This indicates an attack attempt to exploit a Security Bypass vulnerability in Apache HTTP Server.
The vulnerability is due to improper validation of user supplied data when vulnerable module handles a maliciously crafted HTTP request. A remote attacker may be able to exploit this to bypass defined rules on vulnerable web server via an HTTP request.
Apache Software Foundation HTTP Server 2.4.1-2.4.10
Apache Software Foundation HTTP Server 2.2.0-2.2.27
Security Bypass: Remote attackers can bypass security checks of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.