
III. Examples
IV.  Solutions
V.   Conclusion




I. Introduction

When people visit your website, certain information is passed from
the users web browser to your web server/script. This information 
contains data such as what browser they are using, the last site 
visited, the file they requested, and other information. This paper 
was written to help you understand how an attacker can use these
information fields to exploit your web statistics software.





II. Type of Threats


Public Statistic Threats:

   If you have hit reports on your site, and they are viewable by the public,
then there are a few risks you must be aware of.


* SSI Tag Insertion
        - Command Execution
        - Page Includes


* HTML Insertion
        - Links to unwanted sites (Spammed references)
        - Possible Alteration of statistical page
        - JavaScript Insertion
        - Possible falsification of logs 
        - Popup Windows (Tricked Advertising)

* Other(Maybe)
        - Java
        - Active X
        - Python
        - TCL
        - VBscript
        - Other Markup Language Insertion
        - PHP
        - ASP
        - SQL/Database injection


Private Statistics Threats:

   Same as above, except if cookie theft is possible, it could allow an attacker
access to administrative tools.





III. Examples

   The threats of modified http headers vary depending on what language
the software is written in, what file format the output is displayed in, 
and the server permissions.



  A. SSI

   For example, if I have a script that prints the output in a .shtml file, then it
*may* be possible to insert file includes, and depending on server configuration,
execution of commands.

Below is an example of such an attack.


su-2.05# telnet localhost 80
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
GET / HTTP/1.0
Referer: <!--#virtual include="somefile.log"-->
User-Agent: <!--#exec cmd="/bin/id"-->

HTTP/1.1 200 OK
Date: Mon, 17 Dec 2001 20:39:02 GMT
Server:
Connection: close
Content-Type: text/html


   In this example the attacker is inserting SSI tags into the Referrer and User-Agent fields.
Depending on whether the software outputs this information as text or in image form, this
could lead to possible file includes, or command execution. (Of course these examples
could be interchangeable). If the logs are shown as text and displayed in a shtml file,
and the referrer, or user agent fields are shown (most of the time they are), then
these two requests will be included in the file. The next time a visitor views
these logs, the SSI tags will be executed by the web server, and should display 
the results of the "id" command, as well as the contents of "somefile.log".
(Once again depending on server configuration).


  B. Html

   Inserting html is less of a threat than SSI, but it still has its concerns.
If a attacker can insert html, then there is a good chance JavaScript can also be inserted.

  - Fake Logs

   Sometimes an attacker will flood your logs with false entries to hide his presence.
Another possibility of html insertion would be falsification of logs. If the attacker
manages to insert tags like </html> into the request, it could hide his presence to a visitor's
web browser. Since his tag would be inserted in the middle or beginning of the document, if the attacker
puts this tag everything afterwards will be invisible to a normal user. (Unless you select View Source).


  - Stolen Cookies

With JavaScript insertion allowed it may be possible for an attacker to steal
cookies from a visiting user. This of course could lead to possible session hijacking,
and depending on the site, user and password information to be leaked.

su-2.05# telnet localhost 80
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
GET / HTTP/1.0
Referer: <javascript-that-is-evil-so-there's-no-need-for-examples>
User-Agent: </html>


   If a user visits this stats page and the referrer is outputted then
it may be possible to steal users cookies.



   C. Other

   ASP, JSP, python, and other languages are all possible using this attack method.
Once again this depends on a few contributing factors. This method isn't as 
'one dimensional' as other exploitation. Database command, and content injection
also may be possible.



   D. Additional

   Some software may only show the top 20, 50, or 100 Referrers and user agents. 
This means that just because an attacker has sent a request with malicious
headers, that it may not execute. Of course since most statistical software
shows the amount of hits per field, the attacker could send a flood of requests
to get his/her evil headers to be displayed. While this isn't a very efficient
attack, it still has its purposes. I've managed to get this attack type
working in 3 different sites.





IV. Solutions



1.(BEST WAY) Stripping out metacharacters like <>":;'}{][|\)(*&^%$#!`

This can help prevent html, JavaScript, php, sql injection, 
and SSI insertion.



2. Replacing < and > with &gt &lt

By replacing < and > with &lt and &gt this helps prevent tag execution.
One obvious problem would be scripts with potential backtick "`" problems. 





V. Conclusion

This paper was written to show awareness of this particular threat. 
I'm sure this paper doesn't cover *every* use for this attack, but I hope 
it helps web developers create safer web applications. 


Published to the Public January 2002
Copyright 2002 Cgisecurity.com


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