Where can we learn what this forging is and what countermeasures can be taken against it? Is this unique to Yahoo, or a web account problem of a more general nature?

Oh, I see...
Yahoo: One Billion More Accounts Hacked — Krebs on Security

Never used yahoo except as 'play' email accounts so far no more spam than usual, have separate .org accounts for business only use, except for the Ashley Madison boondoggle in which that account was three jobs and two cities ago. So all the hackers got there was very obsolete info on me from what is practically the dark ages, net wise. Only the odd extortion attempt which was forwarded to the real FBI and a counter-threat to the would-be extortionist that I hoped that a black-ops team would come to knock his door down and kill his n- ass! Peace ever since. All you gotta do is set Thunderbird or Navigator[if anyone still uses it] or any other to view message source code, then you can read where it originally came from and whether anon mail services were used. If the latter, there are ways to find the true origins, but that's for another time.

BTW Yahoo keeps bugging me via the one email addy to change the pw, but already did that ages previous to their reminders.