Rails: Filter sensitive information from external network requests

This was lifted from an existing [blog post](https://thoughtbot.com/blog/parameter-filtering).

Author: stevepolitodesign

rails/README.md

@@ -84,6 +84,7 @@ Guidance on ActiveRecord, ActiveModel, and other model objects.
 ## Security
 
 - Set [config.sandbox_by_default][sandbox] to `true` in production-like environments to avoid accidental writing to the production database.
+- Filter sensitive information from external network requests. [Example](/rails/how-to/filter-sensitive-information-from-external-network-requests.md).
 
 [sandbox]: https://guides.rubyonrails.org/configuring.html#config-sandbox-by-default
 

rails/how-to/filter-sensitive-information-from-external-network-requests.md

@@ -0,0 +1,94 @@
+# Filter sensitive information from external network requests
+
+If you’re using Faraday, it’s your responsibility to [filter sensitive
+information][1] when logging requests. This does not happen by default.
+
+```ruby
+conn = Faraday.new(url: "http://httpbingo.org") do |builder|
+  builder.response :logger do | logger |
+    logger.filter(/(api_key=)([^&]+)/, '\1[REMOVED]')
+  end
+end
+```
+
+However, since Rails provides an [API for filtering parameters][2], we can
+create a [custom formatter][3] to re-use that configuration.
+
+```ruby
+conn = Faraday.new(url: "http://httpbingo.org") do |builder|
+  builder.response :logger, nil, {
+    formatter: ApplicationFormatter
+  }
+end
+
+class ApplicationFormatter < Faraday::Logging::Formatter
+  def request(env)
+    info("Request") { log_url(env.url) }
+    info("Request") { log_body(env.body) } if env.body && log_body?
+  end
+
+  def response(env)
+    info("Response") { log_url(env.url) }
+    info("Response") { log_body(env.body) } if env.body && log_body?
+  end
+
+  private
+
+  # Re-uses existing configuration from config/initializers/filter_parameter_logging.rb
+  def filter_parameters
+    @filter_parameters ||= Rails.configuration.filter_parameters
+  end
+
+  # Filters parameters
+  def parameter_filter(**options)
+    ActiveSupport::ParameterFilter.new(filter_parameters, **options)
+  end
+
+  def parse_json(json)
+    JSON.parse(json, object_class: HashWithIndifferentAccess)
+  end
+
+  def log_body?
+    @options[:bodies]
+  end
+
+  def log_body(body)
+    result = walk(body)
+
+    parameter_filter.filter(result).pretty_inspect
+  end
+
+  def log_url(url)
+    filtered_url = filter_url(url)
+
+    filtered_url.to_s
+  end
+
+  def filter_url(url)
+    return url if url.query.nil?
+
+    params = URI.decode_www_form(url.query).to_h
+    filtered_params = parameter_filter(mask: "FILTERED").filter(params)
+    url.query = URI.encode_www_form(filtered_params)
+  end
+
+  def walk(obj)
+    case obj
+    when Hash
+      obj.transform_values { walk(_1) }
+    when Array
+      obj.map { walk(_1) }
+    when String
+      parse_json(obj)
+    else
+      obj
+    end
+  rescue JSON::ParserError
+    obj
+  end
+end
+```
+
+[1]: https://lostisland.github.io/faraday/#/middleware/included/logging?id=filter-sensitive-information
+[2]: https://guides.rubyonrails.org/action_controller_advanced_topics.html#parameters-filtering
+[3]: https://lostisland.github.io/faraday/#/middleware/included/logging?id=customize-the-formatter