this post was submitted on 13 May 2025
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it's a combination of multiple issues.
lemmy federation has improved significantly over the past years, so if this was happening with lemmy instances today, especially online ones involved, this would be much less of an issue.
the original user posting this stuff was on a kbin instance. kbin/mbin still do not support federating bans of users. kbin is basically dead, mbin is tracking that here. when this was originally removed on kbin this never federated out to other platforms.
the next problem is that the original instance is no longer there, so attempting to address this with community bans from lemmys side is not working anymore if the user isn't known to the instance anymore, as it can't be refetched from the source. if the instances that the related communities are hosted on purged this user in the past they wouldn't be able to federate out a community ban anymore.
another problem is that lemmy is typically configured in a way where it creates a local copy of thumbnails if no thumbnail url is provided by the source, which is what lead to a local copy of this material. in the end i consider this only a secondary issue, as while most people would rather not have this stored on their servers at all, if you allow media uploads you can never be 100% sure about the content uploaded to your server. this is therefore typically something where providers are expected to take action once they become aware of it. some providers are also taking preventative measures like scanning uploads, comparing to hash databases of known csam, etc. had the original instance ban or community bans been performed properly this would at least have removed public access to the stuff, as the media filenames are randomly generated and not guessable.
it's generally not impossible to prevent stuff from returning to your instance once you have taken it down properly, but in cases where federation didn't work, which could be for a wide range of reasons, including your instance being misconfigured during maintenance, your instance being defederated from an instance involved in the removal, and others, it may require local action. if i ban a user then no new content form that user is going to come to LW until they are unbanned again. this includes manually fetching posts or other content, so if i purge an old post of theirs the post wouldn't be able to come back until the user gets unbanned.