This indicates an attack attempt to exploit an OS Command Injection vulnerability in Wordpress Landing Pages plugin.
The vulnerability is caused by a lack of sanitizing of the "url" parameter that is passed to "server.php". A remote attacker can exploit this to send a crafted request to execute OS commands on a vulnerable server.
Landing Pages 1.8.8 to 1.9.0
System Compromise: Remote attackers can gain control of vulnerable systems.
Upgrade to the latest version available from the website.