Hi Alexander, Much appreciate the quick response and advice. May I ask for just a bit more? It would be useful to find out more precisely how the intrusion worked to know where its method should be appropriately reported. > 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. Checking the files in /etc/pam.d/ and /lib/x86_64-linux-gnu/security/, they exactly match the same sets of files on another Ubuntu system of the same vintage with no public exposure, both in files there and each file's particulars. There is no LDAP, NIS or AD use by the system. The string "backup" is not in any file in /etc/pam.d/; the two places it obviously shows up in /lib/x86_64-linux-gnu/security/*.so files are in message strings, not references to the userid. > 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. Likely right about port forwarding. When I caught the intrusion in action "backup" did have an open ssh connection; unsure if "backup" had a shell. Postfix was seeing the email origin as local. > > 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. Is there a valid use case where standard systems users (sync, backup, etc.) should have ssh access? If not, blocking by default should be prudent. On the other hand, if there are valid, standard uses for ssh for such users, what are normal ways to enable them? I should see if any of them were used here. Highjacking a server for spam is a pedestrian goal; this evidences something the hands of the script kiddies which requires better defense. Neither OSSEC (Wazuh) on the system, nor regular scans with Nessus and OpenVAS, gave warning. It would be good to find a signature for it; a start would be adding a watch in OSSEC for system-level userids showing up in auth.log, but there's some additional trick enabling those to bypass standard security. Thanks again, Whit _______________________________________________ openssh-unix-dev mailing list openssh-unix-dev@xxxxxxxxxxx https://lists.mindrot.org/mailman/listinfo/openssh-unix-dev
