diff --git a/.gitignore b/.gitignore
index b89b11b4f..96a88d934 100644
--- a/.gitignore
+++ b/.gitignore
@@ -34,3 +34,6 @@ BRANCH
 .rubocop-https*
 .env*
 
+node_modules
+yarn.lock
+
diff --git a/.release-please-manifest.json b/.release-please-manifest.json
index 931d27504..8e34c5c9b 100644
--- a/.release-please-manifest.json
+++ b/.release-please-manifest.json
@@ -1,3 +1,3 @@
 {
-  ".": "3.3.4"
+  ".": "3.3.7"
 }
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8b78a9257..da81f33bf 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,67 @@
 
 This file contains all the latest changes and updates to Postal.
 
+## [3.3.7](https://github.com/postalserver/postal/compare/3.3.6...3.3.7) (2026-06-03)
+
+
+### Bug Fixes
+
+* **http:** prevent SSRF in outbound webhook and HTTP endpoint requests ([11c9814](https://github.com/postalserver/postal/commit/11c9814474f956619da35e8385ef7fab9f304de0))
+* **message-db:** prevent SQL injection via condition keys (GHSA-x2hq-rfpg-3xr5) ([4314a6e](https://github.com/postalserver/postal/commit/4314a6ec1e2812daa67dd20effd1db1769c1f8e8))
+* **specs:** stub IPv6 support in address guard IPv6 literal spec ([029bfe0](https://github.com/postalserver/postal/commit/029bfe098d9b8c0b5cafc49eac33e767f5748cd3))
+
+
+### Miscellaneous Chores
+
+* **deps:** upgrade rack & rails ([0445e5c](https://github.com/postalserver/postal/commit/0445e5c509870dfe9c16366c53dee3fc02ad3904))
+* update security vulnerability reporting instructions ([8be1e27](https://github.com/postalserver/postal/commit/8be1e27fec489ab659ef5e909f705932028b1694))
+
+## [3.3.6](https://github.com/postalserver/postal/compare/3.3.5...3.3.6) (2026-04-28)
+
+
+### Bug Fixes
+
+* **messages:** sandbox rendered email HTML as extra XSS defence ([cad2aa6](https://github.com/postalserver/postal/commit/cad2aa6808519a3ff25215f09f4966d9fa3bb372))
+
+
+### Miscellaneous Chores
+
+* ignore node modules and yarn.lock ([b611d57](https://github.com/postalserver/postal/commit/b611d577af79b8e1e75b6d47fa04d1ba03e34eec))
+
+
+### Code Refactoring
+
+* **auth:** tighten return_to validation ([84f4e20](https://github.com/postalserver/postal/commit/84f4e20f05db2d11b0144f95960c956f8221e657))
+* **helpers:** escape interpolated values in select options ([9243524](https://github.com/postalserver/postal/commit/924352403553dcfcc569876ca76c219493fac9d6))
+* **tracking:** remove unused src image proxy ([dca7f90](https://github.com/postalserver/postal/commit/dca7f90b9046247c0d953567be35921167e79d87))
+
+## [3.3.5](https://github.com/postalserver/postal/compare/3.3.4...3.3.5) (2026-02-01)
+
+
+### Bug Fixes
+
+* **deliveries:** escape delivery details to prevent HTML injection ([11419f9](https://github.com/postalserver/postal/commit/11419f99140e13688a9613cab3ee03f8d3cbae45))
+* **health_server:** use rackup handler instead of rack handler ([7c47422](https://github.com/postalserver/postal/commit/7c47422c865e738c4d6af0fed1cca4405288341f))
+* oidc scopes are invalid when concatenated ([#3332](https://github.com/postalserver/postal/issues/3332)) ([9c5f96a](https://github.com/postalserver/postal/commit/9c5f96ae90cf06dcd5db776806865752f667bd95))
+* typo in process logging ([#3212](https://github.com/postalserver/postal/issues/3212)) ([b7e5232](https://github.com/postalserver/postal/commit/b7e5232e077b3c9b7a999dcb6676fba0ec61458e))
+* typo in the credentials page ([fd3c7cc](https://github.com/postalserver/postal/commit/fd3c7ccdf6dc4ee0a76c9523cbd735159e4b8000))
+* update url for v2 config ([#3225](https://github.com/postalserver/postal/issues/3225)) ([e00098b](https://github.com/postalserver/postal/commit/e00098b8003cf37f2708f536871b3ade377aed2d))
+
+
+### Documentation
+
+* **process.rb:** add help about time unit used by metric ([#3339](https://github.com/postalserver/postal/issues/3339)) ([f5325c4](https://github.com/postalserver/postal/commit/f5325c49ff1152ad53eaaec98717ad3412d379ae))
+
+
+### Miscellaneous Chores
+
+* **deps:** upgrade puma, net-imap and other deps ([c03c44b](https://github.com/postalserver/postal/commit/c03c44b442a29aa9881c1e1aae60bead9776a6b6))
+* **dockerfile:** reduce container size ([86de372](https://github.com/postalserver/postal/commit/86de372382bd62bdd5d1372254f8817b0360bd56))
+* remove version from docker-compose.yml ([c78000c](https://github.com/postalserver/postal/commit/c78000ca8f2998aa04648f465060768db6467de6))
+* upgrade resolv to 0.6.2 ([d00d978](https://github.com/postalserver/postal/commit/d00d978872a96369544303d08f6a9d11cdf56b62))
+* upgrade to rails 7.1 and ruby 3.4 ([#3457](https://github.com/postalserver/postal/issues/3457)) ([ab6d443](https://github.com/postalserver/postal/commit/ab6d4430baa33a05f1aa66e776cc2a5bcaa0ede8))
+* upgrade uri gem to 1.0.3 ([f193b8e](https://github.com/postalserver/postal/commit/f193b8e77fc096382ab7aaa6a2c29641b4cb12df))
+
 ## [3.3.4](https://github.com/postalserver/postal/compare/3.3.3...3.3.4) (2024-06-20)
 
