> I'm not really sure what Yeti 2022 is exactly

Sometimes there are problems with certificates in the Web PKI (approximately the certificates your web browser trusts to determine that this is really news.ycombinator.com, for example). It's a lot easier to discover such problems early, and detect if they've really stopped happening after someone says "We fixed it" if you have a complete list of every certificate.

The issuing CAs could just have like a huge tarball you download, and promise to keep it up-to-date. But you just know that the same errors of omission that can cause the problem you were looking for, can cause that tarball to be lacking the certificates you'd need to see.

So, some people at Google conceived of a way to build an append-only log system, which issues signed receipts for the items logged. They built test logs by having the Google crawler, which already gets sent certificates by any HTTPS site it visits as part of the TLS protocol, log every new certificate it saw.

Having convinced themselves that this idea is at least basically viable, Google imposed a requirement that in order to be trusted in their Chrome browser, all certificates must be logged from a certain date. There are some fun chicken-and-egg problems (which have also been solved, which is why you didn't need to really do anything even if you maintain an HTTPS web server) but in practice today this means if it works in Chrome it was logged. This is not a policy requirement, not logging certificates doesn't mean your CA gets distrusted - it just means those certificates won't work in Chrome until they're logged and the site presents the receipts to Chrome.

The append-only logs are operated by about half a dozen outfits, some you've heard of (e.g. Let's Encrypt, Google itself) and some maybe not (Sectigo, Trust Asia). Google decided the rule for Chrome is, it must see at least one log receipt (these are called SCTs) from Google, and one from any "qualified log" that's not Google.

After a few years operating these logs, Google were doing fine, but some other outfits realised hey, these logs just grow, and grow, and grow without end, they're append-only, that's the whole point, but it means we can't trim 5 year old certificates that nobody cares about. So, they began "sharding" the logs. Instead of creating Kitten log, with a bunch of servers and a single URL, make Kitten 2018, and Kitten 2019, and Kitten 2020 and so on. When people want to log a certificate, if it expires in 2018, that goes in Kitten 2018, and so on. This way, by the end of 2018 you can switch Kitten 2018 to read-only, since there can't be new certificates which have already expired, that's nonsense. And eventually you can just switch it off. Researchers would be annoyed if you did it in January 2019, but by 2021 who cares?

So, Yeti 2022 is the shard of DigiCert's Yeti log which only holds certificates that expire in 2022. DigiCert sells lots of "One year" certificates, so those would be candidates for Yeti 2022. DigiCert also operate Yeti 2021 and 2023 for example. They also have a "Nessie" family with Nessie 2022 still working normally.

Third parties run "verifiers" which talk to a log and want to see that is in fact a consistent append-only log. They ask it for a type of cryptographic hash of all previous state, which will inherit from an older hash of the same sort, and so on back to when the log was empty. They also ask to see all the certificates which were logged, if the log operates correctly, they can calculate the forward state and determine that the log is indeed a record of a list of certificates, in order, and the hashes match. They remember what the log said, and if it were to subsequently contradict itself, that's a fatal error. For example if it suddenly changed its mind about which certificate was logged in January, that's a fatal error, or if there was a "fork" in the list of hashes, that's a fatal error too. This ensures the append-only nature of the log.

Yeti 2022 failed those verification tests, beginning at the end of June, because in fact it had somehow logged one certificate for *.molabtausnipnimo.ml but it has mistakenly calculated a SHA256 hash which was one bit different and then all subsequent work assumed the (bad) hash was correct. There's no way to rewind and fix that.

In principle if you knew a way to make a bogus certificate which matched that bad hash you could overwrite the real certificate with that one. But we haven't the faintest idea how to begin going about that so it's not an option.

So yes, this was mission failure for Yeti 2022. This log shard will be set read-only and eventually decommissioned. New builds of Chrome (and presumably Safari) will say Yeti 2022 can't be trusted past this failure. But the overall Certificate Transparency system is fine, it was designed to be resilient against failure of just one log.