This indicates an attack attempt to exploit a Security Bypass vulnerability in Eclipse Mosquitto.
The vulnerability is due to an error when the vulnerable software handles special characters in user names or client id. An attacker can exploit this by sending a crafted request to the target server and bypass the security feature.
Eclipse Mosquitto versions before 1.4.12
Security Bypass: Remote attackers can bypass security features of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.