This indicates an attack attempt to exploit a Denial of Service vulnerability in Digium Asterisk.
The vulnerability is due to an error when the vulnerable module handles malicious SIP requests. A remote attacker may be able to exploit this to cause a denial of service condition on the affected system.
Digium Asterisk Open Source 14.x - prior to 14.7.5
Digium Asterisk Open Source 15.x - prior to 15.1.5
Digium Asterisk Open Source 13.x - prior to 13.18.5
Digium Certified Asterisk 13.18 - prior to 13.18-cert2
Denial of Service: Remote attackers can crash vulnerable systems.
Apply the most recent upgrade or patch from the vendor.