Arts & Crafts doesn't get a lot of traffic. I just noticed an unusual number of new users on the Users tab. There's 66 pages of new users. Starting with the last row of new users on the first page to the end of the list (about 2,600 accounts), they are all odd usernames, either entirely what looks like Chinese, or Chinese mixed with snippets of numbers and occasional Latin characters, often containing the same snippets. They include a mix of the default gravitars.
Spot checking the accounts, many or most have a profile "About" section populated with similar random garbage or nonsense, also in Chinese, apparently random content, user instructions, and other filler snatched from activity on the web. The accounts appear to have been created in batches on particular days, many 6 or 7 days ago, apparently by some automated process. It looks like all of these bogus registered accounts have no actual activity, and the initial rep of 1 point.
In some cases, the accounts are linked to similar accounts on other SE sites. I spot checked Super User, which gets way more traffic. Only the last 700+ new users were these same kinds of accounts, and many were a few weeks older than the ones on Arts & Crafts. On Meta SE, it looks like most of the last 3,000 new Users are these same bogus accounts. It looks like they started about the same time (40 some days ago), but the most recent batch I spotted here was 8 days ago.
My recollection is that the New Users tab covers only something like the last 45 days, so this activity could have been going on for longer, and there could be lots more in the system. I didn't bother checking other sites, but it's obvious that this has been done on a lot of sites based on the linked accounts.
It isn't clear what threat, if any, these accounts represent. It might simply be a test to see if they can overwhelm the system, or probing to see if they can get away with it unnoticed. I don't know what impact tens of thousands of bogus accounts has on system resources. If the perpetrator were to use the accounts to flood sites with automated garbage posts, it would be a job to get ahead of it, especially if moderators and active users are not aware that the scope is not individual posts.
Right now, the immediate effects are just distortions of site stats and some inconvenient navigating. But these are obviously not legitimate accounts, and we might want to do some large scale deletion before they are used for mischief. Be aware, though, that there appear to be some legitimate accounts scattered in with user names in Chinese characters, so clearing these would need to be a bit surgical.
Given the breadth of scope, any action would probably need to be from SE staff. Is this something people were aware of, and should anything be done about it?