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Stop MitM on the first SSH connection, on any VPS or cloud provider

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Re: Stop MitM on the first SSH connection, on any VPS or cloud provider

#21

Post by gruez »

ekr____ wrote: post #20
gruez wrote: post #17

Earlier quoted context omitted.

I was about to downvote this for being obviously false, but after some research this does appear to be true, because ssh uses some channel binding mechanism to prevent your public key authentication from being replayed/reused by the "man" in the middle.

This is one of those situations where it's necessary to be very precise about the security properties. Specifically, if you bind authentication to the connection, then an attacker who impersonates the server (in this case because it's the first connection, but in other settings because they have a fake certificate), then client authentication is not portable to another connection, so the attacker can't mount a classi…

>* If you use SSH to copy a secret such as an API key to the server, then the attacker still knows the API key.

That's much harder to pull off though, because you need to replicate the environment close enough so that the victim doesn't suspect anything. Do they put their config files in /var/lib or random docker volumes? Do they use docker compose or docker-compose, etc.

Re: Stop MitM on the first SSH connection, on any VPS or cloud provider

#22

Post by ekr____ »

gruez wrote: post #21
ekr____ wrote: post #20

Earlier quoted context omitted.

This is one of those situations where it's necessary to be very precise about the security properties. Specifically, if you bind authentication to the connection, then an attacker who impersonates the server (in this case because it's the first connection, but in other settings because they have a fake certificate), then client authentication is not portable to another connection, so the attacker can't mount a classi…

>* If you use SSH to copy a secret such as an API key to the server, then the attacker still knows the API key. That's much harder to pull off though, because you need to replicate the environment close enough so that the victim doesn't suspect anything. Do they put their config files in /var/lib or random docker volumes? Do they use docker compose or docker-compose, etc.

Sure. I'm not saying it's not better to use public key authentication (it is!). Just that it's still possible to have problems.

Re: Stop MitM on the first SSH connection, on any VPS or cloud provider

#23

Post by daveguy »

londons_explore wrote: post #2

A big class of attacker is nation state attackers who do not want to risk discovery. A big way to deter them is to keep remote log files which, if analyzed, will reveal any attack. For example, if both ssh-client and ssh-server kept a fingerprint of the session key in some append-only logfile, then a later administrator could compare the logfiles to know if an MITM happened. Suddenly, nation state attackers won't be…

Because log processing is handled in the kernel/root/system? Is this a trick question?

See also: rsyslogd

Re: Stop MitM on the first SSH connection, on any VPS or cloud provider

#25

Post by athrowaway3z »

So this is a tangent on a thought i had after reading the title, but it might be a cool idea that I'll not have the time to do anything with so feel free to use it:

Human checkable fingerprints for pubkeys/hashes dont really work. None of the schemes i've seen hold up under somebody willing to spend compute to get a near-enough collision to fool most people most of the time.

But we can take those random bits and transform them and feed them into a seeded image generation LLM, and then have a person remember/compare the deterministic output image.

You might even make the case its the perfect machine to create memorable-2-human image artifact from random data.

Re: Stop MitM on the first SSH connection, on any VPS or cloud provider

#26

Post by washingupliquid »

I'm supposed to believe MitM with the same exact keypair is somehow possible? Private keys are never exchanged. Did everybody forget how crypto works?

Yes you implicitly trust the public key on first login.... then just... immediately compare it with what's on your box?

Might as well seal your doors with duct tape to prevent ghosts from entering your home because this is equally effective.

Re: Stop MitM on the first SSH connection, on any VPS or cloud provider

#27

Post by anygivnthursday »

flumpcakes wrote: post #8
INTPenis wrote: post #6

The author essentially bootstraps their servers with a known trusted host key, so that first connection is recognized, instead of having to trust a new and recently generated host key when you first connect. It's a neat little trick if you're often deploying VPS in shared cloud environments.

This is something I have struggled with: How to deploy secrets during bootstrap to a new virtual machine running in the Cloud that does not leave a trace in the infrastructure. And in a way that I can completely automate the deployment. One answer is providing the secrets in cloudinit - but this leaves a trail on the host/provider's infrastructure, I do not know if those configs I paste into the portal then get saved…

You can bootstrap from your custom ISO with some embedded starter key, upload ISO, loads into RAM and opens SSH, connect and run a playbook to encrypt the root drive where you deploy your OS with your SSH key. If you went with encrypted root, you might need to enter pass via console, or dropbear server in a pre-boot env you connect to via SSH to enter the key if you dont trust the console, or setup some custom network unlock mechanism, etc. But once unlocked your provider can still dump your keys from memory. There are also things like AMD SEV-SNP for some more confidential use cases.

Re: Stop MitM on the first SSH connection, on any VPS or cloud provider

#28

Post by cesarb »

hnlmorg wrote: post #4
londons_explore wrote: post #2

A big class of attacker is nation state attackers who do not want to risk discovery. A big way to deter them is to keep remote log files which, if analyzed, will reveal any attack. For example, if both ssh-client and ssh-server kept a fingerprint of the session key in some append-only logfile, then a later administrator could compare the logfiles to know if an MITM happened. Suddenly, nation state attackers won't be…

Couldn’t the MITM ssh server just forward the client’s fingerprint to the legitimate server? If so, the legitimate server wouldn’t have anything in their logs that would help detect such an attack. OpenSSH does log other telemetry though.

> Couldn’t the MITM ssh server just forward the client’s fingerprint to the legitimate server?

The client sends not only the public key, but also a signature, and that signature depends on the output from the key exchange, so it's "bound" to the shared keys negotiated between the client and the server. If the MITM server does separate key exchanges with the client (pretending to be the real server) and the server (pretending to be the real client), the signature won't match; if it forwards the key exchange between the real client and the real server, it won't be able to decrypt the packets.

That's the best thing about SSH public key authentication (and HTTPS client certificates): even when MITM can impersonate the server to the client (because the client didn't verify the host key), it can't impersonate the client to the real server.

Re: Stop MitM on the first SSH connection, on any VPS or cloud provider

#29

Post by cesarb »

projektfu wrote: post #19
zuzululu wrote: post #11

but how real is this threat? you buy a box from hetzner or some VPS provider and its rooted when you try to ssh ?

You get a box and someone is sitting on its IP address (in the middle), proxying to the real one, so everything is getting logged. Other comments say that this mitm stops working when you use public key authentication.

> Other comments say that this mitm stops working when you use public key authentication.

It doesn't completely stop working; a MITM can still pretend to be the server, it just can't authenticate to the real server on your behalf. You could be doing all your work in a fake server controlled by the attacker, while the real server sits there untouched.

Re: Stop MitM on the first SSH connection, on any VPS or cloud provider

#30

Post by dspillett »

asalahli wrote: post #9

It baffles me why VPS providers don't display the fingerprints on their dashboards or expose them through the API.

A few do. Though a more common way of securing the loop is to take the desired user public key during the build process.

Assuming you trust that the host control panel (or API server) hasn't been hacked, which you are assuming anyway if you trust a host fingerprint given to you that way, that should be as secure. For a small bit of extra assurance, to protect from an extra very unlikely attack, generate a new key pair just for this VM's creation so that you know you aren't connecting to some other VM that happens to have a known public key of yours and you've been redirected to by DNS poisoning.

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