This indicates a possible attempt to obtain information from a system using a vulnerability in Apache Tomcat Web Server.
There is an information disclosure vulnerability in Apache Jakarta Tomcat before 3.3.1a (when using JDK 1.3.1 or earlier). It can be exploited by sending a URL containing a NULL character to the server. As a result a remote attacker can list directories or obtain source code for a JSP file.
Apache Software Foundation Tomcat 3.0 - 3.3.1 (except V 3.3.1 a)
Apply the appropriate patch from the vendor or upgrade to a non-vulnerable version if available.