Fix unbounded recursion in account discovery (#22025)

* Fix trying to fetch posts from other users when fetching featured posts

* Rate-limit discovery of new subdomains

* Put a limit on recursively discovering new accounts

12 files changed, 147 insertions, 18 deletions

diff --git a/Gemfile b/Gemfile
index acb1ac53c..a399f5102 100644
--- a/Gemfile
+++ b/Gemfile
@@ -66,6 +66,7 @@ gem 'oj', '~> 3.13'
 gem 'ox', '~> 2.14'
 gem 'parslet'
 gem 'posix-spawn'
+gem 'public_suffix', '~> 5.0'
 gem 'pundit', '~> 2.2'
 gem 'premailer-rails'
 gem 'rack-attack', '~> 6.6'
diff --git a/Gemfile.lock b/Gemfile.lock
index 390f8d9f0..f7d87d2a6 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -819,6 +819,7 @@ DEPENDENCIES
   private_address_check (~> 0.5)
   pry-byebug (~> 3.10)
   pry-rails (~> 0.3)
+  public_suffix (~> 5.0)
   puma (~> 5.6)
   pundit (~> 2.2)
   rack (~> 2.2.4)
diff --git a/app/lib/activitypub/activity/create.rb b/app/lib/activitypub/activity/create.rb
index 73882e134..b15e66ca2 100644
--- a/app/lib/activitypub/activity/create.rb
+++ b/app/lib/activitypub/activity/create.rb
@@ -222,7 +222,7 @@ class ActivityPub::Activity::Create < ActivityPub::Activity
     return if tag['href'].blank?
 
     account = account_from_uri(tag['href'])
-    account = ActivityPub::FetchRemoteAccountService.new.call(tag['href']) if account.nil?
+    account = ActivityPub::FetchRemoteAccountService.new.call(tag['href'], request_id: @options[:request_id]) if account.nil?
 
     return if account.nil?
 
diff --git a/app/lib/activitypub/activity/update.rb b/app/lib/activitypub/activity/update.rb
index 5b3238ece..e7c3bc9bf 100644
--- a/app/lib/activitypub/activity/update.rb
+++ b/app/lib/activitypub/activity/update.rb
@@ -18,7 +18,7 @@ class ActivityPub::Activity::Update < ActivityPub::Activity
   def update_account
     return reject_payload! if @account.uri != object_uri
 
-    ActivityPub::ProcessAccountService.new.call(@account.username, @account.domain, @object, signed_with_known_key: true)
+    ActivityPub::ProcessAccountService.new.call(@account.username, @account.domain, @object, signed_with_known_key: true, request_id: @options[:request_id])
   end
 
   def update_status
@@ -28,6 +28,6 @@ class ActivityPub::Activity::Update < ActivityPub::Activity
 
     return if @status.nil?
 
-    ActivityPub::ProcessStatusUpdateService.new.call(@status, @object)
+    ActivityPub::ProcessStatusUpdateService.new.call(@status, @object, request_id: @options[:request_id])
   end
 end
diff --git a/app/models/concerns/domain_materializable.rb b/app/models/concerns/domain_materializable.rb
index 88337f8c0..0eac6878e 100644
--- a/app/models/concerns/domain_materializable.rb
+++ b/app/models/concerns/domain_materializable.rb
@@ -3,11 +3,24 @@
 module DomainMaterializable
   extend ActiveSupport::Concern
 
+  include Redisable
+
   included do
     after_create_commit :refresh_instances_view
   end
 
   def refresh_instances_view
-    Instance.refresh unless domain.nil? || Instance.where(domain: domain).exists?
+    return if domain.nil? || Instance.exists?(domain: domain)
+
+    Instance.refresh
+    count_unique_subdomains!
+  end
+
+  def count_unique_subdomains!
+    second_and_top_level_domain = PublicSuffix.domain(domain, ignore_private: true)
+    with_redis do |redis|
+      redis.pfadd("unique_subdomains_for:#{second_and_top_level_domain}", domain)
+      redis.expire("unique_subdomains_for:#{second_and_top_level_domain}", 1.minute.seconds)
+    end
   end
 end
diff --git a/app/services/activitypub/fetch_featured_collection_service.rb b/app/services/activitypub/fetch_featured_collection_service.rb
index 50a187ad9..a746ef4d6 100644
--- a/app/services/activitypub/fetch_featured_collection_service.rb
+++ b/app/services/activitypub/fetch_featured_collection_service.rb
@@ -46,9 +46,9 @@ class ActivityPub::FetchFeaturedCollectionService < BaseService
       next unless item.is_a?(String) || item['type'] == 'Note'
 
       uri = value_or_id(item)
-      next if ActivityPub::TagManager.instance.local_uri?(uri)
+      next if ActivityPub::TagManager.instance.local_uri?(uri) || invalid_origin?(uri)
 
