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Similar to how we analyzed Twitter in our self-generated CSAM report, we did a brief analysis of public timelines of prominent servers, processing media with PhotoDNA and SafeSearch. The results were legitimately jaw-dropping: our first pDNA alerts started rolling in within minutes. The true scale of the problem is much larger, as inferred by cross-referencing CSAM-related hashtags with SafeSearch level 5 nudity matches.

Hits were primarily on a not-to-be-named Japanese instance, but a secondary test to see how far they propagated did show them getting federated to other servers. A number of matches were also detected in posts originating from the big mainstream servers. Some of the posts that triggered matches were removed eventually, but the origin servers did not seem to consistently send "delete" events when that happened, which I hope doesn't mean the other servers just continued to store it.

The Japanese server problem is often thought to mean "lolicon" or CG-CSAM, but it appears that servers that allow computer-generated imagery of kids also attracts users posting and trading "IRL" materials (their words, clear from post and match metadata), as well as grooming and swapping of CSAM chat group identifiers. This is not altogether surprising, but it is another knock against the excuses of lolicon apologists.

Traditionally the solution here has been to defederate from freezepeach servers and...well, all of Japan. This is commonly framed as a feature and not a bug, but it's a blunt instrument and it allows the damage to continue. With the right tooling, it might be possible to get the large Japanese servers to at least crack down on material that's illegal there (which non-generated/illustrated CSAM is).

I have argued for a while that the Fediverse is way behind in this area; part of this lack of tooling and reliance on user reports, but part is architectural. CSAM-scanning systems work one of two ways: hosted like PhotoDNA, or privately distributed hash databases. The former is a problem because all servers hitting PhotoDNA at once for the same images doesn't scale. The latter is a problem because widely distributed hash databases allow for crafting evasions or collisions.

I think for this particular issue to be resolved, a couple things need to happen: one, an ActivityPub implementation of content scanning attestation should be developed, allowing the origin servers to perform scanning via a remote service and other servers to verify it happened. Second, for the hash databases that are privately distributed (e.g. Take It Down, NCMEC's NCII database), someone should probably take on making these into a hosted service.

There are some other things that would be helpful in controlling proliferation: for example, easy UI for admins to do hashtag and keyword blocks, instead of relying on users to track a changing threat landscape. These could be distributed or subscription-based across servers, though how public those lists should be is up for debate. That subscription model could also be used for general "fediblock" lists.