This indicates an attack attempt to exploit a Denial of Service Vulnerability in Eclipse Mosquitto.
The vulnerability is due to an error when the vulnerable software attempts to handle a maliciously crafted web page. A remote attacker may be able to exploit this to cause a denial of service condition on the affected system.
Eclipse Mosquitto versions from 1.5 to 1.5.2
Denial of Service: Remote attackers can crash vulnerable systems.
Apply the most recent upgrade or patch from the vendor.