This indicates an attack attempt to exploit a Denial of Service vulnerability in Digium Asterisk.
The vulnerability is due to an input validation error when handling closing of connection while the server is in a certain state. A remote attack may be able to exploit this to cause a denial of service condition on the affected system.
Digium Asterisk Open Source 1.8 prior to 18.104.22.168
Digium Asterisk Open Source 10 prior to 10.4.1
Denial of Service: Remote attackers can crash vulnerable systems.
Apply patch available from the website.