Does a known security issue allow ssh login via system accounts?

[Alexander Wuerstlein]

On 2022-02-28T18:55, Whit Blauvelt <whit at transpect.com> wrote:
> Hi,
> 
> If this is not the right place to ask this, please redirect me. Hopefully it
> is a known vulnerability, due to out of date software. We had a server
> running OpenSSH_8.6p1 compiled on Ubuntu 16.04.7 which was compromised last
> week. The intruder managed to achieve this:
> 
>   Feb 24 08:13:52 localhost sshd[32276]: Accepted password for backup from 5.161.47.185 port 37962 ssh2
> 
> This despite that /etc/passwd has:
> 
>   backup:x:34:34:backup:/var/backups:/usr/sbin/nologin
> 
> And /etc/shadow has:
> 
>   backup:*:16359:0:99999:7:::
> 
> Either the /usr/bin/nologin or the "*" in the second field of /etc/shadow
> should have been enough to prevent "Accepted password for backup." The
> /usr/sbin/nologin is the standard version for that Ubuntu generation, byte
> identical.

There are a few more mechanisms that may allow password logins despite
the measures you named: When using PAM authentication (which I would
assume, since you are talking about Ubuntu) all the configuration in
/etc/pam.d regarding ssh logins, so /etc/pam.d/ssh and
/etc/pam.d/common* will be in effect. If e.g. some module there
specifies a different password and shell for backup, e.g. from LDAP,
NIS, ActiveDirectory or different passwd/shadow files, those might
apply. You said below that pam.d is "standard", but installed packages
might modify configuration there, so I'd suggest having another look
there.

Also, depending on your configuration, /usr/sbin/nologin may not be
sufficient to prevent port-forwarding without an open shell, thus
enabling a spammer to forward some SMTP port for sending spam without
authentication by "looking like" coming from an internal server.  This
is also hard to see and distinguish from an attacker having a shell in
the logs.

> Adding this to sshd_config was effective:
> 
>   DenyUsers backup
> 
> If that's still not the default for system-level users like "backup", would
> adding it be a reasonble feature request? Or is that on the distros to
> define their default sshd_config settings?

I'd consider it good practice to selectively 'AllowGroup ssh-login' or
something to that effect for any internet-exposed system. But common
distros are far more open than that.  I don't think OpenSSH is the right
place for a policy change, since as mentioned before, there is a lot
going on in PAM that will affect what OpenSSH does here, without being
part of OpenSSH.


Ciao,

Alexander Würstlein.