Fix `Update` importing old previously-unknown activities and treating them as recent ones (#36848)

app/lib/activitypub/activity/update.rb

@@ -1,6 +1,9 @@
 # frozen_string_literal: true
 
 class ActivityPub::Activity::Update < ActivityPub::Activity
+  # Updates to unknown objects older than that are ignored
+  OBJECT_AGE_THRESHOLD = 1.day
+
   def perform
     @account.schedule_refresh_if_stale!
 
@@ -28,6 +31,9 @@ class ActivityPub::Activity::Update < ActivityPub::Activity
 
     @status = Status.find_by(uri: object_uri, account_id: @account.id)
 
+    # Ignore updates for old unknown objects, since those are updates we are not interested in
+    return if @status.nil? && object_too_old?
+
     # We may be getting `Create` and `Update` out of order
     @status ||= ActivityPub::Activity::Create.new(@json, @account, **@options).perform
 
@@ -35,4 +41,10 @@ class ActivityPub::Activity::Update < ActivityPub::Activity
 
     ActivityPub::ProcessStatusUpdateService.new.call(@status, @json, @object, request_id: @options[:request_id])
   end
+
+  def object_too_old?
+    @object['published'].present? && @object['published'].to_datetime < OBJECT_AGE_THRESHOLD.ago
+  rescue Date::Error
+    false
+  end
 end

spec/lib/activitypub/activity_spec.rb

@@ -34,6 +34,8 @@ RSpec.describe ActivityPub::Activity do
       }
     end
 
+    let(:publication_date) { 1.hour.ago.utc }
+
     let(:create_json) do
       {
         '@context': [
@@ -52,7 +54,7 @@ RSpec.describe ActivityPub::Activity do
             'https://www.w3.org/ns/activitystreams#Public',
           ],
           content: 'foo',
-          published: '2025-05-24T11:03:10Z',
+          published: publication_date.iso8601,
           quote: ActivityPub::TagManager.instance.uri_for(quoted_status),
         },
       }.deep_stringify_keys
@@ -77,7 +79,7 @@ RSpec.describe ActivityPub::Activity do
             'https://www.w3.org/ns/activitystreams#Public',
           ],
           content: 'foo',
-          published: '2025-05-24T11:03:10Z',
+          published: publication_date.iso8601,
           quote: ActivityPub::TagManager.instance.uri_for(quoted_status),
           quoteAuthorization: approval_uri,
         },