 
diff --git a/Gemfile b/Gemfile
index 48d8bf75c..4d3c0deac 100644
--- a/Gemfile
+++ b/Gemfile
@@ -29,7 +29,7 @@ gem "ostruct"
 gem "prometheus-client"
 gem "puma"
 gem "rackup"
-gem "rails", "= 7.1.5.2"
+gem "rails", "= 7.1.6"
 gem "resolv"
 gem "secure_headers"
 gem "sentry-rails"
diff --git a/Gemfile.lock b/Gemfile.lock
index a4ad73ca5..16adf02bf 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -2,35 +2,36 @@ GEM
   remote: https://rubygems.org/
   specs:
     abbrev (0.1.2)
-    actioncable (7.1.5.2)
-      actionpack (= 7.1.5.2)
-      activesupport (= 7.1.5.2)
+    actioncable (7.1.6)
+      actionpack (= 7.1.6)
+      activesupport (= 7.1.6)
       nio4r (~> 2.0)
       websocket-driver (>= 0.6.1)
       zeitwerk (~> 2.6)
-    actionmailbox (7.1.5.2)
-      actionpack (= 7.1.5.2)
-      activejob (= 7.1.5.2)
-      activerecord (= 7.1.5.2)
-      activestorage (= 7.1.5.2)
-      activesupport (= 7.1.5.2)
+    actionmailbox (7.1.6)
+      actionpack (= 7.1.6)
+      activejob (= 7.1.6)
+      activerecord (= 7.1.6)
+      activestorage (= 7.1.6)
+      activesupport (= 7.1.6)
       mail (>= 2.7.1)
       net-imap
       net-pop
       net-smtp
-    actionmailer (7.1.5.2)
-      actionpack (= 7.1.5.2)
-      actionview (= 7.1.5.2)
-      activejob (= 7.1.5.2)
-      activesupport (= 7.1.5.2)
+    actionmailer (7.1.6)
+      actionpack (= 7.1.6)
+      actionview (= 7.1.6)
+      activejob (= 7.1.6)
+      activesupport (= 7.1.6)
       mail (~> 2.5, >= 2.5.4)
       net-imap
       net-pop
       net-smtp
       rails-dom-testing (~> 2.2)
-    actionpack (7.1.5.2)
-      actionview (= 7.1.5.2)
-      activesupport (= 7.1.5.2)
+    actionpack (7.1.6)
+      actionview (= 7.1.6)
+      activesupport (= 7.1.6)
+      cgi
       nokogiri (>= 1.8.5)
       racc
       rack (>= 2.2.4)
@@ -38,35 +39,36 @@ GEM
       rack-test (>= 0.6.3)
       rails-dom-testing (~> 2.2)
       rails-html-sanitizer (~> 1.6)
-    actiontext (7.1.5.2)
-      actionpack (= 7.1.5.2)
-      activerecord (= 7.1.5.2)
-      activestorage (= 7.1.5.2)
-      activesupport (= 7.1.5.2)
+    actiontext (7.1.6)
+      actionpack (= 7.1.6)
+      activerecord (= 7.1.6)
+      activestorage (= 7.1.6)
+      activesupport (= 7.1.6)
       globalid (>= 0.6.0)
       nokogiri (>= 1.8.5)
-    actionview (7.1.5.2)
-      activesupport (= 7.1.5.2)
+    actionview (7.1.6)
+      activesupport (= 7.1.6)
       builder (~> 3.1)
+      cgi
       erubi (~> 1.11)
       rails-dom-testing (~> 2.2)
       rails-html-sanitizer (~> 1.6)
-    activejob (7.1.5.2)
-      activesupport (= 7.1.5.2)
+    activejob (7.1.6)
+      activesupport (= 7.1.6)
       globalid (>= 0.3.6)
-    activemodel (7.1.5.2)
-      activesupport (= 7.1.5.2)
-    activerecord (7.1.5.2)
-      activemodel (= 7.1.5.2)
-      activesupport (= 7.1.5.2)
+    activemodel (7.1.6)
+      activesupport (= 7.1.6)
+    activerecord (7.1.6)
+      activemodel (= 7.1.6)
+      activesupport (= 7.1.6)
       timeout (>= 0.4.0)
-    activestorage (7.1.5.2)
-      actionpack (= 7.1.5.2)
-      activejob (= 7.1.5.2)
-      activerecord (= 7.1.5.2)
-      activesupport (= 7.1.5.2)
+    activestorage (7.1.6)
+      actionpack (= 7.1.6)
+      activejob (= 7.1.6)
+      activerecord (= 7.1.6)
+      activesupport (= 7.1.6)
       marcel (~> 1.0)
-    activesupport (7.1.5.2)
+    activesupport (7.1.6)
       base64
       benchmark (>= 0.3)
       bigdecimal
@@ -93,10 +95,11 @@ GEM
       execjs (~> 2)
     base64 (0.3.0)
     bcrypt (3.1.20)
-    benchmark (0.4.1)
-    bigdecimal (3.2.3)
+    benchmark (0.5.0)
+    bigdecimal (4.1.2)
     bindata (2.5.0)
-    builder (3.2.4)
+    builder (3.3.0)
+    cgi (0.5.1)
     chronic (0.10.2)
     coffee-rails (5.0.0)
       coffee-script (>= 2.2.0)
@@ -105,8 +108,8 @@ GEM
       coffee-script-source
       execjs
     coffee-script-source (1.12.2)
-    concurrent-ruby (1.3.5)
-    connection_pool (2.5.4)
+    concurrent-ruby (1.3.6)
+    connection_pool (3.0.2)
     crack (1.0.0)
       bigdecimal
       rexml
@@ -115,7 +118,7 @@ GEM
       activerecord (>= 5.a)
       database_cleaner-core (~> 2.0)
     database_cleaner-core (2.0.1)
-    date (3.4.1)
+    date (3.5.1)
     diff-lcs (1.6.2)
     domain_name (0.6.20240107)
     dotenv (3.0.2)
@@ -123,8 +126,8 @@ GEM
     dynamic_form (1.2.0)
     email_validator (2.2.4)
       activemodel
-    erb (5.0.2)
-    erubi (1.12.0)
+    erb (6.0.4)
+    erubi (1.13.1)
     execjs (2.7.0)
     factory_bot (6.4.6)
       activesupport (>= 5.0.0)
@@ -149,11 +152,12 @@ GEM
     hashdiff (1.1.0)
     hashie (5.0.0)
     highline (2.1.0)
-    i18n (1.14.7)
+    i18n (1.14.8)
       concurrent-ruby (~> 1.0)
-    io-console (0.8.1)
-    irb (1.15.2)
+    io-console (0.8.2)
+    irb (1.18.0)
       pp (>= 0.6.0)
+      prism (>= 1.3.0)
       rdoc (>= 4.0.0)
       reline (>= 0.4.2)
     jquery-rails (4.5.1)
@@ -189,42 +193,45 @@ GEM
     konfig-config (3.0.0)
       hashie
     logger (1.7.0)
-    loofah (2.24.1)
+    loofah (2.25.1)
       crass (~> 1.0.2)
       nokogiri (>= 1.12.0)
-    mail (2.8.1)
+    mail (2.9.0)
+      logger
       mini_mime (>= 0.1.1)
       net-imap
       net-pop
       net-smtp
-    marcel (1.1.0)
+    marcel (1.2.1)
     mini_mime (1.1.5)
-    minitest (5.25.5)
+    minitest (6.0.6)
+      drb (~> 2.0)
+      prism (~> 1.5)
     mutex_m (0.3.0)
     mysql2 (0.5.6)
     net-http (0.4.1)
       uri
-    net-imap (0.5.11)
+    net-imap (0.6.4)
       date
       net-protocol
     net-pop (0.1.2)
       net-protocol
     net-protocol (0.2.2)
       timeout
-    net-smtp (0.4.0.1)
+    net-smtp (0.5.1)
       net-protocol
     nifty-utils (1.1.7)
     nilify_blanks (1.4.0)
       activerecord (>= 4.0.0)
       activesupport (>= 4.0.0)
-    nio4r (2.7.4)
-    nokogiri (1.18.10-aarch64-linux-gnu)
+    nio4r (2.7.5)
+    nokogiri (1.19.3-aarch64-linux-gnu)
       racc (~> 1.4)
-    nokogiri (1.18.10-arm64-darwin)
+    nokogiri (1.19.3-arm64-darwin)
       racc (~> 1.4)
-    nokogiri (1.18.10-x86_64-darwin)
+    nokogiri (1.19.3-x86_64-darwin)
       racc (~> 1.4)
-    nokogiri (1.18.10-x86_64-linux-gnu)
+    nokogiri (1.19.3-x86_64-linux-gnu)
       racc (~> 1.4)
     omniauth (2.1.2)
       hashie (>= 3.4.6)
@@ -253,18 +260,19 @@ GEM
     parallel (1.22.1)
     parser (3.2.1.1)
       ast (~> 2.4.1)
-    pp (0.6.2)
+    pp (0.6.3)
       prettyprint
     prettyprint (0.2.0)
+    prism (1.9.0)
     prometheus-client (4.2.2)
-    psych (5.2.6)
+    psych (5.4.0)
       date
       stringio
     public_suffix (5.0.4)
     puma (7.0.4)
       nio4r (~> 2.0)
     racc (1.8.1)
-    rack (3.2.1)
+    rack (3.2.6)
     rack-oauth2 (2.2.1)
       activesupport
       attr_required
@@ -276,49 +284,52 @@ GEM
       base64 (>= 0.1.0)
       logger (>= 1.6.0)
       rack (>= 3.0.0, < 4)
-    rack-session (2.1.1)
+    rack-session (2.1.2)
       base64 (>= 0.1.0)
       rack (>= 3.0.0)
-    rack-test (2.1.0)
+    rack-test (2.2.0)
       rack (>= 1.3)
-    rackup (2.2.1)
+    rackup (2.3.1)
       rack (>= 3)
-    rails (7.1.5.2)
-      actioncable (= 7.1.5.2)
-      actionmailbox (= 7.1.5.2)
-      actionmailer (= 7.1.5.2)
-      actionpack (= 7.1.5.2)
-      actiontext (= 7.1.5.2)
-      actionview (= 7.1.5.2)
-      activejob (= 7.1.5.2)
-      activemodel (= 7.1.5.2)
-      activerecord (= 7.1.5.2)
-      activestorage (= 7.1.5.2)
-      activesupport (= 7.1.5.2)
+    rails (7.1.6)
+      actioncable (= 7.1.6)
+      actionmailbox (= 7.1.6)
+      actionmailer (= 7.1.6)
+      actionpack (= 7.1.6)
+      actiontext (= 7.1.6)
+      actionview (= 7.1.6)
+      activejob (= 7.1.6)
+      activemodel (= 7.1.6)
+      activerecord (= 7.1.6)
+      activestorage (= 7.1.6)
+      activesupport (= 7.1.6)
       bundler (>= 1.15.0)
-      railties (= 7.1.5.2)
-    rails-dom-testing (2.2.0)
+      railties (= 7.1.6)
+    rails-dom-testing (2.3.0)
       activesupport (>= 5.0.0)
       minitest
       nokogiri (>= 1.6)
-    rails-html-sanitizer (1.6.2)
-      loofah (~> 2.21)
+    rails-html-sanitizer (1.7.0)
+      loofah (~> 2.25)
       nokogiri (>= 1.15.7, != 1.16.7, != 1.16.6, != 1.16.5, != 1.16.4, != 1.16.3, != 1.16.2, != 1.16.1, != 1.16.0.rc1, != 1.16.0)
-    railties (7.1.5.2)
-      actionpack (= 7.1.5.2)
-      activesupport (= 7.1.5.2)
+    railties (7.1.6)
+      actionpack (= 7.1.6)
+      activesupport (= 7.1.6)
+      cgi
       irb
       rackup (>= 1.0.0)
       rake (>= 12.2)
       thor (~> 1.0, >= 1.2.2)
+      tsort (>= 0.2)
       zeitwerk (~> 2.6)
     rainbow (3.1.1)
-    rake (13.1.0)
-    rdoc (6.14.2)
+    rake (13.4.2)
+    rdoc (7.2.0)
       erb
       psych (>= 4.0.0)
+      tsort
     regexp_parser (2.7.0)
-    reline (0.6.2)
+    reline (0.6.3)
       io-console (~> 0.5)
     resolv (0.6.2)
     rexml (3.4.4)
@@ -387,17 +398,18 @@ GEM
       actionpack (>= 5.2)
       activesupport (>= 5.2)
       sprockets (>= 3.0.0)
-    stringio (3.1.7)
+    stringio (3.2.0)
     swd (2.0.3)
       activesupport (>= 3)
       attr_required (>= 0.0.5)
       faraday (~> 2.0)
       faraday-follow_redirects
     temple (0.10.3)
-    thor (1.3.0)
+    thor (1.5.0)
     tilt (2.3.0)
     timecop (0.9.8)
-    timeout (0.4.3)
+    timeout (0.6.1)
+    tsort (0.2.0)
     turbolinks (5.2.1)
       turbolinks-source (~> 5.2)
     turbolinks-source (5.2.0)
@@ -423,7 +435,7 @@ GEM
       base64
       websocket-extensions (>= 0.1.0)
     websocket-extensions (0.1.5)
-    zeitwerk (2.6.13)
+    zeitwerk (2.8.2)
 
 PLATFORMS
   aarch64-linux
@@ -467,7 +479,7 @@ DEPENDENCIES
   prometheus-client
   puma
   rackup
-  rails (= 7.1.5.2)
+  rails (= 7.1.6)
   resolv
   rspec
   rspec-rails
diff --git a/SECURITY.md b/SECURITY.md
index 7936c0808..23e945be0 100644
--- a/SECURITY.md
+++ b/SECURITY.md
@@ -11,5 +11,5 @@ We only support updates to the 3.x versions of Postal.
 