-      status = ActivityPub::FetchRemoteStatusService.new.call(uri, on_behalf_of: local_follower)
+      status = ActivityPub::FetchRemoteStatusService.new.call(uri, on_behalf_of: local_follower, expected_actor_uri: @account.uri, request_id: @options[:request_id])
       next unless status&.account_id == @account.id
 
       status.id
diff --git a/app/services/activitypub/fetch_remote_account_service.rb b/app/services/activitypub/fetch_remote_account_service.rb
index ca7a8c6ca..7aba8269e 100644
--- a/app/services/activitypub/fetch_remote_account_service.rb
+++ b/app/services/activitypub/fetch_remote_account_service.rb
@@ -2,7 +2,7 @@
 
 class ActivityPub::FetchRemoteAccountService < ActivityPub::FetchRemoteActorService
   # Does a WebFinger roundtrip on each call, unless `only_key` is true
-  def call(uri, id: true, prefetched_body: nil, break_on_redirect: false, only_key: false, suppress_errors: true)
+  def call(uri, id: true, prefetched_body: nil, break_on_redirect: false, only_key: false, suppress_errors: true, request_id: nil)
     actor = super
     return actor if actor.nil? || actor.is_a?(Account)
 
diff --git a/app/services/activitypub/fetch_remote_actor_service.rb b/app/services/activitypub/fetch_remote_actor_service.rb
index db09c38d8..a25fa54c4 100644
--- a/app/services/activitypub/fetch_remote_actor_service.rb
+++ b/app/services/activitypub/fetch_remote_actor_service.rb
@@ -10,7 +10,7 @@ class ActivityPub::FetchRemoteActorService < BaseService
   SUPPORTED_TYPES = %w(Application Group Organization Person Service).freeze
 
   # Does a WebFinger roundtrip on each call, unless `only_key` is true
-  def call(uri, id: true, prefetched_body: nil, break_on_redirect: false, only_key: false, suppress_errors: true)
+  def call(uri, id: true, prefetched_body: nil, break_on_redirect: false, only_key: false, suppress_errors: true, request_id: nil)
     return if domain_not_allowed?(uri)
     return ActivityPub::TagManager.instance.uri_to_actor(uri) if ActivityPub::TagManager.instance.local_uri?(uri)
 
@@ -35,7 +35,7 @@ class ActivityPub::FetchRemoteActorService < BaseService
 
     check_webfinger! unless only_key
 
-    ActivityPub::ProcessAccountService.new.call(@username, @domain, @json, only_key: only_key, verified_webfinger: !only_key)
+    ActivityPub::ProcessAccountService.new.call(@username, @domain, @json, only_key: only_key, verified_webfinger: !only_key, request_id: request_id)
   rescue Error => e
     Rails.logger.debug "Fetching actor #{uri} failed: #{e.message}"
     raise unless suppress_errors
diff --git a/app/services/activitypub/fetch_remote_status_service.rb b/app/services/activitypub/fetch_remote_status_service.rb
index 803098245..21b9242f8 100644
--- a/app/services/activitypub/fetch_remote_status_service.rb
+++ b/app/services/activitypub/fetch_remote_status_service.rb
@@ -4,7 +4,8 @@ class ActivityPub::FetchRemoteStatusService < BaseService
   include JsonLdHelper
 
   # Should be called when uri has already been checked for locality
-  def call(uri, id: true, prefetched_body: nil, on_behalf_of: nil)
+  def call(uri, id: true, prefetched_body: nil, on_behalf_of: nil, expected_actor_uri: nil, request_id: nil)
+    @request_id = request_id
     @json = begin
       if prefetched_body.nil?
         fetch_resource(uri, id, on_behalf_of)
@@ -30,6 +31,7 @@ class ActivityPub::FetchRemoteStatusService < BaseService
     end
 
     return if activity_json.nil? || object_uri.nil? || !trustworthy_attribution?(@json['id'], actor_uri)
+    return if expected_actor_uri.present? && actor_uri != expected_actor_uri
     return ActivityPub::TagManager.instance.uri_to_resource(object_uri, Status) if ActivityPub::TagManager.instance.local_uri?(object_uri)
 
     actor = account_from_uri(actor_uri)
@@ -40,7 +42,7 @@ class ActivityPub::FetchRemoteStatusService < BaseService
     # activity as an update rather than create
     activity_json['type'] = 'Update' if equals_or_includes_any?(activity_json['type'], %w(Create)) && Status.where(uri: object_uri, account_id: actor.id).exists?
 
