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 application handles crafted SIP packet. 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 13.15.1
Digium Asterisk Open Source prior to 14.4.1
Digium Certified Asterisk prior to 13.13-cert4
Teluu Ltd. PJPROJECT prior to commit 595cac8
Denial of Service: Remote attackers can crash vulnerable systems.
Apply the most recent upgrade or patch from the vendor.