Replayability means something different in this context. First, we do know the backdoor will pass the payload to system, so in general it is like an attacker has access to bash, presumably as root since it is sshd.

Replayability means, if someone were to catch a payload in action which did use the exploit, you can’t resend the attacker’s data and have it work. It might contain something like a date or other data specific only to the context it came from. This makes a recorded attack less helpful for developing a test… since you can’t replay it.

> It might contain something like a date or other data specific only to the context it came from.

In all these modern protocols, including SSHv2 / SecSH (Sean Connery fans at the IETF evidently) both parties deliberately introduce random elements into a signed conversation as a liveness check - precisely to prevent replaying previous communications.

TLS 1.3's zero round-trip (ORT) mode cannot do this, which is why it basically says you'd better be damn sure you've figured out exactly why it's safe to use this, including every weird replay scenario and why it's technically sound in your design or else you must not enable it. We may yet regret the whole thing and just tell everybody to refuse it.

What could be done, I think, is patch the exploit into logging the payload (and perhaps some network state?) instead of executing it to be able to analyse it. Analyse it, in the unlikely case that the owner of the key would still try their luck using it after discovery, on a patched system.

What it does: it's full RCE, remote code execution, it does whatever the attacker decides to upload. No mystery there.

> see what it does

it does whatever the decrypted/signed payload tells the backdoor to execute - it's sent along with the key.

The backdoor is just that - a backdoor to let in that payload (which will have come from the attacker in the future when they're ready to use this backdoor).