 ## Reporting a Vulnerability
 
-If you discover a vulnerability in Postal, please do not post an issue on GitHub. Instead you should send an
-e-mail to security@postalserver.io with details. We will get back to you directly.
+If you discover a vulnerability in Postal, please do not post an issue on GitHub. Please,
+instead, [create a new security advisory through GitHub](https://github.com/postalserver/postal/security/advisories/new).
diff --git a/app/controllers/application_controller.rb b/app/controllers/application_controller.rb
index d2c36cda5..24498f7ed 100644
--- a/app/controllers/application_controller.rb
+++ b/app/controllers/application_controller.rb
@@ -62,10 +62,13 @@ def append_info_to_payload(payload)
   end
 
   def url_with_return_to(url)
-    if params[:return_to].blank? || !params[:return_to].starts_with?("/")
+    return_to = params[:return_to]
+    if return_to.blank? ||
+       !return_to.start_with?("/") ||
+       return_to.start_with?("//", "/\\")
       url_for(url)
     else
-      params[:return_to]
+      return_to
     end
   end
 
diff --git a/app/controllers/legacy_api/messages_controller.rb b/app/controllers/legacy_api/messages_controller.rb
index 6d78c1618..239eee3cd 100644
--- a/app/controllers/legacy_api/messages_controller.rb
+++ b/app/controllers/legacy_api/messages_controller.rb
@@ -15,12 +15,9 @@ class MessagesController < BaseController
     #                   OR an error if the message does not exist.
     #
     def message
-      if api_params["id"].blank?
-        render_parameter_error "`id` parameter is required but is missing"
-        return
-      end
+      message = find_message
+      return if performed?
 
-      message = @current_credential.server.message(api_params["id"])
       message_hash = { id: message.id, token: message.token }
       expansions = api_params["_expansions"]
 
@@ -111,12 +108,9 @@ def message
     #                   OR an error if the message does not exist.
     #
     def deliveries
-      if api_params["id"].blank?
-        render_parameter_error "`id` parameter is required but is missing"
-        return
-      end
+      message = find_message
+      return if performed?
 
-      message = @current_credential.server.message(api_params["id"])
       deliveries = message.deliveries.map do |d|
         {
           id: d.id,
@@ -136,5 +130,37 @@ def deliveries
                    id: api_params["id"]
     end
 
+    private
+
+    # Look up the message referenced by the request's `id` parameter.
+    #
+    # The legacy API only ever identifies a message by its integer ID. The
+    # request body is parsed as JSON, so without validation a JSON object or
+    # array supplied for `id` would arrive as a Ruby Hash/Array and be passed
+    # straight through to the message database as a raw set of SQL conditions.
+    # We therefore reject anything that is not a simple scalar before it can
+    # reach the database and coerce the value to an integer ID.
+    #
+    # Renders an error and returns nil when the parameter is missing or is not
+    # a scalar; otherwise returns the matched message (raising NotFound when no
+    # message matches, which the actions rescue).
+    #
+    # @return [Postal::MessageDB::Message, nil]
+    def find_message
+      id = api_params["id"]
+
+      if id.blank?
+        render_parameter_error "`id` parameter is required but is missing"
+        return
+      end
+
+      unless id.is_a?(String) || id.is_a?(Integer)
+        render_parameter_error "`id` parameter must be a string or integer"
+        return
+      end
+
+      @current_credential.server.message(id.to_i)
+    end
+
   end
 end
diff --git a/app/controllers/messages_controller.rb b/app/controllers/messages_controller.rb
index ea0e940ce..9d59f08e4 100644
--- a/app/controllers/messages_controller.rb
+++ b/app/controllers/messages_controller.rb
@@ -89,6 +89,18 @@ def deliveries
   end
 
   def html_raw
+    override_content_security_policy_directives(
+      default_src: %w('none'),
+      script_src: %w('none'),
+      style_src: %w('unsafe-inline'),
+      img_src: %w(* data:),
+      font_src: %w(*),
+      frame_ancestors: %w('self'),
+      form_action: %w('none'),
+      base_uri: %w('none')
+    )
+    response.headers["X-Content-Type-Options"] = "nosniff"
+    response.headers["Referrer-Policy"] = "no-referrer"
     render html: @message.html_body_without_tracking_image.html_safe
   end
 
diff --git a/app/helpers/application_helper.rb b/app/helpers/application_helper.rb
index c6acf4563..17d124217 100644
--- a/app/helpers/application_helper.rb
+++ b/app/helpers/application_helper.rb
@@ -3,6 +3,7 @@
 module ApplicationHelper
 
   def format_delivery_details(server, text)
+    text = h(text)
     text.gsub!(/<msg:(\d+)>/) do
       id = ::Regexp.last_match(1).to_i
       link_to("message ##{id}", organization_server_message_path(server.organization, server, id), class: "u-link")
@@ -32,7 +33,7 @@ def domain_options_for_select(server, selected_domain = nil, options = {})
         s << "<optgroup label='Server Domains'>"
         server_domains.each do |domain|
           selected = domain == selected_domain ? "selected='selected'" : ""
-          s << "<option value='#{domain.id}' #{selected}>#{domain.name}</option>"
+          s << "<option value='#{h(domain.id)}' #{selected}>#{h(domain.name)}</option>"
         end
         s << "</optgroup>"
       end
@@ -42,7 +43,7 @@ def domain_options_for_select(server, selected_domain = nil, options = {})
         s << "<optgroup label='Organization Domains'>"
         organization_domains.each do |domain|
           selected = domain == selected_domain ? "selected='selected'" : ""
-          s << "<option value='#{domain.id}' #{selected}>#{domain.name}</option>"
+          s << "<option value='#{h(domain.id)}' #{selected}>#{h(domain.name)}</option>"
         end
         s << "</optgroup>"
       end
@@ -59,7 +60,7 @@ def endpoint_options_for_select(server, selected_value = nil, options = {})
         http_endpoints.each do |endpoint|
           value = "#{endpoint.class}##{endpoint.uuid}"
           selected = value == selected_value ? "selected='selected'" : ""
-          s << "<option value='#{value}' #{selected}>#{endpoint.description}</option>"
+          s << "<option value='#{h(value)}' #{selected}>#{h(endpoint.description)}</option>"
         end
         s << "</optgroup>"
       end
@@ -70,7 +71,7 @@ def endpoint_options_for_select(server, selected_value = nil, options = {})
         smtp_endpoints.each do |endpoint|
           value = "#{endpoint.class}##{endpoint.uuid}"
           selected = value == selected_value ? "selected='selected'" : ""
-          s << "<option value='#{value}' #{selected}>#{endpoint.description}</option>"
+          s << "<option value='#{h(value)}' #{selected}>#{h(endpoint.description)}</option>"
         end
         s << "</optgroup>"
       end
@@ -81,7 +82,7 @@ def endpoint_options_for_select(server, selected_value = nil, options = {})
         address_endpoints.each do |endpoint|
           value = "#{endpoint.class}##{endpoint.uuid}"
           selected = value == selected_value ? "selected='selected'" : ""
-          s << "<option value='#{value}' #{selected}>#{endpoint.address}</option>"
+          s << "<option value='#{h(value)}' #{selected}>#{h(endpoint.address)}</option>"
         end
         s << "</optgroup>"
       end
@@ -93,7 +94,7 @@ def endpoint_options_for_select(server, selected_value = nil, options = {})
 
           selected = (selected_value == mode ? "selected='selected'" : "")
           text = t("route_modes.#{mode.underscore}")
-          s << "<option value='#{mode}' #{selected}>#{text}</option>"
+          s << "<option value='#{h(mode)}' #{selected}>#{h(text)}</option>"
         end
         s << "</optgroup>"
       end
diff --git a/app/lib/smtp_server/server.rb b/app/lib/smtp_server/server.rb
index 90902b5a8..fe5a63d4e 100644
--- a/app/lib/smtp_server/server.rb
+++ b/app/lib/smtp_server/server.rb
@@ -129,7 +129,12 @@ def run_event_loop
                 end
                 # We know who the client is, welcome them.
                 client.logger&.debug "Client identified as #{client_ip_address}"
-                new_io.print("220 #{Postal::Config.postal.smtp_hostname} ESMTP Postal/#{client.trace_id}")
+                # Write the greeting line and its terminating CRLF in a single write. Using
+                # #print emits the string and the "$\" output record separator ("\r\n") as two
+                # separate writes, which can be sent as two TCP segments. Some clients read the
+                # greeting text before the CRLF arrives, issue EHLO early, and then mis-handle the
+                # stray CRLF prepended to the EHLO reply. Sending the line atomically avoids this.
+                new_io.write("220 #{Postal::Config.postal.smtp_hostname} ESMTP Postal/#{client.trace_id}\r\n")
               end
               # Register the client and its socket with nio4r
               monitor = @io_selector.register(new_io, :r)
diff --git a/app/models/http_endpoint.rb b/app/models/http_endpoint.rb
index 8cd9044f3..393b78536 100644
--- a/app/models/http_endpoint.rb
+++ b/app/models/http_endpoint.rb
@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require "uri"
+
 # == Schema Information
 #
 # Table name: http_endpoints
@@ -38,6 +40,7 @@ class HTTPEndpoint < ApplicationRecord
 
   validates :name, presence: true
   validates :url, presence: true
+  validate :url_must_be_http_or_https
   validates :encoding, inclusion: { in: ENCODINGS }
   validates :format, inclusion: { in: FORMATS }
   validates :timeout, numericality: { greater_than_or_equal_to: 5, less_than_or_equal_to: 60 }
@@ -56,4 +59,17 @@ def update_routes
     routes.each { |r| r.update(endpoint: nil, mode: "Reject") }
   end
 
