Merge #2450

2450: Introduce TLS_PERMISSIVE for port 25 r=mergify[bot] a=nextgens

## What type of PR?

Feature

## What does this PR do?

This new advanced setting to harden cipher configuration on port 25. Changing the default is strongly discouraged, please read the documentation before doing so.

This specific feature has been requested numerous times... and while it's a terrible idea, I'm getting tired of explaining why every time. Those that would rather go through the fun of tracing missing emails tomorrow than picking a fight with their auditor today can enable it.

### Related issue(s)
- close #2449
- close #1945
- #1617

## Prerequisites
Before we can consider review and merge, please make sure the following list is done and checked.
If an entry in not applicable, you can check it or remove it from the list.

- [x] In case of feature or enhancement: documentation updated accordingly
- [x] Unless it's docs or a minor change: add [changelog](https://mailu.io/master/contributors/workflow.html#changelog) entry file.


Co-authored-by: Florent Daigniere <nextgens@freenetproject.org>

core/admin/mailu/configuration.py

@@ -74,6 +74,7 @@ DEFAULT_CONFIG = {
     'PERMANENT_SESSION_LIFETIME': 30*24*3600,
     'SESSION_COOKIE_SECURE': True,
     'CREDENTIAL_ROUNDS': 12,
+    'TLS_PERMISSIVE': True,
     'TZ': 'Etc/UTC',
     # Host settings
     'HOST_IMAP': 'imap',

core/nginx/conf/nginx.conf

@@ -317,9 +317,11 @@ mail {
       ssl_certificate /certs/letsencrypt/live/mailu/fullchain.pem;
       ssl_certificate /certs/letsencrypt/live/mailu-ecdsa/fullchain.pem;
       {% endif %}
+      {% if TLS_PERMISSIVE %}
       ssl_protocols TLSv1 TLSv1.1 TLSv1.2 TLSv1.3;
       ssl_ciphers ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384:DHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA:ECDHE-RSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES256-SHA256:AES128-GCM-SHA256:AES256-GCM-SHA384:AES128-SHA256:AES256-SHA256:AES128-SHA:AES256-SHA:DES-CBC3-SHA;
       ssl_prefer_server_ciphers on;
+      {% endif %}
       starttls on;
       {% endif %}
       protocol smtp;

core/nginx/config.py

@@ -9,6 +9,8 @@ args = os.environ.copy()
 
 log.basicConfig(stream=sys.stderr, level=args.get("LOG_LEVEL", "WARNING"))
 
+args['TLS_PERMISSIVE'] = str(args.get('TLS_PERMISSIVE')).lower() not in ('false', 'no')
+
 # Get the first DNS server
 with open("/etc/resolv.conf") as handle:
     content = handle.read().split()

docs/configuration.rst

@@ -217,6 +217,8 @@ but slows down the performance of modern devices.
 
 .. _`android handsets older than 7.1.1`: https://community.letsencrypt.org/t/production-chain-changes/150739
 
+The ``TLS_PERMISSIVE`` (default: true) setting controls whether ciphers and protocols offered on port 25 for STARTTLS are optimized for maximum compatibility. We **strongly recommend** that you do **not** change this setting on the basis that any encryption beats no encryption. If you are subject to compliance requirements and are not afraid of losing emails as a result of artificially reducing compatibility, set it to 'false'. Keep in mind that servers that are running a software stack old enough to not be compatible with the current TLS requirements will either a) deliver in plaintext b) bounce emails c) silently drop emails; moreover, modern servers will benefit from various downgrade protections (DOWNGRD, RFC7507) making the security argument mostly a moot point.
+
 .. _reverse_proxy_headers:
 
 The ``REAL_IP_HEADER`` (default: unset) and ``REAL_IP_FROM`` (default: unset) settings 

towncrier/newsfragments/2449.feature

@@ -0,0 +1 @@
+Introduce TLS_PERMISSIVE, a new advanced setting to harden cipher configuration on port 25. Changing the default is strongly discouraged, please read the documentation before doing so.