This indicates an attack attempt to exploit an Authentication Bypass Vulnerability in WordPress Plugin Userpro.
The vulnerability is due to an error in the vulnerable application when handling a maliciously crafted HTTP request. An attacker can exploit this to gain administrator privileges in the affected system.
UserPro plugin before 22.214.171.124 for WordPress
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor: