csp plug: add support for certificate transparency

config/config.exs

@@ -179,7 +179,8 @@ config :pleroma, :suggestions,
 config :pleroma, :csp,
   enabled: true,
   sts: false,
-  sts_max_age: 31_536_000
+  sts_max_age: 31_536_000,
+  ct_max_age: 2_592_000
 
 config :cors_plug,
   max_age: 86_400,

config/config.md

@@ -85,3 +85,4 @@ This section is used to configure Pleroma-FE, unless ``:managed_config`` in ``:i
 * ``enabled``: Whether the managed content security policy is enabled
 * ``sts``: Whether to additionally send a `Strict-Transport-Security` header
 * ``sts_max_age``: The maximum age for the `Strict-Transport-Security` header if sent
+* ``ct_max_age``: The maximum age for the `Except-CT` header if sent

lib/pleroma/plugs/csp_plug.ex

@@ -44,10 +44,12 @@ defmodule Pleroma.Plugs.CSPPlug do
   end
 
   defp maybe_send_sts_header(conn, true) do
-    max_age = Config.get([:csp, :sts_max_age])
+    max_age_sts = Config.get([:csp, :sts_max_age])
+    max_age_ct = Config.get([:csp, :ct_max_age])
 
     merge_resp_headers(conn, [
-      {"strict-transport-security", "max-age=#{max_age}; includeSubDomains"}
+      {"strict-transport-security", "max-age=#{max_age_sts}; includeSubDomains"},
+      {"expect-ct", "enforce, max-age=#{max_age_ct}"}
     ])
   end