app/services/post_status_service.rb

@@ -93,7 +93,7 @@ class PostStatusService < BaseService
 
     raise Mastodon::ValidationError, I18n.t('media_attachments.validations.too_many') if @options[:media_ids].size > 4
 
-    @media = MediaAttachment.where(status_id: nil).where(id: @options[:media_ids].take(4).map(&:to_i))
+    @media = @account.media_attachments.where(status_id: nil).where(id: @options[:media_ids].take(4).map(&:to_i))
 
     raise Mastodon::ValidationError, I18n.t('media_attachments.validations.images_and_video') if @media.size > 1 && @media.find(&:video?)
   end

spec/services/post_status_service_spec.rb

@@ -167,7 +167,7 @@ RSpec.describe PostStatusService, type: :service do
 
   it 'attaches the given media to the created status' do
     account = Fabricate(:account)
-    media = Fabricate(:media_attachment)
+    media = Fabricate(:media_attachment, account: account)
 
     status = subject.call(
       account,
@@ -178,6 +178,19 @@ RSpec.describe PostStatusService, type: :service do
     expect(media.reload.status).to eq status
   end
 
+  it 'does not attach media from another account to the created status' do
+    account = Fabricate(:account)
+    media = Fabricate(:media_attachment, account: Fabricate(:account))
+
+    status = subject.call(
+      account,
+      text: "test status update",
+      media_ids: [media.id],
+    )
+
+    expect(media.reload.status).to eq nil
+  end
+
   it 'does not allow attaching more than 4 files' do
     account = Fabricate(:account)