+  private
+
+  def url_must_be_http_or_https
+    return if url.blank?
+
+    uri = URI.parse(url)
+    return if uri.is_a?(URI::HTTP) && uri.host.present?
+
+    errors.add(:url, "must be an HTTP or HTTPS URL")
+  rescue URI::InvalidURIError
+    errors.add(:url, "must be a valid HTTP or HTTPS URL")
+  end
+
 end
diff --git a/app/views/messages/html.html.haml b/app/views/messages/html.html.haml
index 357f2cb50..826f7e176 100644
--- a/app/views/messages/html.html.haml
+++ b/app/views/messages/html.html.haml
@@ -14,4 +14,4 @@
         This means that we no longer store the raw data for this e-mail
         or the e-mail didn't include a HTML part.
 - else
-  %iframe{:width => "100%", :height => "100%", :src => html_raw_organization_server_message_path(organization, @server, @message.id)}
+  %iframe{:width => "100%", :height => "100%", :sandbox => "allow-popups allow-popups-to-escape-sandbox", :referrerpolicy => "no-referrer", :src => html_raw_organization_server_message_path(organization, @server, @message.id)}
diff --git a/doc/config/environment-variables.md b/doc/config/environment-variables.md
index 940424e04..56fc70a5e 100644
--- a/doc/config/environment-variables.md
+++ b/doc/config/environment-variables.md
@@ -18,6 +18,7 @@ This document contains all the environment variables which are available for thi
 | `POSTAL_SIGNING_KEY_PATH` | String | Path to the private key used for signing | $config-file-root/signing.key |
 | `POSTAL_SMTP_RELAYS` | Array of strings | An array of SMTP relays in the format of smtp://host:port | [] |
 | `POSTAL_TRUSTED_PROXIES` | Array of strings | An array of IP addresses to trust for proxying requests to Postal (in addition to localhost addresses) | [] |
+| `POSTAL_ALLOWED_REQUEST_DESTINATIONS` | Array of strings | Hostnames or IP/CIDR ranges that outbound webhook and HTTP endpoint requests are permitted to reach even when they resolve to a private, loopback, link-local or otherwise reserved address. All other such destinations are blocked to prevent SSRF. | [] |
 | `POSTAL_QUEUED_MESSAGE_LOCK_STALE_DAYS` | Integer | The number of days after which to consider a lock as stale. Messages with stale locks will be removed and not retried. | 1 |
 | `POSTAL_BATCH_QUEUED_MESSAGES` | Boolean | When enabled queued messages will be de-queued in batches based on their destination | true |
 | `WEB_SERVER_DEFAULT_PORT` | Integer | The default port the web server should listen on unless overriden by the PORT environment variable | 5000 |
diff --git a/doc/config/yaml.yml b/doc/config/yaml.yml
index f3a735a9f..4e0406d38 100644
--- a/doc/config/yaml.yml
+++ b/doc/config/yaml.yml
@@ -29,6 +29,8 @@ postal:
   smtp_relays: []
   # An array of IP addresses to trust for proxying requests to Postal (in addition to localhost addresses)
   trusted_proxies: []
+  # Hostnames or IP/CIDR ranges that outbound webhook and HTTP endpoint requests are permitted to reach even when they resolve to a private, loopback, link-local or otherwise reserved address. All other such destinations are blocked to prevent SSRF.
+  allowed_request_destinations: []
   # The number of days after which to consider a lock as stale. Messages with stale locks will be removed and not retried.
   queued_message_lock_stale_days: 1
   # When enabled queued messages will be de-queued in batches based on their destination
diff --git a/lib/postal/config_schema.rb b/lib/postal/config_schema.rb
index e3c6415d5..871d094a7 100644
--- a/lib/postal/config_schema.rb
+++ b/lib/postal/config_schema.rb
@@ -92,6 +92,14 @@ module Postal
         transform { |ip| IPAddr.new(ip) }
       end
 
+      string :allowed_request_destinations do
+        array
+        description "Hostnames or IP/CIDR ranges that outbound webhook and HTTP " \
+                    "endpoint requests are permitted to reach even when they resolve " \
+                    "to a private, loopback, link-local or otherwise reserved address. " \
+                    "All other such destinations are blocked to prevent SSRF."
+      end
+
       integer :queued_message_lock_stale_days do
         description "The number of days after which to consider a lock as stale. Messages with stale locks will be removed and not retried."
         default 1
diff --git a/lib/postal/http.rb b/lib/postal/http.rb
index 8a32d9b58..4f17febd6 100644
--- a/lib/postal/http.rb
+++ b/lib/postal/http.rb
@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require "net/https"
+require "resolv"
 require "uri"
 
 module Postal
@@ -47,19 +48,24 @@ def self.request(method, url, options = {})
 
       request["User-Agent"] = options[:user_agent] || "Postal/#{Postal.version}"
 
-      connection = Net::HTTP.new(uri.host, uri.port)
-
-      if uri.scheme == "https"
-        connection.use_ssl = true
-        connection.verify_mode = OpenSSL::SSL::VERIFY_PEER
-        ssl = true
-      else
-        ssl = false
-      end
+      timeout = options[:timeout] || 60
+      ssl = uri.scheme == "https"
 
       begin
-        timeout = options[:timeout] || 60
         Timeout.timeout(timeout) do
+          connect_address = AddressGuard.safe_connect_address(uri.host)
+
+          connection = Net::HTTP.new(uri.host, uri.port)
+          # Pin the connection to the address we validated above so that the socket
+          # cannot be redirected to a different (e.g. internal) address via a DNS
+          # rebinding race between the check and the connection.
+          connection.ipaddr = connect_address
+
+          if uri.scheme == "https"
+            connection.use_ssl = true
+            connection.verify_mode = OpenSSL::SSL::VERIFY_PEER
+          end
+
           result = connection.request(request)
           {
             code: result.code.to_i,
@@ -68,6 +74,13 @@ def self.request(method, url, options = {})
             secure: ssl
           }
         end
+      rescue BlockedDestinationError => e
+        {
+          code: -4,
+          body: e.message,
+          headers: {},
+          secure: ssl
+        }
       rescue OpenSSL::SSL::SSLError
         {
           code: -3,
@@ -75,7 +88,7 @@ def self.request(method, url, options = {})
           headers: {},
           secure: ssl
         }
-      rescue SocketError, Errno::ECONNRESET, EOFError, Errno::EINVAL, Errno::ENETUNREACH, Errno::EHOSTUNREACH, Errno::ECONNREFUSED => e
+      rescue Resolv::ResolvError, SocketError, SystemCallError, EOFError => e
         {
           code: -2,
           body: e.message,
diff --git a/lib/postal/http/address_guard.rb b/lib/postal/http/address_guard.rb
new file mode 100644
index 000000000..ed863d2ab
--- /dev/null
+++ b/lib/postal/http/address_guard.rb
@@ -0,0 +1,202 @@
+# frozen_string_literal: true
+
+require "ipaddr"
+require "resolv"
+require "socket"
+
+module Postal
+  module HTTP
+    # Guards outbound HTTP requests against SSRF by resolving the destination
+    # host and refusing to connect to private, loopback, link-local, multicast
+    # or otherwise reserved addresses (for example cloud metadata endpoints).
+    #
+    # Administrators can permit specific destinations by adding hostnames or
+    # IP/CIDR ranges to the `postal.allowed_request_destinations` config option.
+    class AddressGuard
+
+      # IP ranges that outbound requests are never allowed to reach unless the
+      # destination has been explicitly allowlisted.
+      BLOCKED_RANGES = [
+        # IPv4
+        "0.0.0.0/8",        # "this host on this network"
+        "10.0.0.0/8",       # RFC1918 private
+        "100.64.0.0/10",    # RFC6598 carrier-grade NAT
+        "127.0.0.0/8",      # loopback
+        "169.254.0.0/16",   # link-local (incl. 169.254.169.254 metadata)
+        "172.16.0.0/12",    # RFC1918 private
+        "192.0.0.0/24",     # IETF protocol assignments
+        "192.168.0.0/16",   # RFC1918 private
+        "198.18.0.0/15",    # benchmarking
+        "224.0.0.0/4",      # multicast
+        "240.0.0.0/4",      # reserved
+        # IPv6
+        "::/128",           # unspecified
+        "::1/128",          # loopback
+        "::ffff:0:0/96",    # IPv4-mapped (also re-checked against the v4 list)
+        "fc00::/7",         # unique-local
+        "fe80::/10",        # link-local
+        "ff00::/8", # multicast
+      ].map { |range| IPAddr.new(range) }.freeze
+
+      class << self
+
+        # Resolve and validate the given host, returning the IP address the
+        # connection should be pinned to (as a string). Pinning the connection
+        # to the validated address prevents a DNS-rebinding race between the
+        # check here and the actual connection.
+        #
+        # @param [String] host the hostname or IP literal from the request URL
+        # @raise [Postal::HTTP::BlockedDestinationError] if the host cannot be
+        #   resolved or any resolved address is not permitted
+        # @raise [SocketError] if the host only resolves to addresses whose
+        #   family this server cannot reach (e.g. IPv6 with no IPv6 support)
+        # @return [String] the validated IP address to connect to
+        def safe_connect_address(host)
+          new(host).safe_connect_address
+        end
+
+        # Whether this server has IPv6 connectivity (a global IPv6 address on
+        # one of its interfaces). Memoized as it does not change at runtime.
+        def ipv6_supported?
+          return @ipv6_supported unless @ipv6_supported.nil?
+
+          @ipv6_supported = local_families.include?(:ipv6)
+        end
+
+        # Whether this server has IPv4 connectivity. Defaults to true unless the
+        # host clearly only has IPv6, so that a host reporting no global
+        # addresses at all (e.g. inside a minimal container) still attempts IPv4
+        # as it did before this guard existed.
+        def ipv4_supported?
+          return @ipv4_supported unless @ipv4_supported.nil?
+
+          families = local_families
+          @ipv4_supported = families.include?(:ipv4) || !families.include?(:ipv6)
+        end
+
+        private
+
+        def local_families
+          families = []
+          Socket.ip_address_list.each do |address|
+            families << :ipv4 if address.ipv4? && !address.ipv4_loopback?
+            families << :ipv6 if address.ipv6? && !address.ipv6_loopback? && !address.ipv6_linklocal?
+          end
+          families.uniq
+        end
+
+      end
+
+      # @param [String] host
+      def initialize(host)
+        @host = host.to_s
+      end
+
+      def safe_connect_address
+        if @host.empty?
+          raise BlockedDestinationError, "No host was given for the request"
+        end
+
+        addresses = resolve
+        if addresses.empty?
+          raise BlockedDestinationError, "Could not resolve '#{@host}' to any IP address"
+        end
+
+        # Reject the whole request if *any* resolved address is blocked. This is
+        # checked before the reachability filtering below so that a blocked
+        # destination is always reported as such, regardless of which address
+        # families this particular server can reach. It also defeats DNS
+        # responses that mix a public and a private address to slip past.
+        addresses.each do |address|
+          next unless blocked?(address)
+
+          raise BlockedDestinationError,
+                "Destination '#{@host}' (#{address}) is not permitted"
+        end
+
+        # Only connect to an address whose family this server can actually
+        # reach. Otherwise we might pin the connection to an IPv6 address on a
+        # host without IPv6 connectivity and fail to connect even when a usable
+        # IPv4 address was available.
+        usable = addresses.select { |address| family_reachable?(address) }
+        if usable.empty?
+          raise SocketError,
+                "'#{@host}' only resolves to addresses this server cannot reach " \
+                "(#{addresses.join(', ')})"
+        end
+
+        # Prefer IPv4 for predictability; only use IPv6 when it is the only
+        # reachable option.
+        (usable.find(&:ipv4?) || usable.first).to_s
+      end
+
+      private
+
+      # @return [Array<IPAddr>]
+      def resolve
+        return [IPAddr.new(@host)] if ip_literal?
+
+        Resolv.getaddresses(@host).filter_map do |address|
+          IPAddr.new(address)
+        rescue IPAddr::InvalidAddressError
+          nil
+        end
+      end
+
+      def ip_literal?
+        IPAddr.new(@host)
+        true
+      rescue IPAddr::InvalidAddressError
+        false
+      end
+
+      # @param [IPAddr] address
+      def family_reachable?(address)
+        if address.ipv6? && !address.ipv4_mapped?
+          self.class.ipv6_supported?
+        else
+          self.class.ipv4_supported?
+        end
+      end
+
+      # @param [IPAddr] address
+      def blocked?(address)
+        return false if allowlisted?(address)
+
+        # IPv4-mapped IPv6 addresses (::ffff:a.b.c.d) must be checked against the
+        # IPv4 rules using the embedded address, otherwise they bypass the list.
+        if address.ipv6? && address.ipv4_mapped?
+          mapped = address.native
+          return true if mapped.ipv4? && BLOCKED_RANGES.any? { |range| range.include?(mapped) }
+        end
+
+        BLOCKED_RANGES.any? { |range| range.include?(address) }
+      end
+
+      # @param [IPAddr] address
+      def allowlisted?(address)
+        allowlist.any? do |entry|
+          if entry.is_a?(IPAddr)
+            entry.include?(address)
+          else
+            entry.casecmp?(@host)
+          end
+        end
+      end
+
+      # Allowlist entries are kept as strings in config. An entry that parses as
+      # an IP/CIDR is matched against the resolved address; anything else is
+      # matched against the request hostname (case-insensitively).
+      #
+      # @return [Array<IPAddr, String>]
+      def allowlist
+        @allowlist ||= Array(Postal::Config.postal.allowed_request_destinations).map do |entry|
+          IPAddr.new(entry.to_s)
+        rescue IPAddr::InvalidAddressError
+          entry.to_s
+        end
+      end
+
+    end
+  end
+end
diff --git a/lib/postal/http/blocked_destination_error.rb b/lib/postal/http/blocked_destination_error.rb
new file mode 100644
index 000000000..c08a4446e
--- /dev/null
+++ b/lib/postal/http/blocked_destination_error.rb
@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+module Postal
+  module HTTP
+    # Raised when an outbound request would be sent to an address that is not
+    # permitted (a private, loopback, link-local or otherwise reserved address
+    # that has not been explicitly allowlisted). Used as an SSRF guard.
+    class BlockedDestinationError < StandardError
+    end
+  end
+end
diff --git a/lib/postal/message_db/database.rb b/lib/postal/message_db/database.rb
index 52f456986..415a88deb 100644
--- a/lib/postal/message_db/database.rb
+++ b/lib/postal/message_db/database.rb
@@ -70,7 +70,7 @@ def new_message(attributes = {})
       # Return the total size of all stored messages
       #
       def total_size
-        query("SELECT SUM(size) AS size FROM `#{database_name}`.`raw_message_sizes`").first["size"] || 0
+        query("SELECT SUM(size) AS size FROM #{escape_identifier(database_name)}.`raw_message_sizes`").first["size"] || 0
       end
 
