Okay let me elaborate how I envision that attack to work:

1. attacker wants to use your yubikey-backed ssh key, let's say for running ssh-copy-id once with their own key so they can gain access to your server

2. thus they need to trick you into touching the key when they run that command

3. the best way to trick you is to wait until you do something where you'd normally need to touch that key yourself

4. so they alias ssh to a script that detects when you're trying to connect to this server yourself, and invoke ssh-copy-id instead, which prompts you to touch the yubikey and you do

5. spit out a reasonable looking error (something that makes you think "bloody DNS, it's always DNS, innit" or something silly like that); then they undo the alias so you succeed on the next try and suspect nothing

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palata 16 hours ago | parent [-]

That's a valid attack, but one thing is that they only get access this one time, and you may realise that something wrong happened (maybe not).

But they won't get your private key.

	▲	Liskni_si 16 hours ago \| parent [-]
		> But they won't get your private key. Indeed, that was my point exactly a couple posts up the thread. :-) > you may realise that something wrong happened I think I can iterate on the exact mechanics to make this less likely. I mean it's getting off-topic but the one thing that comes to mind is to enable ControlMaster for all ssh connections which allows any second ssh invocation to skip the auth and just re-use the existing connection. ssh-copy-id is near instant then and doesn't ask anything. At that point you might—rightly so—argue that they're no longer tricking the user into authorising a different operation. Just a reminder that if someone can run code as your local user, they can easily and sneakily gain access elsewhere. Even if you need a yubikey touch to connect there. The original attack idea of timing the yubikey touch for when you normally expect to touch it might still be relevant for a scenario like ssh-agent forwarding to a malicious box. They can't run code as your local user, but can still perhaps trigger the agent to interact with the yubikey. Maybe.