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Comment by staticassertion | Hacker News Reader

> if an attack can get into the bastion host, they can disable the firewall, so it cannot be used to limit egress.

This assumes that the attacker can get unconstrained root access to the system. It's fine to assume that attackers will but it's not as if you can't make that difficult.

Agree with the rest of what you said though.

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At least in the DoD and IC environments I've worked in that had bastion hosts, the bastion host was severely locked down:

- Shell compiled without built-ins

- No coreutils

- No sudo

- Root account disabled

- Read-only root filesystem

- No user home directories

- Destroyed and rebuilt from template every X hours on some maintenance schedule

Effectively, all you can do is ssh in, ssh out, and forward ports. It might be theoretically possible, but as far as I know, no one has ever compromised one, especially since you can already only get to the bastion from a government VPN anyway, and authentication to that requires a smart card, so there are an awful lot of things you need to compromise to get to that point.

This also answers the suggestion down the page of "why don't you just apply the same controls to every host and not have a bastion." Because the bastion is unusable and you want to actually use your other hosts.

This setup should be easier than it currently is. Any bastion host that’s used for more than jumping is asking for trouble.

From a defense standpoint, one should consider "shell on a box" to usually mean attackers can get root on a box. If they can get persistence, they can wait for a kernel CVE to abuse.

Now, if you're just using a bastion as a jump host, you don't need to offer shells on it. Just allow people to proxy a port to behind the bastion and be done with it.

    PermitTTY no
    ForceCommand /usr/sbin/nologin
    AllowTcpForwarding yes
    AllowAgentForwarding no

I think it's probably reasonable when performing your incident response or even threat modeling to assume the attacker has or could escalate privileges. The linked article doesn't discuss anything that would make that harder, although perhaps practices like staying patched and minimizing attack surface are somewhat assumed (they do bring up choosing your OS based on minimizing attack surface for example).

There's also a lot you can do to harden that boundary. You can harden your kernel, you can execute user's shells in constrained environments like docker containers or restricted shells, leverage sandboxing technologies like apparmor or selinux, etc.

The user/root boundary can be a lot thinner than people expect, so I get why you'd want to point out that reliance on the attacker not escalating should be met with an evaluation of that boundary, but I think it may be understating the boundary to unconditionally not trust a host based firewall, or to say that getting onto the bastion itself is enough to disable the firewall when it does indeed require escalation.

Is finer grained control over TCP forwarding possible? e.g. allow forwarding only to certain TCP ports?

"PermitOpen *:22"

I suggest:

   AllowAgentForwarding no
   AllowTcpForwarding yes
   X11Forwarding no
   PermitTunnel no
   GatewayPorts no
   PermitOpen *:22
   ForceCommand echo 'Nope'

Then:

    ssh -J finaluser@finalhost user@bastion

You can nicely use your local agent, etc. Bastion is relatively hardened. Etc.

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