       #
@@ -151,11 +151,11 @@ def select(table, options = {})
         if options[:count]
           sql_query << " COUNT(id) AS count"
         elsif options[:fields]
-          sql_query << (" " + options[:fields].map { |f| "`#{f}`" }.join(", "))
+          sql_query << (" " + options[:fields].map { |f| escape_identifier(f) }.join(", "))
         else
           sql_query << " *"
         end
-        sql_query << " FROM `#{database_name}`.`#{table}`"
+        sql_query << " FROM #{escape_identifier(database_name)}.#{escape_identifier(table)}"
         if options[:where].present?
           sql_query << (" " + build_where_string(options[:where], " AND "))
         end
@@ -163,7 +163,7 @@ def select(table, options = {})
           direction = (options[:direction] || "ASC").upcase
           raise Postal::Error, "Invalid direction #{options[:direction]}" unless %w[ASC DESC].include?(direction)
 
-          sql_query << " ORDER BY `#{options[:order]}` #{direction}"
+          sql_query << " ORDER BY #{escape_identifier(options[:order])} #{direction}"
         end
 
         if options[:limit]
@@ -211,7 +211,7 @@ def select_with_pagination(table, page, options = {})
       # Will return the total number of affected rows.
       #
       def update(table, attributes, options = {})
-        sql_query = "UPDATE `#{database_name}`.`#{table}` SET"
+        sql_query = "UPDATE #{escape_identifier(database_name)}.#{escape_identifier(table)} SET"
         sql_query << " #{hash_to_sql(attributes)}"
         if options[:where]
           sql_query << (" " + build_where_string(options[:where]))
@@ -227,8 +227,8 @@ def update(table, attributes, options = {})
       # Will return the ID of the new item.
       #
       def insert(table, attributes)
-        sql_query = "INSERT INTO `#{database_name}`.`#{table}`"
-        sql_query << (" (" + attributes.keys.map { |k| "`#{k}`" }.join(", ") + ")")
+        sql_query = "INSERT INTO #{escape_identifier(database_name)}.#{escape_identifier(table)}"
+        sql_query << (" (" + attributes.keys.map { |k| escape_identifier(k) }.join(", ") + ")")
         sql_query << (" VALUES (" + attributes.values.map { |v| escape(v) }.join(", ") + ")")
         with_mysql do |mysql|
           query_on_connection(mysql, sql_query)
@@ -243,8 +243,8 @@ def insert_multi(table, keys, values)
         if values.empty?
           nil
         else
-          sql_query = "INSERT INTO `#{database_name}`.`#{table}`"
-          sql_query << (" (" + keys.map { |k| "`#{k}`" }.join(", ") + ")")
+          sql_query = "INSERT INTO #{escape_identifier(database_name)}.#{escape_identifier(table)}"
+          sql_query << (" (" + keys.map { |k| escape_identifier(k) }.join(", ") + ")")
           sql_query << " VALUES "
           sql_query << values.map { |v| "(" + v.map { |r| escape(r) }.join(", ") + ")" }.join(", ")
           query(sql_query)
@@ -260,7 +260,7 @@ def insert_multi(table, keys, values)
       # Will return the total number of affected rows.
       #
       def delete(table, options = {})
-        sql_query = "DELETE FROM `#{database_name}`.`#{table}`"
+        sql_query = "DELETE FROM #{escape_identifier(database_name)}.#{escape_identifier(table)}"
         sql_query << (" " + build_where_string(options[:where], " AND "))
         with_mysql do |mysql|
           query_on_connection(mysql, sql_query)
@@ -351,32 +351,41 @@ def build_where_string(attributes, joiner = ", ")
 
       def hash_to_sql(hash, joiner = ", ")
         hash.map do |key, value|
+          column = escape_identifier(key)
           if value.is_a?(Array) && value.all? { |v| v.is_a?(Integer) }
-            "`#{key}` IN (#{value.join(', ')})"
+            "#{column} IN (#{value.join(', ')})"
           elsif value.is_a?(Array)
             escaped_values = value.map { |v| escape(v) }.join(", ")
-            "`#{key}` IN (#{escaped_values})"
+            "#{column} IN (#{escaped_values})"
           elsif value.is_a?(Hash)
             sql = []
             value.each do |operator, inner_value|
               case operator
               when :less_than
-                sql << "`#{key}` < #{escape(inner_value)}"
+                sql << "#{column} < #{escape(inner_value)}"
               when :greater_than
-                sql << "`#{key}` > #{escape(inner_value)}"
+                sql << "#{column} > #{escape(inner_value)}"
               when :less_than_or_equal_to
-                sql << "`#{key}` <= #{escape(inner_value)}"
+                sql << "#{column} <= #{escape(inner_value)}"
               when :greater_than_or_equal_to
-                sql << "`#{key}` >= #{escape(inner_value)}"
+                sql << "#{column} >= #{escape(inner_value)}"
               end
             end
             sql.empty? ? "1=1" : sql.join(joiner)
           else
-            "`#{key}` = #{escape(value)}"
+            "#{column} = #{escape(value)}"
           end
         end.join(joiner)
       end
 
