This indicates an attack attempt to exploit a Denial of Service vulnerability in Asterisk Open Source and Certified Asterisk.
The vulnerability is due to an error when the SIP channel driver fails to properly handle an ACK with SDP received after the channel has been terminated. A remote attacker may be able to exploit this to cause a denial of service condition on the affected system.
Digium Asterisk Open Source Prior to 184.108.40.206
Digium Asterisk Open Source Prior to 11.5.1
Digium Certified Asterisk Prior to 1.8.15-cert3
Digium Certified Asterisk Prior to 11.2-cert2
Denial of Service: Remote attackers can crash vulnerable systems or services
Apply the most recent upgrade or patch from the vendor.