-    ActivityPub::Activity.factory(activity_json, actor).perform
+    ActivityPub::Activity.factory(activity_json, actor, request_id: request_id).perform
   end
 
   private
@@ -52,7 +54,7 @@ class ActivityPub::FetchRemoteStatusService < BaseService
 
   def account_from_uri(uri)
     actor = ActivityPub::TagManager.instance.uri_to_resource(uri, Account)
-    actor = ActivityPub::FetchRemoteAccountService.new.call(uri, id: true) if actor.nil? || actor.possibly_stale?
+    actor = ActivityPub::FetchRemoteAccountService.new.call(uri, id: true, request_id: @request_id) if actor.nil? || actor.possibly_stale?
     actor
   end
 
diff --git a/app/services/activitypub/process_account_service.rb b/app/services/activitypub/process_account_service.rb
index 99bcb3835..2da9096c7 100644
--- a/app/services/activitypub/process_account_service.rb
+++ b/app/services/activitypub/process_account_service.rb
@@ -6,6 +6,9 @@ class ActivityPub::ProcessAccountService < BaseService
   include Redisable
   include Lockable
 
+  SUBDOMAINS_RATELIMIT = 10
+  DISCOVERIES_PER_REQUEST = 400
+
   # Should be called with confirmed valid JSON
   # and WebFinger-resolved username and domain
   def call(username, domain, json, options = {})
@@ -15,9 +18,12 @@ class ActivityPub::ProcessAccountService < BaseService
     @json        = json
     @uri         = @json['id']
     @username    = username
-    @domain      = domain
+    @domain      = TagManager.instance.normalize_domain(domain)
     @collections = {}
 
+    # The key does not need to be unguessable, it just needs to be somewhat unique
+    @options[:request_id] ||= "#{Time.now.utc.to_i}-#{username}@#{domain}"
+
     with_lock("process_account:#{@uri}") do
       @account            = Account.remote.find_by(uri: @uri) if @options[:only_key]
       @account          ||= Account.find_remote(@username, @domain)
@@ -25,7 +31,18 @@ class ActivityPub::ProcessAccountService < BaseService
       @old_protocol       = @account&.protocol
       @suspension_changed = false
 
-      create_account if @account.nil?
+      if @account.nil?
+        with_redis do |redis|
+          return nil if redis.pfcount("unique_subdomains_for:#{PublicSuffix.domain(@domain, ignore_private: true)}") >= SUBDOMAINS_RATELIMIT
+
+          discoveries = redis.incr("discovery_per_request:#{@options[:request_id]}")
+          redis.expire("discovery_per_request:#{@options[:request_id]}", 5.minutes.seconds)
+          return nil if discoveries > DISCOVERIES_PER_REQUEST
+        end
+
+        create_account
+      end
+
       update_account
       process_tags
 
@@ -150,7 +167,7 @@ class ActivityPub::ProcessAccountService < BaseService
   end
 
   def check_featured_collection!
-    ActivityPub::SynchronizeFeaturedCollectionWorker.perform_async(@account.id, { 'hashtag' => @json['featuredTags'].blank? })
+    ActivityPub::SynchronizeFeaturedCollectionWorker.perform_async(@account.id, { 'hashtag' => @json['featuredTags'].blank?, 'request_id' => @options[:request_id] })
   end
 
   def check_featured_tags_collection!
@@ -254,7 +271,7 @@ class ActivityPub::ProcessAccountService < BaseService
 
   def moved_account
     account   = ActivityPub::TagManager.instance.uri_to_resource(@json['movedTo'], Account)
-    account ||= ActivityPub::FetchRemoteAccountService.new.call(@json['movedTo'], id: true, break_on_redirect: true)
+    account ||= ActivityPub::FetchRemoteAccountService.new.call(@json['movedTo'], id: true, break_on_redirect: true, request_id: @options[:request_id])
     account
   end
 
diff --git a/app/services/activitypub/process_status_update_service.rb b/app/services/activitypub/process_status_update_service.rb
index a0605b1a3..fad19f87f 100644
--- a/app/services/activitypub/process_status_update_service.rb
+++ b/app/services/activitypub/process_status_update_service.rb
@@ -5,7 +5,7 @@ class ActivityPub::ProcessStatusUpdateService < BaseService
   include Redisable
   include Lockable
 
-  def call(status, json)
+  def call(status, json, request_id: nil)
     raise ArgumentError, 'Status has unsaved changes' if status.changed?
 
