This indicates an attack attempt to exploit a Denial of Service vulnerability in Apache HTTPD.
The vulnerability is due to an input validation error when handling malformed HTTP requests. As a result, a remote attack may be able to exploit this to cause a denial-of-service condition on the affected system.
Apache Software Foundation HTTP Server 2.2.21 and earlier versions.
Denial of Service: Remote attackers can crash vulnerable systems.
Apply patch available from the website.