The problem comes when a Plex Media Server 
discovers a local router that has SSDP support 
enabled. When this happens, the Plex Media 
Server will add a NAT forwarding rule to the 
router, exposing its Plex Media SSDP (PMSSDP) 
service directly on the internet on UDP port 32414

To FIX this once >

Open portainer > Local > Container > plex


Duplicate/Edit >

Network ports configuration
Remove all ports but leave → 32400


scroll down a little bit

And press

The blue Deploy the Docker.

So now Plex is only available to use 32400

And ya more is not needed;)

All other Ports are unused now

Source :

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Update your plex Server


(Security) Mitigate against potential DDoS amplification by only responding to UDP requests from LAN