Software Engineering Institute

The problem occurs when the web server responds with a "302 Moved Temporarily" redirection error. One easy way to obtain this error is to request a URL for a directory while omitting the trailing slash. The Location: header contained in this response is composed in part from the Host: header contained in the request. By carefully manipulating the length of the Host: header before and after URL encoding, the attacker can cause the resulting Location: header to contain information in adjacent memory on the web server.

The advisory from @Stake describing this problem in more detail is available from:

http://www.atstake.com/research/advisories/2001/a041601-1.txt

A remote attacker can obtain sensitive information from the memory of the web server, including userids, passwords, cookies or authentication data belonging to other users of the web server. With this data the attacker may be able to falsely authenticate themselves to the web server as other users. In some cases, the attacker may be able to prevent the normal operation of the web server using this vulnerability.

Upgrade your Web Server

System administrators are encouraged to upgrade their systems to a non-vulnerable version of the web server software. Information about upgrading your web server is available from iPlanet at:

http://www.iplanet.com/products/iplanet_web_enterprise/iwsalert4.16.html

Filter HTTP Requests with Large Headers

Sites that are able to deploy a monitoring system between the Internet and their web server may be able to detect and block packets with large amounts of header data. Possible mechanisms include an NSAPI filter, an active intrusion detection system, or a reverse-proxy web server. The @Stake advisory contains more detailed suggestions for detecting and monitoring malicious HTTP requests of this type.