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

[Alexander Wuerstlein]

On 2022-02-28T22:16, Whit Blauvelt <whit at transpect.com> wrote:
> 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.

The string 'backup' wouldn't have to necessarily appear in the config.
Also, the .so files should be irrelevant, those are binaries, not config.

To check, I'd look at: PAM-Config (/etc/pam.d/*). That that config is
the same as another machine doesn't mean that there isn't a problem in
there. It just might mean that an attacker didn't introduce the problem
(or the problem was introduced in all machines). Then I'd look at
/etc/nsswitch.conf to check for other passwd/group/shadow mechanisms
than 'file'. Then I'd look at the relevant configs for those mechanisms.
Then I'd compare "getent passwd" to /etc/passwd, same for group and
shadow. There might be sssd in use, check config and cache.  And check
/etc/shells, this should only contain proper shells, not anything like
/usr/bin/nologin, /bin/false, ...

The unfortunate problem is that Linux AAA has become a messy stack of
things that might cause problems. In the extreme case Authentication
might e.g. call into:
SSH -> /etc/pam.d/ssh -> /etc/pam.d/common* -> pam_unix.so ->
/etc/nsswitch.conf -> sssd (maybe cached) -> LDAP -> Kerberos

And there can be side-branches, e.g. SSH will also check for SSH-Keys,
PAM may try multiple modules one after another and can have
conditionals, same for nsswitch (but not as complex), same for sssd (but
even more complex).

In the sshd_config, there might also be Match-blocks or other
configuration that makes certain users be treated specially.

It is hard to say what your exact problem is withouth having seen all of
the above. There are just too many things that can go wrong and a lot of
"traditional nuances" in the configs one has to watch out for.

> > > 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.

I don't see any common use cases for system users to be accessible via
SSH directly. Usually one uses /bin/false as a shell to disable login
for those users. /usr/bin/nologin does the same, but with a message.
This is what most distros do.  I've seen problems with a homegrown
version of /usr/bin/nologin that displayed a multipage message with
'press any key to continue' and timeouts, which caused the exact
portforwarding problem you might have.

Another approach would be AllowGroups in the sshd_config, or appropriate
PAM configuration (there are tons of possibilities here).


Ciao,

Alexander Würstlein.