Waste and overflows can be nasty. That’s super cheap thin plastic, it should be done in abs. The threaded connection there may be loose can you get your hand on it and tighten it? Is there an access panel directly behind it?

The threaded connection didn't seem loose. No, there is no access panel.

Not surprised if inappropriate material was used, house was "rehabbed" by an amateur flipper before I bought it six and a half years ago.

Yea, I’m a plumber. I see all kinds of garbage work done by people flipping houses it’s ridiculous. Can you sneak your phone in there and point it down and see if it drains into black abs piping? Honestly I would replace that white shit with a new OS&B waste and overflow kit. It might be worthwhile having a plumber come and do it. There are some little tricks to it etc. and you’ll need to buy glue and a short length of abs and plumbers putty if you do it yourself. You don’t need to use silicone anywhere on the waste and overflow. Some people get silicone crazy lol. Plumbers putty on the chrome waste up top and there is a rubber gasket that goes underneath. It’s a shitty job and you’ll need access, as would the plumber who comes to do it. Often times there’s an access panel behind the waste and overflow, if there isn’t, it’s not a bad idea to cut one in. Lots of connections there that can leak, even after it’s fixed properly. Good to have access

I had this same thing happen to me. In my case it was old cast iron pipes that had rusted through.

I would definitely suggest getting a plumber to check it out. You may also want to reach out to your insurance company. They wouldn't cover the cost of fixing the pipes, but they did pay to fix the damage to the ceiling below the tub.

The cast iron is all below this point.

Unfortunately the one side of that is the top of the stairwell, the other side is the brick exterior wall, no room for an access panel.

I added some additional photos to the imgur gallery.

I also doublechecked the connection, the plastic nut definitely seems tight. Everything above that connection is dry. If there were a backup, could it force its way out even if that plastic nut is tight?

As a contractor once quoted to me, "Aunt Bea, call the man."

A backup can definitely force something to leak when it normally wouldn’t. But that would probably show itself in the tub with a slow drain. Do you have access from below?

We had a similar problem in our multiple flats building. Our bathrooms are aligned with the above unit that was leaking. But anyways, you need to open up the ceiling where it is leaking down from and hopefully it is right there above as opposed to water traveling across the room since maybe not level. If you are not much of a plumber then have a pro do it either after you open it up or have him do it all. I suppose you can do the opening and drywalling and painting if you want to save some money.

The drain doesn't appear completely drained at this time. Not sure if it was draining slowly when this issue made itself obvious. I suppose it doesn't hurt to check at this point.

I don't have access from below. There's a drywall ceiling in the downstairs bathroom, then the subfloor. One of the cast iron pipes nearby has a point that is terminated with a cap that can be unscrewed, but I'm doubtful I could apply enough leverage in the limited space to open it.

I went back and ran water. It looks like it is in fact a cast iron supply line. FML.

Well shit. I ran some water. Looks like the leak is the cast iron supply line in the wall, not the plastic. FML.

How do you mean supply line? Do you mean the cast iron drainage?

No I mean the line running from source to the faucet or shower head. The leak is actually above and behind where I thought originally.

Ah I see! If you have any issues with that and some pics I can try to help. Sounds like they are galvanized metal water lines? Could be a much more difficult issue I’m afraid to say! Unless it is the line running to the shower head, it’s probably pex. But you’ll need access to get at it. Check the shower head arm that comes out of the tub at the top, I’ve seen those loose and leaking, or no Teflon tape lol. You’ll need to possibly unthread it from where it goes into the wall. Just thinking of things I normally check, starting small and working your way up.

The line does run to both the tub faucet and the shower head. When I bought the place flow was constricted, had to use a low flow head. I'm figuring at this point this means having someone rip out the wall, and I probably might as well redo the entire (small but only full) bathroom the way it should have been done in the first place to address various other concerns. I had hoped to avoid this until I could buy a second house and move first, but then rates jumped.

Thanks for all your input!

No worries! I’m more than happy to help folks who have questions with plumbing stuff! Plumbers are expensive lol