The design of telnet and ssh where you have a daemon running as root is bad security that as shown here is a liability, a ticking time bomb ready to give attackers root.
The design of telnet and ssh where you have a daemon running as root is bad security that as shown here is a liability, a ticking time bomb ready to give attackers root.
Oldschool telnetd didn’t actually run as root; rather, it just set up a PTY for the incoming socket to talk to, and then fork-exec’ed a /bin/login subprocess to live inside that pty. /bin/login is setuid-root, so it’s “where the security lived.”
I think we all collectively decided that that was a bad idea at some point — probably because /bin/login was never designed under the assumption that it would have to deal with arbitrary binary network traffic being thrown at it (it really only expects keyboard input.) So we switched to doing auth directly in our network daemons, since at least then “people who are aware the code is network-facing” would be maintaining it.
Literally how else is a remote login daemon supposed to work though?
1. Start with root to bind the port below 1024.
2. give up root because you don't need it any further.
3. Only accept non-root logins
4. when a user creates a session, if they need root within the session they can obtain it via sudo or su.
That still needs a way to change users, and OpenSSH already has privilege separation. That hardens the process somewhat to reduce the amount of code running in the process which can change the uid for a session but fundamentally something needs permission to call setuid() or the equivalent.
Yes, but changing users is a function of the shell (or maybe more specifically /usr/bin/login), not the SSH daemon.
Congratulations, you've created a server that lets people have shells running as the user running telnetd.
You presumably want them to run as any (non root) user. The capability you need for that, to impersonate arbitrary (non-root) users on the system, is pretty damn close to being root.
I'm not sure that you need root because of the port - I think login itself needs to run as root, otherwise it cant login to anything other than the account its running under.
You still need to have privileges to become the userid of the user logging in. Openssh does do privsep, but you still need a privileged daemon.
The remote daemon has its own account and is given a privilege that allows it to connect a network socket to a pseudo terminal.
Those are already unprivileged operations, but how does it start the initial process in that terminal with the correct privileges for a different user?
The kernel could authenticate the user before starting it.
How does it do that?
There are many ways from passkeys to SAML. Though for complex methods we may want a dedicated services outside the kernel.
Any breach of the daemon will still give access to a system that can approve/deny user logins. Breaching the daemon therefore allows permission escalation, because you can simply jump to an account. Chain with any local vuln of your choice to completely own the box.
It doesn't matter what user it is running as.
If this was so easy to deal with, someone would have done it. Instead, we get endless HN comments about people that act like they can do better but never submit a PR.
Breaching the daemon only allows for the attacker to get access to the login. User accounts should still be secured requiring authentication.
>If this was so easy to deal with, someone would have done it.
Sadly this is not the case. There is a lot of inertia towards solutions like ssh or sudo. It may be easy to delete them, but actually getting such a changed accepted is no trivial task.
> Breaching the daemon only allows for the attacker to get access to the login
Yes, but potentially any login. See the problem? If you compromise the gatekeeper, you are now the keymaster. Or whatever :)
I'll admit it is still problematic. But at least there is only 1 gatekeeper instead of 2.
How is that better?
OpenSSH has been moving quite quickly in the direction of multiple, privilege separated processes, each also heavily sandboxed with pledge and unveil
What do you think proper architecture would be, given that ssh needs a capability to let root logins?
I suppose it could be via a proper PAM module, which is widely supported.
Too bad the first PAM RFC was published about the same time the first be version of ssh was released.
> ssh needs a capability to let root logins
One can disable root login via SSH in /etc/ssh/sshd_config. sshd also drops root priviledges once it's running IIRC.
I use use sudo or doas as a regular user once logged in.
I think a proper architecture would not even have a root account. The server would just expose an authenticated endpoint that allows for configuration and updates to be pushed for it.
You are thinking 20 years ahead. In 1995 most servers were still pets, not cattle.
Does ssh need to allow root logins?
Sshing as a regular user and then sudo to root works 95% of the time…
How does SSH become an arbitrary user without effective root?