Do not use interactive service if running as admin

Commit 791aea49e6 cherry-picked from
master. Commit message altered to make it more relevant for this version.

Connecting to a named pipe server while running with admin rights is not
secure in some windows versions. Even if interactive service is not installed,
the GUI attempts to connect to the service pipe making it possible to exploit
this. Windows XP is known to be vulnerable. See also
http://nsylvain.blogspot.ca/2008/01/namedpipe-impersonation-attack.html

Signed-off-by: Selva Nair <selva.nair@gmail.com>

misc.c

@@ -190,3 +190,29 @@ ForceForegroundWindow(HWND hWnd)
 
     return ret;
 }
+
+/*
+ * Check user has admin rights
+ * Taken from https://msdn.microsoft.com/en-us/library/windows/desktop/aa376389(v=vs.85).aspx
+ * Returns true if the calling process token has the local Administrators group enabled
+ * in its SID.  Assumes the caller is not impersonating and has access to open its own 
+ * process token.
+ */
+BOOL IsUserAdmin(VOID)
+{
+    BOOL b;
+    SID_IDENTIFIER_AUTHORITY NtAuthority = {SECURITY_NT_AUTHORITY};
+    PSID AdministratorsGroup;
+
+    b = AllocateAndInitializeSid (&NtAuthority, 2, SECURITY_BUILTIN_DOMAIN_RID,
+                                  DOMAIN_ALIAS_RID_ADMINS, 0, 0, 0, 0, 0, 0,
+                                  &AdministratorsGroup);
+    if(b)
+    {
+        if (!CheckTokenMembership(NULL, AdministratorsGroup, &b))
+            b = FALSE;
+        FreeSid(AdministratorsGroup);
+    }
+
+    return(b);
+}

misc.h

@@ -30,4 +30,6 @@ BOOL streq(LPCSTR, LPCSTR);
 BOOL wcsbegins(LPCWSTR, LPCWSTR);
 
 BOOL ForceForegroundWindow(HWND);
+
+BOOL IsUserAdmin(VOID);
 #endif

openvpn.c

@@ -692,10 +692,11 @@ StartOpenVPN(connection_t *c)
         (o.proxy_source != config ? _T("--management-query-proxy ") : _T("")));
 
     /* Try to open the service pipe */
+    if (!IsUserAdmin())
       service = CreateFile(_T("\\\\.\\pipe\\openvpn\\service"),
                 GENERIC_READ | GENERIC_WRITE, 0, NULL, OPEN_EXISTING, 0, NULL);
 
-    if (service != INVALID_HANDLE_VALUE)
+    if (service && service != INVALID_HANDLE_VALUE)
     {
         DWORD size = _tcslen(c->config_dir) + _tcslen(options) + sizeof(c->manage.password) + 3;
         TCHAR startup_info[1024];