Enable filtering of query string parameters

Adds a configurable web_request event query string filter via ActiveSupport::ParameterFilter mechanism.

lib/dfe/analytics.rb

@@ -18,6 +18,7 @@
 require 'dfe/analytics/middleware/request_identity'
 require 'dfe/analytics/middleware/send_cached_page_request_event'
 require 'dfe/analytics/railtie'
+require 'active_support/parameter_filter'
 
 module DfE
   module Analytics
@@ -63,6 +64,8 @@ def self.config
         pseudonymise_web_request_user_id
         entity_table_checks_enabled
         rack_page_cached
+        filter_web_request_events
+        web_request_event_filter
       ]
 
       @config ||= Struct.new(*configurables).new
@@ -86,6 +89,8 @@ def self.configure
       config.pseudonymise_web_request_user_id ||= false
       config.entity_table_checks_enabled      ||= false
       config.rack_page_cached                 ||= proc { |_rack_env| false }
+      config.filter_web_request_events        ||= false
+      config.web_request_event_filter         ||= ActiveSupport::ParameterFilter.new(Rails.application.config.filter_parameters)
     end
 
     def self.initialize!
@@ -244,5 +249,13 @@ def self.rack_page_cached?(rack_env)
     def self.entity_table_checks_enabled?
       config.entity_table_checks_enabled
     end
+
+    def self.filter_web_request_events?
+      config.filter_web_request_events
+    end
+
+    def self.web_request_event_filter
+      config.web_request_event_filter
+    end
   end
 end

lib/dfe/analytics/event.rb

@@ -1,10 +1,13 @@
 # frozen_string_literal: true
 
 require 'active_support/values/time_zone'
+require 'dfe/analytics/query_string_utilities'
 
 module DfE
   module Analytics
     class Event
+      include QueryStringUtilities
+
       EVENT_TYPES = %w[
         web_request create_entity update_entity delete_entity import_entity initialise_analytics entity_table_check
       ].freeze
@@ -37,7 +40,7 @@ def with_request_details(rack_request)
           request_user_agent: ensure_utf8(rack_request.user_agent),
           request_method: rack_request.method,
           request_path: ensure_utf8(rack_request.path),
-          request_query: hash_to_kv_pairs(Rack::Utils.parse_query(rack_request.query_string)),
+          request_query: hash_to_kv_pairs(query_string_to_hash(rack_request.query_string)),
           request_referer: ensure_utf8(rack_request.referer),
           anonymised_user_agent_and_ip: anonymised_user_agent_and_ip(rack_request)
         )

lib/dfe/analytics/query_string_utilities.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+module DfE
+  module Analytics
+    # Utility methods for query string processing
+    module QueryStringUtilities
+      def query_string_to_hash(query_string)
+        hash = Rack::Utils.parse_query(query_string)
+        hash = filter_query_string(hash) if DfE::Analytics.filter_web_request_events?
+        hash
+      end
+
+      def filter_query_string(hash)
+        DfE::Analytics.web_request_event_filter.filter(hash)
+      end
+    end
+  end
+end

spec/dfe/analytics/event_spec.rb

@@ -209,6 +209,33 @@ def as_json
     end
   end
 
+  describe 'with_request_details using parameter filter' do
+    subject do
+      request = fake_request(
+        query_string: 'some_param=123&some_other_param=456'
+      )
+
+      described_class.new
+        .with_request_details(request)
+        .as_json['request_query']
+    end
+
+    before do
+      allow(DfE::Analytics).to receive(:web_request_event_filter).and_return(
+        ActiveSupport::ParameterFilter.new([:some_param])
+      )
+      allow(DfE::Analytics).to receive(:filter_web_request_events?).and_return(true)
+    end
+
+    it 'filters specified parameter values' do
+      expect(subject).to include({ 'key' => 'some_param', 'value' => ['[FILTERED]'] })
+    end
+
+    it 'does not filter unspecified parameter values' do
+      expect(subject).to include({ 'key' => 'some_other_param', 'value' => ['456'] })
+    end
+  end
+
   def fake_request(overrides = {})
     attrs = {
       uuid: '123',

spec/dfe/analytics/requests_spec.rb

@@ -156,4 +156,32 @@ def index
       end).to have_been_made
     end
   end
+
+  context 'with request filtering' do
+    before do
+      allow(DfE::Analytics).to receive(:web_request_event_filter).and_return(
+        ActiveSupport::ParameterFilter.new([:array_param])
+      )
+      allow(DfE::Analytics).to receive(:filter_web_request_events?).and_return(true)
+    end
+
+    it 'filters out sensitive params' do
+      request = stub_analytics_event_submission
+      DfE::Analytics::Testing.webmock! do
+        perform_enqueued_jobs do
+          get('/example/path',
+              params: { page: '1', per_page: '25', array_param: %w[1 2] },
+              headers: { 'HTTP_USER_AGENT' => 'Test agent' })
+        end
+      end
+
+      expect(request.with do |req|
+        body = JSON.parse(req.body)
+        payload = body['rows'].first['json']
+        expect(payload['request_query']).to include(
+          { 'key' => 'array_param[]', 'value' => ['[FILTERED]'] }
+        )
+      end).to have_been_made
+    end
+  end
 end