Cross-User Defacement

Contributor(s): Zhong, Rezos, KristenS


An attacker can make a single request to a vulnerable server that will cause the server 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 server. 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.

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 (Carriage Return) and LF (Line Feed) characters.

Risk Factors



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


And exemplary code of the redir.php:

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

Crafting appropriate requests:


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


HTTP/1.1 302 Moved Temporarily
Date: Wed, 24 Dec 2003 15:26:41 GMT
Content-Length: 0


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

If user shares a TCP connection (e.g. proxy cache) and will send a request: /index.html the response 2 will be sent to them as an answer to their request.

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

More information can be found in one of the presentations under

  • Validation of the input data (CR and LF).
  • Forbid HTTP headers nesting in one header’s field.