openssh: privilege separation no longer supported on Cygwin? SURPRISE!

[Houder]

On 2017-05-29 11:48, Houder wrote:
> On 2017-05-29 10:39, Marco Atzeri wrote:
>> On 29/05/2017 07:23, Houder wrote:
> 
> [snip]
>>> ... because, that is, I think, what I am seeing:
>>> 
>>>  - the userid of child sshd is still 'cyg_server' ...
>>>  - and I get an elevated shell when I login ...
>>> 
>>> Not what I expected ...
>>> 
>>> Gr. Henri
>>> 
>> 
>> Hi Houder,
>> please read the last Announcement
>> 
>> https://sourceware.org/ml/cygwin-announce/2017-03/msg00028.html
> 
> [snip]
>> It seems you misunderstood the communication:
>> - the possibility to NOT use "privilege separation" is deprecated
>> - "privilege separation" will became mandatory
> 
> Hi Marco,
> 
> Sorry for the misunderstanding. Yes, to my knowledge, PS, privilege
> separation, is now mandatory (using a new mechanism under Linux [1]).
> 
> [1] sandboxing?
> 
> Because of PS, I expect to see an UNprivileged sshd process talking
> to the user process (where the ssh command has been executed).
> 
> But above all, I expect an UNelevated shell when I login in ...
> 
> However, what I get after login (after providing my credentials) is
> an ELEVATED shell (yes, Administrators is part of the group set).
> 
> Now I wonder if this happens because I do NOT observe PS.
> 
> Look below, please ... After executing the ssh command, ssh asks for
> my credentials ... in stead of providing my credentials, I execute
> the ps command in a second terminal. To my surprise, the grandchild
> of the listener is executed using "cyg_server" and not "sshd" ...
> 
> Currently, I am looking at:
> 
>     https://cygwin.com/cygwin-ug-net/ntsec.html#ntsec-setuid-overview

... and read it MULTIPLE times! (and tried, well, about anything)

However, I found a clue here:

     https://sourceware.org/ml/cygwin/2011-10/msg00035.html
     (Re: admin privileges when logging in by ssh? -- by Corinna)

The thread starts here:

     https://cygwin.com/ml/cygwin/2011-09/msg00138.html
     (admin privileges when logging in by ssh? -- by Andrew Schulman)

Above Corinna writes:

"In all cases, password auth and passwordless auth, you should get a 
full
admin token.  In case of password auth and in the passwordless methods
2 and 3, the OS returns a restricted token under UAC, but ...

     that token has a _reference_ to the full admin token attached. 
Cygwin
     fetches this token and uses that when switching the user context.
     =================================================================

Oh? Ah!

The account (Henri) from which I executed the ssh command, is - yes, I
forgot to tell - , a privileged account ...

However when I login to that account, it "normally" starts an UNelevated
shell ... Not so if one executes the ssh command ... apparently.

     And that is a bit of a SURPRISE ... to say the least !!!!!

Even more surprising is when I execute the ssh command from an account
that is NOT privileged (using another account, named jvdwater).

  - yes. this time I get an UNelevated shell (using ssh)
  - however, the userid of the grandchild of the sshd listener, is STILL
    cyg_server ... NOT sshd!

As if the "sshd" account is NEVER, NEVER used during the _whole_ process
(that is, there is NO privilege separation, as far as I can tell).

Regards,

Henri

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