Neohapsis is currently accepting applications for employment. For more information, please visit our website www.neohapsis.com or email hr@neohapsis.com
From: zeno (bugtraqcgisecurity.net)
Date: Wed Jan 23 2002 - 07:28:23 CST

  • Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]


    Below is a paper I wrote on some threats that web statistical software faces
    in regards to header manipulation.

    I've decided to include 1 product affected by this to show that this is very

    Product: w3perl



    Upgrade to 2.86 from 2.85 (bottom right hand corner)

    Product allows insertion of html and javascript which can lead
    to possible cookie theft, possibly faked logs, and other annoying
    javascript stuff.

    Vendor was contacted and provided me with the patch above.

    Below is Paper #4

    - zeno


                                       Author: Zenomorph
                   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
    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
    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
    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