Add a firewall rule that allows access to proxy only from internal lan:
/ip firewall filter add chain=input src-address=!192.168.168.0/24 \
protocol=tcp src-port=1024-65535 dst-port=3128 action=drop \
comment="" disabled=no
example is for lan network = 192.168.168.0/24 and proxy standard
port = 3128
seandsl
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