[stunnel-users] Public domain [PATCH] support environment variables in config file

Pierre Delaage delaage.pierre at free.fr
Wed Dec 21 10:15:15 CET 2016

Hi Dmitry,
Please see answers below...
Yours sincerely,

Le 20/12/2016 19:21, Dmitry Bakshaev a écrit :
> 2016-09-29 19:52 GMT+04:00 Pierre Delaage <delaage.pierre at free.fr 
> <mailto:delaage.pierre at free.fr>>:
> Hi Pierre
>     Shortly speaking : per user envvars are meaningless for WCE
>     ..phones, as there is no way to switch from a user to another on
>     those devices. At a time WCE was used on "handeld PC", where
>     things could be slightly different.
>     So I recommend that, when your customized stunnel is loading conf
>     file, and after you have "tokenized" the file to get env-vars name,
>     you mimic system calls with a stub that always return the same
>     values RELATIVE to "." folder,
>      ie where the stunnel executable has been installed on the phone.
>     This is the SOLE path that is reliable on a WCE phone.
>     Clearly, your envars will be "path to something" I guess :
>     typically path to the cert file.
>     for this example your cert file on a phone will always be in
>     "./mycert.crt".
> 1. again from beginning:
> different OS has different rules,
> different filename syntax: /home/... /etc/... "C:\Document and 
> Settings", "C:\Users",
> different environment variables: "USER" on *nix, "USERNAME" on windows 
> or none on others like WinCE,
> stunnel has platform specific options: chroot=, setuid=, setgid=, pid= 
> ...
> we not target for "one config file for all users in the world",
> we target "one config file for all users on this system",
> and config may use (not necessary) these platform specific features or 
> not.
> for example WinCE may be only specified directly:
> cert = ./mycert.crt
> on Windows through USERPROFILE:
> cert = %USERPROFILE%\.certs\my.p12
> or directly (in personal config file)
> cert = "c:\Users\john\Desktop\New Folder\my.p12"
> on *nix  through HOME:
> cert = ${HOME}/.certs/my.p12
>     For a win32 platform, there will be either a system wide stunnel
>     running (so who will set up envvars to give to the stunnel env ?
> 2. only the necessary and known variables substituted in the 
> configuration file, other values specified as usual.
There were 2 questions in 1 on my side, and you answered only on the 
conf file part, so I repeat my question :
on a win32 system, in your plans, will there be ONE system-wide stunnel 
running (so that per-user envvar customization is useless),
or do you plan to have "multiple" stunnel running simultaneously ? 
(something quite strange unless you customize every ip and/or ports in 
stunnel conf...but why?)
or ... just one stunnel process running, for THE whole system, for the 
currently logged-in user ? (in that case, if THE user is starting 
stunnel, one conf file installed with stunnel, and remotely managed by 
the admin, can be enough; and if the pb is to get access to a personal 
cert file, a generic "./" path into the conf file is enough).
>     another system script ?
> 3. no. standard way to set envvars from Windows GUI (for example, from 
> Google):
> https://www.nextofwindows.com/how-to-addedit-environment-variables-in-windows-7
> administrator may set "System variables" globally,
> regular user may set only "User variables".
> also from
This is not the only way to set envvars; registry hacks can ALSO do the 
job, particularly from malicious code...
>     with the risk that this script be modified by malicious code ?),
>     or a "one user at a time"- wide stunnel running : it will not be
>     possible to get many processes running stunnel at the same time
>     (just because, whatever envvar you use, all your processes may
>     conflict when listening to "accept" sockets) unless ... you
>     customize "accept" port through envvars ?
>     for me this is quite dangerous : customizing accept or -even more
>     dangerous- connect ports through envars is really easy in user
>     space for any malicious code (to redirect the trafic..).
> 4. the administrator decides which of the configuration parameters can 
> be changed by substituting the values of the environment variables,
> choosing between security and management flexibility:
> from nothing to customize "accept" port through envvars, if it is 
> required.
>     So I still do not see clearly the interest for stunnel to decode
>     envvars on the fly in conf file.
>     If you are looking for conf file automated generation on large
>     sites, I already gave the trick in a 1-line sed command :
