by dspillett | 2025-10-28 16:53:33+00:00
> I struggle to see a scenario in which a bad actor is able to give me the address of a bad machine, yet not be able to trick me into their host key being the correct one.

If you aren't bothering to verify then they do not need to trick you at all.

In DayJob we have a lot of clients send feeds and collect automated exports via SFTP, and a few to whom we operate the other way (us pulling data via SFTP or pushing it to their endpoint). HTTPS based APIs are very common and becoming more so, but SFTP is still big in this area (we offer some HTTPS APIs, few use them instead of SFTP).

One possible exploit route, for a malicious actor playing a long and targetted game, that could affect us:

1. Attacker somehow poisons our DNS, or that of a specific prospective client of ours, sending traffic for sftp.ourname.tld to their server, and has access to our mail.

2. Initially they just forward traffic so key verification works for existing users. They monitor for some time to record host addresses that already access the server, so when they start intervening they can keep just forwarding connections from those addresses, so those users see no warnings (and are unaffected by the hack).

3. When they do start to intercept connections from hosts not already on the list make above instead of forwarding everything, existing users are unaffected¹ but new users coming in from entirely different addresses now go to their server and if they are not verifying the key will happily send information through it², authenticating with the initial user+pass we sent or PKI using the public key they sent, with the malicious server connecting through to ours to complete the transfers.

4. Now wait and collect data as no one realises there is a MitM, and later use any PII or other valuable information for ransom/extortion purposes.

Of course there are ways to mitigate this attack route. For one: source address whitelisting, supported by OpenSSH's key based auth as the acceptable source list can be included with the public key so only specific sources can use that key for auth. But they client would have to make effort to do this, and if they aren't going to make the effort to verify the host key then they aren't going to make other efforts either.

We do have some clients who verify the host properly and/or give us source addresses to limit connections to when they provide a public key, we work with financial institutions who are appropriately paranoid about their data and the data of their customers, some even use PGP for data in transit (and in case it is ever stored where it shouldn't be) for an extra level of paranoia. But most do none of this. Most utterly ignore our strong suggestion that they use keys, or change passwords in case of email breach, instead using the password we mail them before first connection for eternity.

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[1] none of our clients are likely to be sending files from dynamic source addresses, at most the source might move around a v4/24 or v6/64, currently I don't think all of them connect from a single IPv4 address, I've had one recently let us know (months in advance) that their source address will be changing.

[2] it can connect to us and send the data