+      # Escape a value for safe use as a MySQL identifier (e.g. a column or
+      # table name). Identifiers are wrapped in backticks and any backtick
+      # within the identifier is doubled so it cannot break out of the quoting
+      # and inject arbitrary SQL.
+      def escape_identifier(identifier)
+        "`" + identifier.to_s.gsub("`", "``") + "`"
+      end
+
     end
   end
 end
diff --git a/lib/tracking_middleware.rb b/lib/tracking_middleware.rb
index bee3a6378..d66473637 100644
--- a/lib/tracking_middleware.rb
+++ b/lib/tracking_middleware.rb
@@ -48,25 +48,11 @@ def dispatch_image_request(request, server_token, message_token)
       Sentry.capture_exception(e) if defined?(Sentry)
     end
 
-    source_image = request.params["src"]
-    case source_image
-    when nil
+    if request.params["src"].nil?
       headers = {}
       headers["Content-Type"] = "image/png"
       headers["Content-Length"] = TRACKING_PIXEL.bytesize.to_s
       [200, headers, [TRACKING_PIXEL]]
-    when /\Ahttps?:\/\//
-      response = Postal::HTTP.get(source_image, timeout: 3)
-      return [404, {}, ["Not found"]] unless response[:code] == 200
-
-      headers = {}
-      headers["Content-Type"] = response[:headers]["content-type"]&.first
-      headers["Last-Modified"] = response[:headers]["last-modified"]&.first
-      headers["Cache-Control"] = response[:headers]["cache-control"]&.first
-      headers["Etag"] = response[:headers]["etag"]&.first
-      headers["Content-Length"] = response[:body].bytesize.to_s
-      [200, headers, [response[:body]]]
-
     else
       [400, {}, ["Invalid/missing source image"]]
     end
diff --git a/spec/apis/legacy_api/messages/deliveries_spec.rb b/spec/apis/legacy_api/messages/deliveries_spec.rb
index 2daf95f6b..cfc69cc29 100644
--- a/spec/apis/legacy_api/messages/deliveries_spec.rb
+++ b/spec/apis/legacy_api/messages/deliveries_spec.rb
@@ -64,6 +64,23 @@
         end
       end
 
+      # Regression test for GHSA-x2hq-rfpg-3xr5 (see message_spec.rb). A JSON
+      # object supplied for `id` must be rejected before reaching the database
+      # rather than being interpreted as a raw set of SQL conditions.
+      context "when the message ID is a JSON object (SQL injection attempt)" do
+        it "rejects it with a parameter error and never reaches the database" do
+          expect_any_instance_of(Server).not_to receive(:message)
+          post "/api/v1/messages/deliveries",
+               headers: { "x-server-api-key" => credential.key,
+                          "content-type" => "application/json" },
+               params: { id: { "id`=0 OR SLEEP(5)#" => "x" } }.to_json
+          expect(response.status).to eq 200
+          parsed_body = JSON.parse(response.body)
+          expect(parsed_body["status"]).to eq "parameter-error"
+          expect(parsed_body["data"]["message"]).to match(/must be a string or integer/)
+        end
+      end
+
       context "when the message ID exists" do
         let(:server) { create(:server) }
         let(:credential) { create(:credential, server: server) }
diff --git a/spec/apis/legacy_api/messages/message_spec.rb b/spec/apis/legacy_api/messages/message_spec.rb
index 202b66b50..c40d756dc 100644
--- a/spec/apis/legacy_api/messages/message_spec.rb
+++ b/spec/apis/legacy_api/messages/message_spec.rb
@@ -63,6 +63,56 @@
         end
       end
 
