In the trapper functionality of Zabbix Server 2.4.x, specifically crafted trapper packets can pass database logic checks, resulting in database writes. An attacker can set up a Man-in-the-Middle server to alter trapper requests made between an active Zabbix proxy and Server to trigger this vulnerability.
https://www.debian.org/security/2017/dsa-3937
https://talosintelligence.com/vulnerability_reports/TALOS-2017-0326