     @json                      = json
@@ -15,6 +15,7 @@ class ActivityPub::ProcessStatusUpdateService < BaseService
     @account                   = status.account
     @media_attachments_changed = false
     @poll_changed              = false
+    @request_id                = request_id
 
     # Only native types can be updated at the moment
     return @status if !expected_type? || already_updated_more_recently?
@@ -191,7 +192,7 @@ class ActivityPub::ProcessStatusUpdateService < BaseService
       next if href.blank?
 
       account   = ActivityPub::TagManager.instance.uri_to_resource(href, Account)
-      account ||= ActivityPub::FetchRemoteAccountService.new.call(href)
+      account ||= ActivityPub::FetchRemoteAccountService.new.call(href, request_id: @request_id)
 
       next if account.nil?
 
diff --git a/spec/services/activitypub/process_account_service_spec.rb b/spec/services/activitypub/process_account_service_spec.rb
index 7728b9ba8..2b20d17b1 100644
--- a/spec/services/activitypub/process_account_service_spec.rb
+++ b/spec/services/activitypub/process_account_service_spec.rb
@@ -109,4 +109,98 @@ RSpec.describe ActivityPub::ProcessAccountService, type: :service do
       end
     end
   end
+
+  context 'discovering many subdomains in a short timeframe' do
+    before do
+      stub_const 'ActivityPub::ProcessAccountService::SUBDOMAINS_RATELIMIT', 5
+    end
+
+    let(:subject) do
+      8.times do |i|
+        domain = "test#{i}.testdomain.com"
+        json = {
+          id: "https://#{domain}/users/1",
+          type: 'Actor',
+          inbox: "https://#{domain}/inbox",
+        }.with_indifferent_access
+        described_class.new.call('alice', domain, json)
+      end
+    end
+
+    it 'creates at least some accounts' do
+      expect { subject }.to change { Account.remote.count }.by_at_least(2)
+    end
+
+    it 'creates no more account than the limit allows' do
+      expect { subject }.to change { Account.remote.count }.by_at_most(5)
+    end
+  end
+
+  context 'accounts referencing other accounts' do
+    before do
+      stub_const 'ActivityPub::ProcessAccountService::DISCOVERIES_PER_REQUEST', 5
+    end
+
+    let(:payload) do
+      {
+        '@context': ['https://www.w3.org/ns/activitystreams'],
+        id: 'https://foo.test/users/1',
+        type: 'Person',
+        inbox: 'https://foo.test/inbox',
+        featured: 'https://foo.test/users/1/featured',
+        preferredUsername: 'user1',
+      }.with_indifferent_access
+    end
+
+    before do
+      8.times do |i|
+        actor_json = {
+          '@context': ['https://www.w3.org/ns/activitystreams'],
+          id: "https://foo.test/users/#{i}",
+          type: 'Person',
+          inbox: 'https://foo.test/inbox',
+          featured: "https://foo.test/users/#{i}/featured",
+          preferredUsername: "user#{i}",
+        }.with_indifferent_access
+        status_json = {
+          '@context': ['https://www.w3.org/ns/activitystreams'],
+          id: "https://foo.test/users/#{i}/status",
+          attributedTo: "https://foo.test/users/#{i}",
+          type: 'Note',
+          content: "@user#{i + 1} test",
+          tag: [
+            {
+              type: 'Mention',
+              href: "https://foo.test/users/#{i + 1}",
+              name: "@user#{i + 1 }",
+            }
+          ],
+          to: [ 'as:Public', "https://foo.test/users/#{i + 1}" ]
+        }.with_indifferent_access
+        featured_json = {
+          '@context': ['https://www.w3.org/ns/activitystreams'],
+          id: "https://foo.test/users/#{i}/featured",
+          type: 'OrderedCollection',
+          totelItems: 1,
+          orderedItems: [status_json],
+        }.with_indifferent_access
+        webfinger = {
+          subject: "acct:user#{i}@foo.test",
+          links: [{ rel: 'self', href: "https://foo.test/users/#{i}" }],
+        }.with_indifferent_access
+        stub_request(:get, "https://foo.test/users/#{i}").to_return(status: 200, body: actor_json.to_json, headers: { 'Content-Type': 'application/activity+json' })
+        stub_request(:get, "https://foo.test/users/#{i}/featured").to_return(status: 200, body: featured_json.to_json, headers: { 'Content-Type': 'application/activity+json' })
+        stub_request(:get, "https://foo.test/users/#{i}/status").to_return(status: 200, body: status_json.to_json, headers: { 'Content-Type': 'application/activity+json' })
+        stub_request(:get, "https://foo.test/.well-known/webfinger?resource=acct:user#{i}@foo.test").to_return(body: webfinger.to_json, headers: { 'Content-Type': 'application/jrd+json' })
+      end
+    end
+
+    it 'creates at least some accounts' do
+      expect { subject.call('user1', 'foo.test', payload) }.to change { Account.remote.count }.by_at_least(2)
+    end
+
+    it 'creates no more account than the limit allows' do
+      expect { subject.call('user1', 'foo.test', payload) }.to change { Account.remote.count }.by_at_most(5)
+    end
+  end
 end