Do I Need to Harden SSH over Tor?
Do I Need to Harden SSH over Tor?
cross-posted from: https://infosec.pub/post/10908807
TLDR:
If I use SSH as a Tor hidden service and do not share the public hostname of that service, do I need any more hardening?
Full Post:
I am planning to setup a clearnet service on a server where my normal "in bound" management will be over SSH tunneled through Wireguard. I also want "out of bound" management in case the incoming ports I am using get blocked and I cannot access my Wireguard tunnel. This is selfhosted on a home network.
I was thinking that I could have an SSH bastion host as a virtual machine, which will expose SSH as a a hidden service. I would SSH into this VM over Tor and then proxy SSH into the host OS from there. As I would only be using this rarely as a backup connection, I do not care about speed or convenience of connecting to it, only that it is always available and secure. Also, I would treat the public hostname like any other secret, as only I need access to it.
Other than setting up secure configs for SSH and Tor themselves, is it worth doing other hardening like running Wireguard over Tor? I know that extra layers of security can't hurt, but I want this backup connection to be as reliable as possible so I want to avoid unneeded complexity.
If you don't have any good reason not to, always set your SSH server to only authenticate with keys.
Anything else is irrelevant.
If you don't have any good reason not to
Spoiler alert: you don't.
Disabling root login and password auth, using a non-standard port and updating regularly works for me for this exact use case.
using a non-standard port
It's really not that hard to find SSH servers running on a non-standard port.
nmapor a similar port-scanning software package can find ports listening for TCP connections. There are software packages -- don't recall names off-the-cuff, but I'm sure that you could go dig one up -- that connect to ports and then aim to identify the protocol from a fingerprint out of a database that they have. The SSH protocol has a very readily-identifiable fingerprint, don't even need specialized software.Let me just bounce to a machine as an example:
$ telnet tals-host.tals-domain.com 22 Trying tals-IP... Connected to tals-host.tals-domain.com Escape character is '^]'. SSH-2.0-OpenSSH_9.6p1 Debian-4 ^] telnet> q Connection closed. $That being said, I don't disagree with your broader point that I wouldn't personally bother with trying to add more layers on top of ssh, as long as you're keeping current on updates.
I feel like the argument for using a nonstandard ssh port these days is that you dodge the low tier automation/bots that are endlessly scanning IPs and port 22 and trying obvious usernames and passwords. I do also question how much it is worth dodging these since presumably you'd have already done the other basics like key only and no root login before this. Maybe there's some value if you want a clean auth.log or equivalent
I think that SSH, hidden or no, is fine. It's intended to be Internet-facing and secure.
All that being said, we did just infamously have the Jia Tan/xz attack, an attempt to stick a backdoor spanning many Linux distros.
Also, I would treat the public hostname like any other secret, as only I need access to it.
If you're worried about someone malicious having access to your network connection, ssh is going to do a DNS lookup to map the hostname to an IP for the client.
Other than setting up secure configs for SSH and Tor themselves, is it worth doing other hardening like running Wireguard over Tor? I know that extra layers of security can’t hurt, but I want this backup connection to be as reliable as possible so I want to avoid unneeded complexity.
If you're going to be running multiple layers of software, keep in mind that compromising the outer layer -- whether it's a port-knocking system, a VPN like Wireguard, some sort of intrusion detection system that inspects packets prior to them hitting the rest of the system, etc -- can be equivalent to compromising the inner. And attackers have aimed to exploit things like buffer overflows in IDSes before -- this is a real thing. So if, for example, there is a Wireguard exploit, it may permit the same sort of compromise that a compromise of sshd does if gaining control of the Wireguard software is as severe as gaining control of the sshd software. So if you're wanting to add another layer and your goal is that compromise of the first layer still leaves another layer to get through, you may want to structure things such that compromise of the VPN software doesn't compromise anything further.
Thanks for the wise words. However I have some questions:
If you’re worried about someone malicious having access to your network connection, ssh is going to do a DNS lookup to map the hostname to an IP for the client.
Are you sure that this is true for Tor? .onion addresses never resolve to an IP address, even for the end user client. If I was on an untrusted network, both for the client and the server, the attacker could find out that I was using Tor, but not know literally anything more than that.
And attackers have aimed to exploit things like buffer overflows in IDSes before – this is a real thing.
I would expect an IDS to be an order of magnitude larger attack surface than Wireguard, and significantly less tested. Although I could also say that about SSH, and we had the recent backdoor. However, I think it is a lot more likely that a bug will cause a security method to be ineffective than actively turn it in to a method for exfiltration or remote access though. For example, with the recent SSH backdoor, if those servers had protected SSH behind Wireguard then they would have been safe even if SSH was compromised.
There are several things you can and should do to harden your server, many of them can be found here.
Acronyms, initialisms, abbreviations, contractions, and other phrases which expand to something larger, that I've seen in this thread:
Fewer Letters More Letters DNS Domain Name Service/System IP Internet Protocol SSH Secure Shell for remote terminal access TCP Transmission Control Protocol, most often over IP VPN Virtual Private Network VPS Virtual Private Server (opposed to shared hosting)
6 acronyms in this thread; the most compressed thread commented on today has 10 acronyms.
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