discourse/app/controllers/admin/site_settings_controller.rb

class Admin::SiteSettingsController < Admin::AdminController
  rescue_from Discourse::InvalidParameters do |e|
    render_json_error e.message, status: 422
  end

  def index
    render_json_dump(site_settings: SiteSetting.all_settings, diags: SiteSetting.diags)
  end

  def update
    params.require(:id)
    id = params[:id]
    value = params[id]
    value.strip! if value.is_a?(String)
    raise_access_hidden_setting(id)
    SiteSetting.set_and_log(id, value, current_user)
    render nothing: true
  end

  private

  def raise_access_hidden_setting(id)
    # note, as of Ruby 2.3 symbols are GC'd so this is considered safe
    if SiteSetting.hidden_settings.include?(id.to_sym)
      raise Discourse::InvalidParameters, "You are not allowed to change hidden settings"
    end
  end

end