Apache Tomcat 4.0.3, and possibly other versions before 4.1.3 beta, allows remote attackers to cause a denial of service (resource exhaustion) via a large number of requests to the server with null characters, which causes the working threads to hang.
http://www.securityfocus.com/bid/5067
http://www.iss.net/security_center/static/9396.php
http://online.securityfocus.com/archive/1/277940
http://archives.neohapsis.com/archives/vulnwatch/2002-q2/0120.html