lib/pleroma/reverse_proxy.ex

@@ -251,6 +251,7 @@ defmodule Pleroma.ReverseProxy do
     |> Enum.filter(fn {k, _} -> k in @keep_resp_headers end)
     |> build_resp_cache_headers(opts)
     |> build_resp_content_disposition_header(opts)
+    |> build_csp_headers()
     |> Keyword.merge(Keyword.get(opts, :resp_headers, []))
   end
 
@@ -316,6 +317,10 @@ defmodule Pleroma.ReverseProxy do
     end
   end
 
+  defp build_csp_headers(headers) do
+    List.keystore(headers, "content-security-policy", 0, {"content-security-policy", "sandbox"})
+  end
+
   defp header_length_constraint(headers, limit) when is_integer(limit) and limit > 0 do
     with {_, size} <- List.keyfind(headers, "content-length", 0),
          {size, _} <- Integer.parse(size),