This indicates an attack attempt to exploit a Denial of Service vulnerability in Apache HTTP Server.
The vulnerability is a result of the application's failure to properly sanitize user supplied input. A remote attacker can exploit this to cause the server to consume excessive resources leading to a Denial of Service condition.
The tool named killapache is exploiting this vulnerability to attack Apache Servers.
Apache Software Foundation HTTP Server all versions.
Denial of Service: Remote attackers can crash vulnerable systems.
Upgrade to Apache 2.2.21