In addition to hardcoded blocklist, Tesseract downloads secret censorship list at runtime without telling anybody

cross-posted from: https://lemmy.world/post/49778547

Also /c/modabuse. Also /c/yepowertrippinbastards. Also lemmy.ml/c/worldnews, comrade, ACAB, and 552 individual accounts across 67 instances, about half of them on lemmy.world.

None of this is in the source code. It’s downloaded at runtime from a file nobody has ever looked at.

If you’re just tuning in

Tesseract is a third-party web frontend for Lemmy, maintained by asimons04 and licensed AGPL-3.0. Admins deploy it on their own servers alongside or instead of lemmy-ui, and there are public instances of it people use to browse Lemmy generally. If you’ve used a Lemmy site that didn’t look like stock Lemmy, there’s a fair chance it was this.

Last week db0 posted a PSA: Tesseract contains a blacklist of instance domains compiled directly into the application. 32 of them. Admins can’t see it, can’t configure it, and aren’t told it’s there. Connect to a listed instance and the app tells you it’s “incompatible,” which is not true.

I went through the code to see how that was implemented. The hardcoded list turns out to be the small half of the system.

There’s a second filter policy fetched over HTTP every time the app loads. It isn’t in the git repository. It’s unauthenticated and world-readable, so anyone can pull it. Right now it carries 552 user accounts, 2,275 username patterns, 54 instances, 97 communities, 289 keyword patterns and 351 domains, with every category set to hide matches rather than flag them. Not collapsed behind a click. Simply absent, with no indication anything was removed.

Verify all of it in ten seconds

curl -s https://tesseract.dubvee.org/tesseract/api/system/policy \
  | base64 -d | gunzip > policy.json

That’s the live policy, base64-wrapped gzip, 111KB of JSON when it unpacks. There’s a stale fallback copy at /data/policy.dat as well.

It filters criticism of moderators

• lemmy.sdf.org/c/modabuse — listed
• lemmy.dbzer0.com/c/yepowertrippinbastards — listed
• lemmy.dbzer0.com/c/YPTBcirclejerk — listed
• community regex power ?tripping?
• keyword censoring me

Call the rest of it whatever you like. This part is not spam defence.

It filters words

The 32 community name patterns include Communis(t|m), Conservativ(e|es|ism), Leftis(t|m), Libertarian(ism)?, ^Green Part(y|ies), Zionis(t|m), (Police|Cops), guillotine and billionaire.

Keywords include comrade, ACAB, neoliberal, proletaria(n|t) and death to.

Filtered communities on instances that aren’t blocked: lemmy.ml/c/worldnews, lemmy.today/c/news, lemmy.ca/c/politicalnewscanada, lemmy.ca/c/usa, infosec.pub/c/strategic_unions.

The 552 users aren’t bots

67 instances. 272 on lemmy.world alone, 40 on sh.itjust.works, 19 on lemmy.ca, and 28 instances contributing exactly one person each.

355 of the 552 usernames are plain alphabetic, twelve characters or under, median length eight. Only 36 look like spam registrations. A bot list looks like the opposite of that.

Seven of them aren’t even Lemmy. There are Mastodon and Friendica accounts in there: people who have never used Lemmy, hidden by a Lemmy frontend, with no possible way of finding out.

I have the list and I’m not posting it. Most of these are ordinary people who got pattern-matched, and 552 names on this comm is a harassment target inside an hour. Run the command above and grep for yourself.

And it lies about it

When the instance block fires you get: “Incompatible Instance. Not Supported. $instance is not compatible with Tesseract.”

Nothing is incompatible. It’s a policy decision dressed as an API error, and it’s what had db0 chasing a version mismatch that never existed.

For the hidden users, communities and keywords, you get no message at all.

Admins can’t switch it off

Tesseract has env vars for PUBLIC_DOMAIN_BLACKLIST, PUBLIC_FAKE_NEWS_BLACKLIST and the shortener lists. There is none for either blocklist. enableToxicMode bypasses the other filters and explicitly not this one.

Self-host it and you cannot disable this, nothing in your config admits it exists, and the contents can change without you pulling a commit.

Before someone says it

A lot of that domain list is real spam defence. It filters conservatism as well as communism. “It targets the left” doesn’t survive the data and I’m not going to pretend it does.

The problem is that spam filtering and political editorial got welded into one undocumented, remotely-updatable blob, shipped hidden, to admins who’ve never read it and users who don’t know it’s there. The spam work is what makes the rest unauditable: “it’s a spam list” answers every individual question and none of the whole.

And /c/modabuse is not spam.

Asks

1. Publish the runtime policy in the repo, or kill the endpoint.
2. Stop reporting a policy block as a technical incompatibility.
3. Tell users when something’s been hidden. One line.
4. Give operators an off switch, like every other blacklist in the codebase has.

It’s AGPL-3.0 and db0 already forked it. That’s the licence working as designed. But forking isn’t disclosure, and the admins who need this are precisely the ones with no reason to go looking.

If you run Tesseract, you are relaying a 111KB moderation policy you have never read, under your instance’s name, to users who don’t know it exists.

Full contents of every list, unedited, in the comments.