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Subject: Re: openssh: privilege separation no longer supported on Cygwin?
To: cygwin AT cygwin DOT com
References: <d436698bbd53eef3cbdda788d4926109 AT xs4all DOT nl> <37b863f6-ce5c-ef13-569f-8044fe485075 AT gmail DOT com> <20e2702ca3837f5d54c558f8e786c717 AT xs4all DOT nl>
From: Marco Atzeri <marco DOT atzeri AT gmail DOT com>
Message-ID: <aa5d0288-9842-fd07-ca9e-619324d00914@gmail.com>
Date: Wed, 31 May 2017 21:52:06 +0200
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On 29/05/2017 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).

Is your user a member of Administrators ?

>
> 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
>
> Regards,
> Henri
>

on my system as reported by lusrmgr.msc
cyg_server is a privileged user member of Administrators
sshd is a normal user as expected reading ssh-host-config.

The cyg_server account can setuid to other users
otherwise you can not change user id:

$ pstree -u
?─┬─cygrunsrv(cyg_server)───sshd───sshd───bash(marco)───pstree
   ├─mintty(marco)───bash───ssh
   └─mintty(marco)───bash

Regards
Marco


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