EDIT
TO EVERYONE ASKING TO OPEN AN ISSUE ON GITHUB, IT HAS BEEN OPEN SINCE JULY 6: https://github.com/LemmyNet/lemmy/issues/3504
June 24 - https://github.com/LemmyNet/lemmy/issues/3236
TO EVERYONE SAYING THAT THIS IS NOT A CONCERN: Everybody has different laws in their countries (in other words, not everyone is American), and whether or not an admin is liable for such content residing in their servers without their knowledge, don't you think it's still an issue anyway? Are you not bothered by the fact that somebody could be sharing illegal images from your server without you ever knowing? Is that okay with you? OR are you only saying this because you're NOT an admin? Different admins have already responded in the comments and have suggested ways to solve the problem because they are genuinely concerned about this problem as much as I am. Thank you to all the hard working admins. I appreciate and love you all.
ORIGINAL POST
You can upload images to a Lemmy instance without anyone knowing that the image is there if the admins are not regularly checking their pictrs database.
To do this, you create a post on any Lemmy instance, upload an image, and never click the "Create" button. The post is never created but the image is uploaded. Because the post isn't created, nobody knows that the image is uploaded.
You can also go to any post, upload a picture in the comment, copy the URL and never post the comment. You can also upload an image as your avatar or banner and just close the tab. The image will still reside in the server.
You can (possibly) do the same with community icons and banners.
Why does this matter?
Because anyone can upload illegal images without the admin knowing and the admin will be liable for it. With everything that has been going on lately, I wanted to remind all of you about this. Don't think that disabling cache is enough. Bad actors can secretly stash illegal images on your Lemmy instance if you aren't checking!
These bad actors can then share these links around and you would never know! They can report it to the FBI and if you haven't taken it down (because you did not know) for a certain period, say goodbye to your instance and see you in court.
Only your backend admins who have access to the database (or object storage or whatever) can check this, meaning non-backend admins and moderators WILL NOT BE ABLE TO MONITOR THESE, and regular users WILL NOT BE ABLE TO REPORT THESE.
Aren't these images deleted if they aren't used for the post/comment/banner/avatar/icon?
NOPE! The image actually stays uploaded! Lemmy doesn't check if the images are used! Try it out yourself. Just make sure to copy the link by copying the link text or copying it by clicking the image then "copy image link".
How come this hasn't been addressed before?
I don't know. I am fairly certain that this has been brought up before. Nobody paid attention but I'm bringing it up again after all the shit that happened in the past week. I can't even find it on the GitHub issue tracker.
I'm an instance administrator, what the fuck do I do?
Check your pictrs images (good luck) or nuke it. Disable pictrs, restrict sign ups, or watch your database like a hawk. You can also delete your instance.
Good luck.
What's the practical difference? In both cases you're culling images based on whether they're orphaned or not.
If you're suggesting that the implementation be based on setting individual timers instead of simply validating the whole database at regular intervals, consider whether or not the complexity of such a system is actually worth the tradeoff.
"Complexity comshmexity", you might say. "Surely it's not a big deal!". Well... what about an image that used to belong to a valid post that later got deleted? Guess you have to take that edge case into account and add a deletion trigger there as well! But what if there were other comments/posts on the same instance hotlinking the same image? Guess you have to scan the whole DB every time before running the deletion trigger to be safe! Wait... wasn't the whole purpose of setting this up with individual jobs to avoid doing a scripted DB scan?
There are mechanisms that exist in a LOT of services for handling TTL expiry and any relevant purging that needs to be done.
That said, a cursory look at the pict-rs project doesn’t appear to have any provision for TTL, so it’s probably going to have to be done as a cron job anyways - or at least triggered by the lemmy service when an image upload isn’t used in an instance-local lemmy post within some reasonable interval.
Note that I’m specifically including “in an an instance-local post” because I am assuming admins don’t want to provide free cloud image hosting to random internet people for arbitrary non-lemmy use.
Note that I at no point allude to hotlinking from outside of the instance. Unless you want it to be possible to create an image post, delete the post, and then have an orphaned image forever (thereby creating an attack vector), you do need to solve that problem. If you solve that problem without considering crossposts and comment hotlinks within the scope of your own instance, you're going to cause breakage. If you're forced to consider these things before triggering the deletion regardless, then you're not saving much on performance.