Add $WebStrictBrowserCache option to disable browser cache

RT systems that store sensitive data may want to disable all
browser cache and back button behavior. This option enables
that and moves these headers to a separate Mason template
for easy override.

See: https://owasp.org/www-project-web-security-testing-guide/v42/4-Web_Application_Security_Testing/04-Authentication_Testing/06-Testing_for_Browser_Cache_Weaknesses

etc/RT_Config.pm.in

@@ -1573,6 +1573,20 @@ off I<unless> user connections to RT are secured by some other method.
 
 Set($WebSecureCookies, 1);
 
+=item C<$WebStrictBrowserCache>
+
+As part of normal operation, browsers typically store some browsing
+history, enabling the Back button to work. Browsers also often
+cache pages in the browsing history to improve performance.
+
+Enable this option if you are using RT with highly sensitive
+information and want to signal the browser to not store any history
+or cache any data. The default is disabled.
+
+=cut
+
+Set($WebStrictBrowserCache, 0);
+
 =item C<$WebHttpOnlyCookies>
 
 Default RT's session cookie to not being directly accessible to

lib/RT/Config.pm

@@ -1889,6 +1889,9 @@ our %META;
     WebSecureCookies => {
         Widget => '/Widgets/Form/Boolean',
     },
+    WebStrictBrowserCache => {
+        Widget => '/Widgets/Form/Boolean',
+    },
     WikiImplicitLinks => {
         Widget => '/Widgets/Form/Boolean',
     },

share/html/Elements/Header

@@ -123,8 +123,7 @@ $lang = $session{'CurrentUser'}->LanguageHandle->language_tag
      && $session{'CurrentUser'}->LanguageHandle
      && $session{'CurrentUser'}->LanguageHandle->language_tag;
 
-$r->headers_out->{'Pragma'} = 'no-cache';
-$r->headers_out->{'Cache-control'} = 'no-cache';
+$m->comp('/Elements/HttpResponseHeaders');
 
 my $id = $m->request_comp->path;
 $id =~ s|^/||g;

share/html/Elements/HttpResponseHeaders

@@ -0,0 +1,99 @@
+%# BEGIN BPS TAGGED BLOCK {{{
+%#
+%# COPYRIGHT:
+%#
+%# This software is Copyright (c) 1996-2023 Best Practical Solutions, LLC
+%#                                          <sales@bestpractical.com>
+%#
+%# (Except where explicitly superseded by other copyright notices)
+%#
+%#
+%# LICENSE:
+%#
+%# This work is made available to you under the terms of Version 2 of
+%# the GNU General Public License. A copy of that license should have
+%# been provided with this software, but in any event can be snarfed
+%# from www.gnu.org.
+%#
+%# This work is distributed in the hope that it will be useful, but
+%# WITHOUT ANY WARRANTY; without even the implied warranty of
+%# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+%# General Public License for more details.
+%#
+%# You should have received a copy of the GNU General Public License
+%# along with this program; if not, write to the Free Software
+%# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+%# 02110-1301 or visit their web page on the internet at
+%# http://www.gnu.org/licenses/old-licenses/gpl-2.0.html.
+%#
+%#
+%# CONTRIBUTION SUBMISSION POLICY:
+%#
+%# (The following paragraph is not intended to limit the rights granted
+%# to you to modify and distribute this software under the terms of
+%# the GNU General Public License and is only of importance to you if
+%# you choose to contribute your changes and enhancements to the
+%# community by submitting them to Best Practical Solutions, LLC.)
+%#
+%# By intentionally submitting any modifications, corrections or
+%# derivatives to this work, or any other work intended for use with
+%# Request Tracker, to Best Practical Solutions, LLC, you confirm that
+%# you are the copyright holder for those contributions and you grant
+%# Best Practical Solutions,  LLC a nonexclusive, worldwide, irrevocable,
+%# royalty-free, perpetual, license to use, copy, create derivative
+%# works based on those contributions, and sublicense and distribute
+%# those contributions and any derivatives thereof.
+%#
+%# END BPS TAGGED BLOCK }}}
+<%INIT>
+
+# Since data in the DB can change at any time, the default headers
+# for dynamic content (content generated from most Mason templates) is:
+#
+# Cache-control: no-cache
+# Pragma: no-cache
+# Expires: [a short time in the past to account for any time drift]
+
+# Pragma is deprecated and usually ignored if Cache-control is sent.
+# Should only be used by HTTP/1.0 clients.
+$r->headers_out->{'Pragma'} = 'no-cache';
+
+my $cache_control = 'no-cache';
+
+my $expires = RT::Date->new(RT->SystemUser);
+$expires->SetToNow;
+
+if ( $MaxAgeSeconds && !RT->Config->Get('WebStrictBrowserCache') ) {
+    $expires->AddSeconds($MaxAgeSeconds);
+
+    # Expires is an older header and has been superseded by Cache-control
+    # and max-age, so set that also. New browsers will use max-age and
+    # ignore Expires.
+
+    # We're allowing a short cache, so replace no-cache with max-age.
+
+    $cache_control = "max-age=$MaxAgeSeconds, private"
+}
+else {
+    # Setting Expires to 0, a common approach to "immediately expired"
+    # doesn't send an Expires header from Mason, so set a little in the past.
+
+    $expires->AddSeconds(-30);
+    $cache_control .= ', max-age=0';
+}
+
+$r->headers_out->{'Expires'} = $expires->RFC2616;
+
+if ( RT->Config->Get('WebStrictBrowserCache') ) {
+
+    # Instruct the browser not to cache or store anything
+    $cache_control .= ', no-store, must-revalidate, s-maxage=0';
+}
+
+$r->headers_out->{'Cache-control'} = $cache_control;
+
+$m->callback( %ARGS, CallbackName => 'End' );
+</%INIT>
+<%ARGS>
+$MaxAgeSeconds => undef  # Time in seconds to allow for cache
+</%ARGS>

share/html/m/_elements/header

@@ -50,8 +50,7 @@ $title => loc('RT for [_1]', RT->Config->Get('rtname'))
 $show_home_button => 1
 </%args>
 <%init>
-$r->headers_out->{'Pragma'} = 'no-cache';
-$r->headers_out->{'Cache-control'} = 'no-cache';
+$m->comp('/Elements/HttpResponseHeaders');
 </%init>
 <html>
 <head>