On February 15th, newly-created Fediverse accounts started posting spam messages from various instances, sending invites to a Discord server for a Japanese troll organization. This spam was widespread across the Fediverse. The posts frame ap12 from “KuronekoServer” as the culprit behind the operation.
Looking at their spam content (in Japanese)
A first step is RBL intergration, a shared blocklist of spam instances that subscribed instances would use to blackhole spam users/traffic/instances. These are used ubiquitously in email spam systems, so there is a precedent in federation systems for it working. We need to stand up an RBL, and then mod Lemmys federation system to work automatically based on the community blocklist.
It does mean that poorly admined instances will get blackholed, breaking their federation, but that’s the cost of a healthy network.
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This is how email servers have worked for decades - there is no silver bullet and this comes closest. If you poorly admin your email instance, say allowing it to be an open relay (same as just allowing open registrations), you get blacklisted everywhere aka defederated. Same if you have a compromise and someone starts spamming out.