+      # Regression tests for GHSA-x2hq-rfpg-3xr5. The request body is parsed as
+      # JSON, so a JSON object/array supplied for `id` would otherwise arrive as
+      # a Ruby Hash/Array and be passed straight through to the message database
+      # as a raw set of SQL conditions (blind SQL injection). These must be
+      # rejected before reaching the database.
+      context "when the message ID is a JSON object (SQL injection attempt)" do
+        it "rejects it with a parameter error and never reaches the database" do
+          expect_any_instance_of(Server).not_to receive(:message)
+          post "/api/v1/messages/message",
+               headers: { "x-server-api-key" => credential.key,
+                          "content-type" => "application/json" },
+               params: { id: { "id`=0 OR SLEEP(5)#" => "x" } }.to_json
+          expect(response.status).to eq 200
+          parsed_body = JSON.parse(response.body)
+          expect(parsed_body["status"]).to eq "parameter-error"
+          expect(parsed_body["data"]["message"]).to match(/must be a string or integer/)
+        end
+      end
+
+      context "when the message ID is a JSON array" do
+        it "rejects it with a parameter error" do
+          post "/api/v1/messages/message",
+               headers: { "x-server-api-key" => credential.key,
+                          "content-type" => "application/json" },
+               params: { id: [1, 2, 3] }.to_json
+          expect(response.status).to eq 200
+          parsed_body = JSON.parse(response.body)
+          expect(parsed_body["status"]).to eq "parameter-error"
+          expect(parsed_body["data"]["message"]).to match(/must be a string or integer/)
+        end
+      end
+
+      context "when the message ID is provided as a numeric string" do
+        let(:message) { MessageFactory.outgoing(server) }
+
+        it "is coerced to an integer and looks the message up" do
+          post "/api/v1/messages/message",
+               headers: { "x-server-api-key" => credential.key,
+                          "content-type" => "application/json" },
+               params: { id: message.id.to_s }.to_json
+          expect(response.status).to eq 200
+          parsed_body = JSON.parse(response.body)
+          expect(parsed_body["status"]).to eq "success"
+          expect(parsed_body["data"]).to match({
+            "id" => message.id,
+            "token" => message.token
+          })
+        end
+      end
+
       context "when the message ID exists" do
         let(:server) { create(:server) }
         let(:credential) { create(:credential, server: server) }
diff --git a/spec/helpers/application_helper_spec.rb b/spec/helpers/application_helper_spec.rb
new file mode 100644
index 000000000..4beb3de98
--- /dev/null
+++ b/spec/helpers/application_helper_spec.rb
@@ -0,0 +1,37 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+RSpec.describe ApplicationHelper, type: :helper do
+  describe "#endpoint_options_for_select" do
+    let(:server) { create(:server) }
+
+    context "when an endpoint has HTML characters in its description" do
+      let(:payload) { %q(x'"><script>alert(1)</script>) }
+
+      before do
+        create(:http_endpoint, server: server, name: payload)
+      end
+
+      it "HTML-escapes the endpoint description in the option text" do
+        html = helper.endpoint_options_for_select(server)
+
+        # The raw payload must not appear verbatim — if it does, the browser
+        # will execute the <script> tag.
+        expect(html).not_to include("<script>alert(1)</script>")
+
+        # Escaped form should appear instead.
+        expect(html).to include("&lt;script&gt;alert(1)&lt;/script&gt;")
+      end
+
+      it "does not allow the payload to break out of the option tag" do
+        html = helper.endpoint_options_for_select(server)
+
+        # The ' and > characters in the payload must be escaped so they
+        # cannot close the opening <option value='...'> or terminate the
+        # element early.
+        expect(html).not_to match(/<option[^>]*>[^<]*<script/)
+      end
+    end
+  end
+end
diff --git a/spec/lib/postal/http/address_guard_spec.rb b/spec/lib/postal/http/address_guard_spec.rb
new file mode 100644
index 000000000..55c6715ac
--- /dev/null
+++ b/spec/lib/postal/http/address_guard_spec.rb
@@ -0,0 +1,192 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+RSpec.describe Postal::HTTP::AddressGuard do
+  describe ".safe_connect_address" do
+    subject(:call) { described_class.safe_connect_address(host) }
+
+    before do
+      allow(Postal::Config.postal).to receive(:allowed_request_destinations).and_return(allowlist)
+    end
+
+    let(:allowlist) { [] }
+
+    context "when given a public IP literal" do
+      let(:host) { "93.184.216.34" }
+
+      it "returns the address to connect to" do
+        expect(call).to eq "93.184.216.34"
+      end
+    end
+
+    context "when given a public IPv6 literal" do
+      let(:host) { "2606:2800:220:1:248:1893:25c8:1946" }
+
+      before { allow(described_class).to receive(:ipv6_supported?).and_return(true) }
+
+      it "returns the address to connect to" do
+        expect(call).to eq "2606:2800:220:1:248:1893:25c8:1946"
+      end
+    end
+
+    [
+      "127.0.0.1",
+      "10.0.0.1",
+      "172.16.5.4",
+      "192.168.1.1",
+      "169.254.169.254", # cloud metadata
+      "100.64.0.1",      # carrier-grade NAT
+      "0.0.0.0",
+      "::1",
+      "fd00::1",         # unique-local IPv6
+      "fe80::1",         # link-local IPv6
+      "::ffff:127.0.0.1", # IPv4-mapped loopback
+    ].each do |blocked|
+      context "when given the blocked address #{blocked}" do
+        let(:host) { blocked }
+
+        it "raises BlockedDestinationError" do
+          expect { call }.to raise_error(Postal::HTTP::BlockedDestinationError)
+        end
+      end
+    end
+
+    context "when given a hostname that resolves to a public address" do
+      let(:host) { "example.com" }
+
+      before do
+        allow(Resolv).to receive(:getaddresses).with(host).and_return(["93.184.216.34"])
+      end
+
+      it "returns the resolved address" do
+        expect(call).to eq "93.184.216.34"
+      end
+    end
+
+    context "when given a hostname that resolves to a private address" do
+      let(:host) { "internal.example.com" }
+
+      before do
+        allow(Resolv).to receive(:getaddresses).with(host).and_return(["10.1.2.3"])
+      end
+
+      it "raises BlockedDestinationError" do
+        expect { call }.to raise_error(Postal::HTTP::BlockedDestinationError)
+      end
+    end
+
+    context "when a hostname resolves to both a public and a private address" do
+      let(:host) { "rebind.example.com" }
+
+      before do
+        allow(Resolv).to receive(:getaddresses).with(host).and_return(["93.184.216.34", "127.0.0.1"])
+      end
+
+      it "raises BlockedDestinationError because one address is blocked" do
+        expect { call }.to raise_error(Postal::HTTP::BlockedDestinationError)
+      end
+    end
+
+    context "when a hostname resolves to both IPv4 and IPv6 addresses" do
+      let(:host) { "dualstack.example.com" }
+      let(:ipv6) { "2606:2800:220:1:248:1893:25c8:1946" }
+
+      before do
+        allow(Resolv).to receive(:getaddresses).with(host).and_return([ipv6, "93.184.216.34"])
+      end
+
+      context "and the server does not support IPv6" do
+        before { allow(described_class).to receive(:ipv6_supported?).and_return(false) }
+
+        it "connects over IPv4" do
+          expect(call).to eq "93.184.216.34"
+        end
+      end
+
+      context "and the server supports IPv6" do
+        before { allow(described_class).to receive(:ipv6_supported?).and_return(true) }
+
+        it "still prefers IPv4 for predictability" do
+          expect(call).to eq "93.184.216.34"
+        end
+      end
+    end
+
+    context "when a hostname resolves only to an IPv6 address" do
+      let(:host) { "v6only.example.com" }
+      let(:ipv6) { "2606:2800:220:1:248:1893:25c8:1946" }
+
+      before do
+        allow(Resolv).to receive(:getaddresses).with(host).and_return([ipv6])
+      end
+
+      context "and the server does not support IPv6" do
+        before { allow(described_class).to receive(:ipv6_supported?).and_return(false) }
+
+        it "raises a SocketError because the address is unreachable" do
+          expect { call }.to raise_error(SocketError)
+        end
+      end
+
+      context "and the server supports IPv6" do
+        before { allow(described_class).to receive(:ipv6_supported?).and_return(true) }
+
+        it "connects over IPv6" do
+          expect(call).to eq ipv6
+        end
+      end
+    end
+
+    context "when a hostname cannot be resolved" do
+      let(:host) { "nope.example.com" }
+
+      before do
+        allow(Resolv).to receive(:getaddresses).with(host).and_return([])
+      end
+
+      it "raises BlockedDestinationError" do
+        expect { call }.to raise_error(Postal::HTTP::BlockedDestinationError, /resolve/)
+      end
+    end
+
+    context "when the host is blank" do
+      let(:host) { "" }
+
+      it "raises BlockedDestinationError" do
+        expect { call }.to raise_error(Postal::HTTP::BlockedDestinationError)
+      end
+    end
+
+    context "when a blocked address is allowlisted by CIDR" do
+      let(:host) { "10.0.0.5" }
+      let(:allowlist) { ["10.0.0.0/8"] }
+
+      it "returns the address" do
+        expect(call).to eq "10.0.0.5"
+      end
+    end
+
+    context "when a blocked address is allowlisted by exact IP" do
+      let(:host) { "127.0.0.1" }
+      let(:allowlist) { ["127.0.0.1"] }
+
+      it "returns the address" do
+        expect(call).to eq "127.0.0.1"
+      end
+    end
+
+    context "when a hostname resolving to a private address is allowlisted by name" do
+      let(:host) { "internal.example.com" }
+      let(:allowlist) { ["internal.example.com"] }
+
+      before do
+        allow(Resolv).to receive(:getaddresses).with(host).and_return(["10.1.2.3"])
+      end
+
+      it "returns the resolved address" do
+        expect(call).to eq "10.1.2.3"
+      end
+    end
+  end
+end
diff --git a/spec/lib/postal/http_spec.rb b/spec/lib/postal/http_spec.rb
new file mode 100644
index 000000000..ce90ff90b
--- /dev/null
+++ b/spec/lib/postal/http_spec.rb
@@ -0,0 +1,80 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+RSpec.describe Postal::HTTP do
+  before do
+    allow(Postal::Config.postal).to receive(:allowed_request_destinations).and_return([])
+  end
+
+  describe ".post" do
+    context "when the host resolves to a blocked address" do
+      before do
+        allow(Resolv).to receive(:getaddresses).with("internal.example.com").and_return(["127.0.0.1"])
+      end
+
+      it "does not make a request and returns a blocked-destination result" do
+        result = described_class.post("http://internal.example.com/hook", json: "{}")
+        expect(result[:code]).to eq(-4)
+        expect(result[:body]).to match(/not permitted/)
+        expect(WebMock).not_to have_requested(:post, "http://internal.example.com/hook")
+      end
+    end
+
+    context "when resolving the host raises an error" do
+      before do
+        allow(Resolv).to receive(:getaddresses).with("example.com").and_raise(Resolv::ResolvError, "resolver failed")
+      end
+
+      it "returns a connection error result" do
+        result = described_class.post("http://example.com/hook", json: "{}")
+        expect(result[:code]).to eq(-2)
+        expect(result[:body]).to match(/resolver failed/)
+        expect(WebMock).not_to have_requested(:post, "http://example.com/hook")
+      end
+    end
+
+    context "when resolving the host exceeds the request timeout" do
+      before do
+        allow(Resolv).to receive(:getaddresses).with("example.com") do
+          sleep 0.2
+          ["93.184.216.34"]
+        end
+      end
+
+      it "returns a timeout result before making a request" do
+        result = described_class.post("http://example.com/hook", json: "{}", timeout: 0.05)
+        expect(result[:code]).to eq(-1)
+        expect(WebMock).not_to have_requested(:post, "http://example.com/hook")
+      end
+    end
+
+    context "when the host resolves to a public address" do
+      before do
+        allow(Resolv).to receive(:getaddresses).with("example.com").and_return(["93.184.216.34"])
+        stub_request(:post, "http://example.com/hook").to_return(status: 200, body: "OK")
+      end
+
+      it "pins the connection to the validated address and performs the request" do
+        expect_any_instance_of(Net::HTTP).to receive(:ipaddr=).with("93.184.216.34").and_call_original
+        result = described_class.post("http://example.com/hook", json: "{}")
+        expect(result[:code]).to eq(200)
+        expect(WebMock).to have_requested(:post, "http://example.com/hook")
+      end
+    end
+
+    context "when the blocked host is allowlisted" do
+      before do
+        allow(Postal::Config.postal).to receive(:allowed_request_destinations).and_return(["internal.example.com"])
+        allow(Resolv).to receive(:getaddresses).with("internal.example.com").and_return(["10.0.0.5"])
+        stub_request(:post, "http://internal.example.com/hook").to_return(status: 200, body: "OK")
+      end
+
+      it "performs the request" do
+        result = described_class.post("http://internal.example.com/hook", json: "{}")
+        expect(result[:code]).to eq(200)
+        expect(WebMock).to have_requested(:post, "http://internal.example.com/hook")
+      end
+    end
+  end
+end
diff --git a/spec/lib/postal/message_db/database_spec.rb b/spec/lib/postal/message_db/database_spec.rb
index ff9eec49e..dc9e96080 100644
--- a/spec/lib/postal/message_db/database_spec.rb
+++ b/spec/lib/postal/message_db/database_spec.rb
@@ -14,5 +14,64 @@
     it "should return the current schema version" do
       expect(database.schema_version).to be_a Integer
     end
+
+    describe "#escape_identifier" do
+      it "wraps a plain identifier in backticks" do
+        expect(database.send(:escape_identifier, "id")).to eq "`id`"
+      end
+
+      it "doubles embedded backticks so the value cannot break out of the quoting" do
+        expect(database.send(:escape_identifier, "id`=0 OR SLEEP(5)#"))
+          .to eq "`id``=0 OR SLEEP(5)#`"
+      end
+
+      it "coerces non-string identifiers to a string" do
+        expect(database.send(:escape_identifier, :token)).to eq "`token`"
+      end
+    end
+
+    describe "#hash_to_sql" do
+      it "builds a simple equality condition" do
+        expect(database.send(:hash_to_sql, { "id" => 5 })).to eq "`id` = '5'"
+      end
+
+      it "builds an IN condition for an array of integers" do
+        expect(database.send(:hash_to_sql, { "id" => [1, 2] })).to eq "`id` IN (1, 2)"
+      end
+
+      it "builds operator conditions for a hash value" do
+        expect(database.send(:hash_to_sql, { "id" => { greater_than: 1 } }))
+          .to eq "`id` > '1'"
+      end
+
+      # Regression tests for GHSA-x2hq-rfpg-3xr5: a backtick in the condition
+      # key must be neutralised so it cannot close the identifier quoting and
+      # inject arbitrary SQL.
+      it "neutralises a backtick injection in an equality key" do
+        sql = database.send(:hash_to_sql, { "id`=0 OR SLEEP(5)#" => "x" })
+        expect(sql).to eq "`id``=0 OR SLEEP(5)#` = 'x'"
+      end
+
+      it "neutralises a backtick injection in an IN key" do
+        sql = database.send(:hash_to_sql, { "id`)#" => %w[a b] })
+        expect(sql).to eq "`id``)#` IN ('a', 'b')"
+      end
+
+      it "neutralises a backtick injection in an operator key" do
+        sql = database.send(:hash_to_sql, { "id`#" => { greater_than: 1 } })
+        expect(sql).to eq "`id``#` > '1'"
+      end
+    end
+
+    describe "#select with a hostile condition key" do
+      # End-to-end proof against the live test database: the injected key is
+      # treated as a single (non-existent) column identifier, so MySQL rejects
+      # the query instead of executing the injected SQL.
+      it "does not allow SQL injection via the condition key" do
+        expect do
+          database.select("messages", where: { "id`=0 OR 1=1#" => "x" }, limit: 1)
+        end.to raise_error(Mysql2::Error)
+      end
+    end
   end
 end
diff --git a/spec/lib/tracking_middleware_spec.rb b/spec/lib/tracking_middleware_spec.rb
new file mode 100644
index 000000000..0335892f3
--- /dev/null
+++ b/spec/lib/tracking_middleware_spec.rb
@@ -0,0 +1,71 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+require "rack/test"
+
+RSpec.describe TrackingMiddleware do
+  include Rack::Test::Methods
+
+  let(:inner_app) { ->(_env) { [200, {}, ["inner"]] } }
+  let(:app) { described_class.new(inner_app) }
+
+  let(:server) { create(:server) }
+  let(:message) do
+    MessageFactory.incoming(server) do |_msg, mail|
+      mail.html_part = Mail::Part.new do
+        content_type "text/html; charset=UTF-8"
+        body "<html><body>hi</body></html>"
+      end
+    end
+  end
+
+  def track_headers
+    { "HTTP_X_POSTAL_TRACK_HOST" => "1" }
+  end
+
+  describe "GET /img/:server_token/:message_token (open tracking pixel)" do
+    before do
+      get "/img/#{server.token}/#{message.token}", {}, track_headers
+    end
+
+    it "returns the tracking pixel PNG" do
+      expect(last_response.status).to eq 200
+      expect(last_response.headers["Content-Type"]).to eq "image/png"
+      expect(last_response.body.bytesize).to be > 0
+    end
+
+    it "records a load for the message" do
+      # Re-fetch the message so loads are read fresh from the DB.
+      reloaded = server.message_db.message(message.id)
+      expect(reloaded.loads.size).to eq 1
+    end
+  end
+
+  describe "GET /img/:server_token/:message_token?src=<url> (image proxy)" do
+    let(:attacker_url) { "http://internal.example.com/secret" }
+
+    before do
+      stub_request(:get, attacker_url).to_return(status: 200, body: "internal-secret")
+    end
+
+    it "does not fetch the URL and returns 400" do
+      get "/img/#{server.token}/#{message.token}", { src: attacker_url }, track_headers
+
+      expect(last_response.status).to eq 400
+      expect(WebMock).not_to have_requested(:get, attacker_url)
+    end
+
+    it "does not fetch the URL even when the message token is invalid" do
+      get "/img/#{server.token}/nonexistent", { src: attacker_url }, track_headers
+
+      expect(WebMock).not_to have_requested(:get, attacker_url)
+    end
+  end
+
+  describe "when the track-host header is missing" do
+    it "passes the request through to the inner app untouched" do
+      get "/img/#{server.token}/#{message.token}"
+      expect(last_response.body).to eq "inner"
+    end
+  end
+end
diff --git a/spec/models/http_endpoint_spec.rb b/spec/models/http_endpoint_spec.rb
new file mode 100644
index 000000000..37a6881d2
--- /dev/null
+++ b/spec/models/http_endpoint_spec.rb
@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+RSpec.describe HTTPEndpoint do
+  describe "validations" do
+    subject(:endpoint) { build(:http_endpoint, url: url) }
+
+    [
+      "https://example.com/messages/~user;v=1?token=a+b#section",
+      "http://example.com:8080/path?x=1&y=2",
+      "https://[2606:2800:220:1:248:1893:25c8:1946]/hook",
+    ].each do |valid_url|
+      context "with #{valid_url}" do
+        let(:url) { valid_url }
+
+        it "is valid" do
+          expect(endpoint).to be_valid
+        end
+      end
+    end
+
+    [
+      "ftp://example.com/hook",
+      "https:///missing-host",
+      "not a url",
+    ].each do |invalid_url|
+      context "with #{invalid_url}" do
+        let(:url) { invalid_url }
+
+        it "is invalid" do
+          expect(endpoint).not_to be_valid
+          expect(endpoint.errors[:url]).to be_present
+        end
+      end
+    end
+  end
+end
diff --git a/spec/rails_helper.rb b/spec/rails_helper.rb
index bf2c9ae9b..9c22de13e 100644
--- a/spec/rails_helper.rb
+++ b/spec/rails_helper.rb
@@ -16,7 +16,7 @@
 DatabaseCleaner.allow_remote_database_url = true
 ActiveRecord::Base.logger = Logger.new("/dev/null")
 
