The default SNMP configuration on ZyXEL Prestige routers, including P-660 and P-661 models with firmware 3.40(AGD.2) through 3.40(AHQ.3), has a Trusted Host value of 0.0.0.0, which allows remote attackers to send SNMP requests from any source IP address.
Weaknesses in this category are typically introduced during the configuration of the software.