ssh and scp on Windows won't take a password from stdin or a variable

The repro, on Windows 11 with the built-in OpenSSH client (`where ssh` points at C:/Windows/System32/OpenSSH/ssh.exe):

1. `echo my-password | ssh user@host "hostname"` — the pipe is ignored. You still get the interactive `user@host's password:` prompt and the command blocks. 2. Piping the string in from PowerShell instead of `echo` — same thing. 3. Put the password in an environment variable and look for a flag to pass it. There isn't one. `ssh` and `scp` have no `--password` option, by design. 4. Reach for `sshpass` and it isn't there. No mainstream Windows package ships it, and its method depends on allocating a PTY and calling `TIOCSCTTY`, which Windows has no equivalent for.

Why this happens: OpenSSH does not read the password from stdin. It opens the controlling terminal directly — `/dev/tty` on Unix, `CONIN$` on Windows — and reads from that. This is deliberate. It exists to stop exactly the scripted password feeding you are attempting. Redirecting or piping stdin changes nothing, because stdin is not where it is looking.

What actually works:

- Key auth. Generate a keypair with `ssh-keygen -t ed25519`, then append the public key to the remote `~/.ssh/authorized_keys` over one password-prompted connection: `type $env:USERPROFILE/.ssh/id_ed25519.pub | ssh user@host "cat >> .ssh/authorized_keys"`. After that every `ssh` and `scp` is non-interactive. This is the right answer for automation.

- If you are stuck with password auth — a box you don't control, a rotating credential — you need a real pseudo-terminal between your script and `ssh`, then write the password when the prompt appears. PuTTY's `plink` accepts `-pw` and works from `cmd` or PowerShell without any of this. Otherwise drive `ssh.exe` under a pty library and match on the `password:` string.

The trap is that stdin redirection looks like it should work and fails silently: no error, just a prompt your script never sees and a process that hangs until killed. If you automate deploys from a Windows box, find this out now rather than in a stuck CI job.