This indicates an attack attempt to exploit a remote Code Execution Vulnerability in Nginx.
The vulnerability is due to an error when vulnerable software handles a HTTP request with unescaped space characters within URIs. A remote attacker can exploit this to bypass security checks of vulnerable system, via a crafted HTTP request.
nginx HTTP Server 1.5.x prior to 1.5.7
nginx HTTP Server 1.x prior 1.4.4
Security Bypass: Remote attackers can bypass security checking of vulnerable systems.
Apply patch available from the website.