Remove the support for using system() when executing external programs or scripts

This patch removes the support for the system() call, and enforces the
usage of execve() on the *nix platform and CreateProcessW() on Windows.
This is to enhance the overall security when calling external scripts.
Using system() is prone to shell expansions, which may lead to security
breaches.  Which is also why the execve() approach has been the default
since commit a82813527551f0e79c6d6ed5a9c1162e3c171bcf which
re-introduced the system() in Nov. 2008.

After having asked on the mailing list and checked around on the IRC
channels, the genereal consensus is that very few uses system() these
days.

The only annoyance I've been made aware of is that this will now
require adding a full path to the script interpreter together with the
script, and not just put in the script name alone.  But to just use the
script name in Windows, you had to configure --script-security with the
'system' flag earlier too.  So my conclusion is that it's better to add
a full path to the script interpreter in Windows and raise the overal
security with OpenVPN, than to continue to have a possible potentially
risky OpenVPN configuration just to make life "easier" for Windows
script users.

Removal of the system() call, also solves a nasty bug related to the
usage of putenv() on the *nix platforms.

For more information please see:
http://thread.gmane.org/gmane.network.openvpn.devel/7090
https://community.openvpn.net/openvpn/ticket/228

Trac-ticket: 228
Signed-off-by: David Sommerseth <davids@redhat.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1351539352-17371-1-git-send-email-dazo@users.sourceforge.net>
URL: http://article.gmane.org/gmane.network.openvpn.devel/7114
(cherry picked from commit 0563473601abfbf2142bfa0ca5b863c5aa7953a2)

1 files changed, 32 insertions, 16 deletions

diff --git a/doc/openvpn.8 b/doc/openvpn.8
index aa653ec..2ed5201 100644
--- a/doc/openvpn.8
+++ b/doc/openvpn.8
@@ -1886,7 +1886,7 @@ is a safety precaution to prevent a LD_PRELOAD style attack
 from a malicious or compromised server.
 .\"*********************************************************
 .TP
-.B \-\-script-security level [method]
+.B \-\-script-security level
 This directive offers policy-level control over OpenVPN's usage of external programs
 and scripts.  Lower
 .B level
@@ -1905,24 +1905,40 @@ Allow calling of built-in executables and user-defined scripts.
 .B 3 \-\-
 Allow passwords to be passed to scripts via environmental variables (potentially unsafe).
 
-The
+OpenVPN releases before v2.3 also supported a
 .B method
-parameter indicates how OpenVPN should call external commands and scripts.
-Settings for
-.B method:
+flag which indicated how OpenVPN should call external commands and scripts.  This
+could be either
+.B execve
+or 
+.B system. 
+As of OpenVPN v2.3, this flag is no longer accepted.  In most *nix environments the execve()
+approach has been used without any issues.
+
+To run scripts in Windows in earlier OpenVPN
+versions you needed to either add a full path to the script interpreter which can parse the
+script or use the
+.B system
+flag to run these scripts.  As of OpenVPN v2.3 it is now a strict requirement to have
+full path to the script interpreter when running non-executables files.
+This is not needed for executable files, such as .exe, .com, .bat or .cmd files.  For
+example, if you have a Visual Basic script, you must use this syntax now:
 
-.B execve \-\-
-(default) Use execve() function on Unix family OSes and CreateProcess() on Windows.
-.br
-.B system \-\-
-Use system() function (deprecated and less safe since the external program command
-line is subject to shell expansion).
+.nf
+.ft 3
+.in +4
+\-\-up 'C:\\\\Windows\\\\System32\\\\wscript.exe C:\\\\Program\\ Files\\\\OpenVPN\\\\config\\\\my-up-script.vbs'
+.in -4
+.ft
+.fi
 
-The
-.B \-\-script-security
-option was introduced in OpenVPN 2.1_rc9.  For configuration file compatibility
-with previous OpenVPN versions, use:
-.B \-\-script-security 3 system
+Please note the single quote marks and the escaping of the backslashes (\\) and
+the space character.
+
+The reason the support for the
+.B system
+flag was removed is due to the security implications with shell expansions
+when executing scripts via the system() call.
 .\"*********************************************************
 .TP
 .B \-\-disable-occ