>     cat stunnel_template.conf | sed -r -e "s/^(.*)$/C\:\\Progra~2\\GnuWin32\\bin\\echo.EXE \1/e" > stunnel.conf
> 5. if machine joined into domain, administrator adds new user into AD, 
> we need manually update/generate new config files? run it from Group 
> Policy?
> also we need regenarete all config files on 
> changing stunnel_template.conf?
> by substituting variables is not necessarily - only one shared config 
> file, centrally managed by the administrator. 
At a time or another, if you are running in an AD environment, users 
will login and execute some script downloaded from the server. 
Refreshing their conf file at that time is like updating any other 
setting or sw in the system. In any case updating stunnel-conf--template 
will require a new download of the template on client systems.
>     You may argue that this trick can lead to same security breach
>     than explained above ...but ... NOT for system-wide stunnel for
>     which -normally- the stunnel.conf file can NOT be modified. Even
>     for system-wide stunnel, any malicious sw that can tweak the
>     system registry will be able to redirect stunnel trafic... 
> 6. if we need static system-wide stunnel.conf, we simple write it 
> without envvars. exactly the same as it is now.
>     So this is why I do not encourage this patch. 
> 7. I think that the discussion about "hack regisrtry", "malicious sw", 
> etc. should be considered in the context of the overall security of 
> the Windows operating system, not stunnel context.
> "by disign" any multiuser operating system has "user privileges 
> separation" for user files, processes, and other objects, like Windows 
> registry, environment variables &etc.
> regular user is not allowed to modify objects of other users.
> when start user session with other credentials through "sudo" on 
> *nix/"run as" on windows,
> system "reset" environment and build it from scratch, merging default 
> "System variables" with "User variables" of the new user.
> on Windows services start under SYSTEM account ("local system" , 
> "LocalSystem" ...), which has own profile, registry branch, 
> environment variables.
> only administrator may change their.
The main pb is that : 1/ system envvars can be overiden in user context 
: and it is quite straightforward in _user _context to ... modify _user 
_variable overiding system variables... by any software (malicious or 
not) (https://support.microsoft.com/en-us/kb/100843) 2/ As user can 
modify THEIR envvars, how an admin can be sure that stunnel per-user 
conf file is working as expected (by the admin...) 3/ per user conf file 
in a system means that stunnel is NOT running as a system wide process, 
but as a per-user process (probably in foreground) : so that one user 
(or malicious code) can easily stop it, modify conf file or restart 
stunnel with another one. In that case, all the work on the template can 
be circumvented. So, for all that reasons, and considering that your 
goal can be achieved differently (sed and/or generic relative path 
etc...), I am still reserved on the patch. Clearly it allows one to 
modify stunnel conf on the fly, for a sw that most of the time is used 
as a service and should not see its conf modified at all. It is lowering 
the security of stunnel (by lowering, not to say suppressing, its conf 
file integrity) and the system.
>     Apart from that, technically speaking, you can go forward by
>     creating a stub for getenv or expandstring w32 functions. To setup
>     a wce dev environment, see this page :
>     http://delaage.pierre.free.fr The link for evc4 is now broken  :
>     use this instead :
>     http://download.microsoft.com/download/C/3/F/C3F8B58B-9753-4C2E-8B96-2DFE3476A2F7/eVC4.exe
>     <http://download.microsoft.com/download/C/3/F/C3F8B58B-9753-4C2E-8B96-2DFE3476A2F7/eVC4.exe>
>     I will refresh this web page later with more recent stunnel codebase. 
> 8. referring to p.1 we do not need to "emulate" envvars on platform 
> that not has it.
> the last version of the patch has already implemented empty stub for 
> ExpandEnvironmentStrings function on WinCE platform.
> p.s.: some interresting links about envvars:
> http://stackoverflow.com/questions/10090014/can-i-trust-my-environment-variables 
> http://security.stackexchange.com/questions/119962/what-are-some-vulnerabilities-of-environment-variables-on-any-platform 
> http://breakingmalware.com/vulnerabilities/elastic-boundaries-elevating-privileges-by-environment-variables-expansion/ 
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