-Dir[File.expand_path("helpers/**/*.rb", __dir__)].each { |f| require f }
+Dir[File.expand_path("helpers/**/*.rb", __dir__)].reject { |f| f.end_with?("_spec.rb") }.each { |f| require f }
 
 ActionMailer::Base.delivery_method = :test
 
diff --git a/spec/requests/messages_controller_spec.rb b/spec/requests/messages_controller_spec.rb
new file mode 100644
index 000000000..524d32b98
--- /dev/null
+++ b/spec/requests/messages_controller_spec.rb
@@ -0,0 +1,58 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+RSpec.describe "MessagesController", type: :request do
+  let(:user) { create(:user, admin: true) }
+  let(:organization) { create(:organization, owner: user) }
+  let(:server) { create(:server, organization: organization) }
+
+  before do
+    post "/login", params: { email_address: user.email_address, password: "passw0rd" }
+  end
+
+  describe "GET /org/:org/servers/:server/messages/:id/html_raw" do
+    let(:xss_payload) { %(<script>alert("XSS")</script>) }
+    let(:message) do
+      payload = xss_payload
+      MessageFactory.incoming(server) do |_msg, mail|
+        mail.html_part = Mail::Part.new do
+          content_type "text/html; charset=UTF-8"
+          body %(<html><body><p>hello</p>#{payload}</body></html>)
+        end
+      end
+    end
+
+    before do
+      get "/org/#{organization.permalink}/servers/#{server.permalink}/messages/#{message.id}/html_raw"
+    end
+
+    it "returns the stored email HTML" do
+      expect(response).to have_http_status(:ok)
+      expect(response.body).to include("hello")
+    end
+
+    it "serves a restrictive Content-Security-Policy that blocks scripts" do
+      csp = response.headers["Content-Security-Policy"]
+      expect(csp).to include("script-src 'none'")
+      expect(csp).to include("default-src 'none'")
+      expect(csp).to include("form-action 'none'")
+      expect(csp).to include("base-uri 'none'")
+    end
+
+    it "sets X-Content-Type-Options and Referrer-Policy on the response" do
+      expect(response.headers["X-Content-Type-Options"]).to eq "nosniff"
+      expect(response.headers["Referrer-Policy"]).to eq "no-referrer"
+    end
+  end
+
+  describe "messages/html view template" do
+    # We assert against the template source rather than rendering it in a
+    # request spec because the full application layout depends on the asset
+    # pipeline which is not configured in this test environment.
+    it "embeds the html_raw view inside a sandboxed iframe" do
+      template = Rails.root.join("app/views/messages/html.html.haml").read
+      expect(template).to match(/%iframe\{[^}]*:sandbox\s*=>/)
+    end
+  end
+end
diff --git a/spec/requests/sessions_controller_spec.rb b/spec/requests/sessions_controller_spec.rb
new file mode 100644
index 000000000..6a98332fa
--- /dev/null
+++ b/spec/requests/sessions_controller_spec.rb
@@ -0,0 +1,71 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+RSpec.describe "SessionsController", type: :request do
+  let(:user) { create(:user) }
+
+  describe "POST /login with return_to" do
+    def login_with(return_to:)
+      post "/login", params: {
+        email_address: user.email_address,
+        password: "passw0rd",
+        return_to: return_to
+      }
+    end
+
+    shared_examples "rejects unsafe return_to" do
+      it "does not redirect to the attacker-controlled location" do
+        login_with(return_to: unsafe_path)
+
+        expect(response).to have_http_status(:found)
+        # Whatever the fallback is, it must be same-origin: a Location that
+        # either omits a host or points at our own host. A browser must not
+        # end up at attacker.example.
+        location = response.location
+        expect(location).not_to include("attacker.example")
+        # Reject protocol-relative and absolute redirects entirely.
+        expect(location).not_to match(%r{\A//})
+        expect(location).not_to match(%r{\Ahttps?://attacker})
+      end
+    end
+
+    context "with a protocol-relative URL (//host)" do
+      let(:unsafe_path) { "//attacker.example/phish" }
+      include_examples "rejects unsafe return_to"
+    end
+
+    context "with a backslash-prefixed URL (/\\host)" do
+      let(:unsafe_path) { "/\\attacker.example/phish" }
+      include_examples "rejects unsafe return_to"
+    end
+
+    context "with an absolute http(s) URL" do
+      let(:unsafe_path) { "https://attacker.example/phish" }
+      include_examples "rejects unsafe return_to"
+    end
+
+    context "with a javascript: URL" do
+      let(:unsafe_path) { "javascript:alert(1)" }
+      include_examples "rejects unsafe return_to"
+    end
+
+    context "with a safe relative path" do
+      it "honours the return_to" do
+        login_with(return_to: "/org/acme/settings")
+        expect(response).to redirect_to("/org/acme/settings")
+      end
+    end
+
+    context "with no return_to" do
+      it "redirects to the default root" do
+        post "/login", params: {
+          email_address: user.email_address,
+          password: "passw0rd"
+        }
+        expect(response).to have_http_status(:found)
+        expect(response.location).not_to match(%r{\A//})
+      end
+    end
+  end
+end
diff --git a/spec/services/webhook_delivery_service_spec.rb b/spec/services/webhook_delivery_service_spec.rb
index 14b0f5967..14a9fa228 100644
--- a/spec/services/webhook_delivery_service_spec.rb
+++ b/spec/services/webhook_delivery_service_spec.rb
@@ -13,6 +13,7 @@
   let(:response_body) { "OK" }
 
   before do
+    allow(Resolv).to receive(:getaddresses).with("example.com").and_return(["93.184.216.34"])
     stub_request(:post, webhook.url).to_return(status: response_status, body: response_body)
   end
 
@@ -116,5 +117,26 @@
         expect { webhook_request.reload }.to raise_error(ActiveRecord::RecordNotFound)
       end
     end
+
+    context "when the webhook URL resolves to a blocked (private) address" do
+      let(:webhook_request) do
+        create(:webhook_request, :locked, webhook: webhook, url: "http://internal.example.com/hook")
+      end
+
+      before do
+        allow(Resolv).to receive(:getaddresses).with("internal.example.com").and_return(["127.0.0.1"])
+      end
+
+      it "does not make a request to the destination" do
+        service.call
+        expect(WebMock).not_to have_requested(:post, "http://internal.example.com/hook")
+      end
+
+      it "records the failure and schedules a retry" do
+        service.call
+        expect(webhook_request.reload.attempts).to eq(1)
+        expect(webhook_request.retry_after).to be_present
+      end
+    end
   end
 end