An issue in the server_handle_regular function of the test_coap_server.c file within the FreeCoAP project allows remote attackers to cause a Denial of Service through specially crafted packets.
https://github.com/keith-cullen/FreeCoAP/issues/36
https://gist.github.com/dqp10515/41ec400b7eecfcae7578d505598ab85f