Add a spam check (#11217)

* Add a spam check

* Use Nilsimsa to generate locality-sensitive hashes and compare using Levenshtein distance

* Add more tests

* Add exemption when the message is a reply to something that mentions the sender

* Use Nilsimsa Compare Value instead of Levenshtein distance

* Use MD5 for messages shorter than 10 characters

* Add message to automated report, do not add non-public statuses to
automated report, add trust level to accounts and make unsilencing
raise the trust level to prevent repeated spam checks on that account

* Expire spam check data after 3 months

* Add support for local statuses, reduce expiration to 1 week, always create a report

* Add content warnings to the spam check and exempt empty statuses

* Change Nilsimsa threshold to 95 and make sure removed statuses are removed from the spam check

* Add all matched statuses into automatic report

2 files changed, 182 insertions, 0 deletions

diff --git a/app/lib/activitypub/activity/create.rb b/app/lib/activitypub/activity/create.rb
index 5849c20d7..56c24680a 100644
--- a/app/lib/activitypub/activity/create.rb
+++ b/app/lib/activitypub/activity/create.rb
@@ -41,6 +41,7 @@ class ActivityPub::Activity::Create < ActivityPub::Activity
 
     resolve_thread(@status)
     fetch_replies(@status)
+    check_for_spam
     distribute(@status)
     forward_for_reply if @status.distributable?
   end
@@ -406,6 +407,18 @@ class ActivityPub::Activity::Create < ActivityPub::Activity
     Account.local.where(username: local_usernames).exists?
   end
 
+  def check_for_spam
+    spam_check = SpamCheck.new(@status)
+
+    return if spam_check.skip?
+
+    if spam_check.spam?
+      spam_check.flag!
+    else
+      spam_check.remember!
+    end
+  end
+
   def forward_for_reply
     return unless @json['signature'].present? && reply_to_local?
     ActivityPub::RawDistributionWorker.perform_async(Oj.dump(@json), replied_to_status.account_id, [@account.preferred_inbox_url])
diff --git a/app/lib/spam_check.rb b/app/lib/spam_check.rb
new file mode 100644
index 000000000..923d48a02
--- /dev/null
+++ b/app/lib/spam_check.rb
@@ -0,0 +1,169 @@
+# frozen_string_literal: true
+
+class SpamCheck
+  include Redisable
+  include ActionView::Helpers::TextHelper
+
+  NILSIMSA_COMPARE_THRESHOLD = 95
+  NILSIMSA_MIN_SIZE          = 10
+  EXPIRE_SET_AFTER           = 1.week.seconds
+
+  def initialize(status)
+    @account = status.account
+    @status  = status
+  end
+
+  def skip?
+    already_flagged? || trusted? || no_unsolicited_mentions? || solicited_reply?
+  end
+
+  def spam?
+    if insufficient_data?
+      false
+    elsif nilsimsa?
+      any_other_digest?('nilsimsa') { |_, other_digest| nilsimsa_compare_value(digest, other_digest) >= NILSIMSA_COMPARE_THRESHOLD }
+    else
+      any_other_digest?('md5') { |_, other_digest| other_digest == digest }
+    end
+  end
+
+  def flag!
+    auto_silence_account!
+    auto_report_status!
+  end
+
+  def remember!
+    # The scores in sorted sets don't actually have enough bits to hold an exact
+    # value of our snowflake IDs, so we use it only for its ordering property. To
+    # get the correct status ID back, we have to save it in the string value
+
+    redis.zadd(redis_key, @status.id, digest_with_algorithm)
+    redis.zremrangebyrank(redis_key, '0', '-10')
+    redis.expire(redis_key, EXPIRE_SET_AFTER)
+  end
+
+  def reset!
+    redis.del(redis_key)
+  end
+
+  def hashable_text
+    return @hashable_text if defined?(@hashable_text)
+
+    @hashable_text = @status.text
+    @hashable_text = remove_mentions(@hashable_text)
+    @hashable_text = strip_tags(@hashable_text) unless @status.local?
+    @hashable_text = normalize_unicode(@status.spoiler_text + ' ' + @hashable_text)
+    @hashable_text = remove_whitespace(@hashable_text)
+  end
+
+  def insufficient_data?
+    hashable_text.blank?
+  end
+
+  def digest
+    @digest ||= begin
+      if nilsimsa?
+        Nilsimsa.new(hashable_text).hexdigest
+      else
+        Digest::MD5.hexdigest(hashable_text)
+      end
+    end
+  end
+
+  def digest_with_algorithm
+    if nilsimsa?
+      ['nilsimsa', digest, @status.id].join(':')
+    else
+      ['md5', digest, @status.id].join(':')
+    end
+  end
+
+  private
+
+  def remove_mentions(text)
+    return text.gsub(Account::MENTION_RE, '') if @status.local?
+
+    Nokogiri::HTML.fragment(text).tap do |html|
+      mentions = @status.mentions.map { |mention| ActivityPub::TagManager.instance.url_for(mention.account) }
+
+      html.traverse do |element|
+        element.unlink if element.name == 'a' && mentions.include?(element['href'])
+      end
+    end.to_s
+  end
+
+  def normalize_unicode(text)
+    text.unicode_normalize(:nfkc).downcase
+  end
+
+  def remove_whitespace(text)
+    text.gsub(/\s+/, ' ').strip
+  end
+
+  def auto_silence_account!
+    @account.silence!
+  end
+
+  def auto_report_status!
+    status_ids = Status.where(visibility: %i(public unlisted)).where(id: matching_status_ids).pluck(:id) + [@status.id] if @status.distributable?
+    ReportService.new.call(Account.representative, @account, status_ids: status_ids, comment: I18n.t('spam_check.spam_detected_and_silenced'))
+  end
+
+  def already_flagged?
+    @account.silenced?
+  end
+
+  def trusted?
+    @account.trust_level > Account::TRUST_LEVELS[:untrusted]
+  end
+
+  def no_unsolicited_mentions?
+    @status.mentions.all? { |mention| mention.silent? || (!@account.local? && !mention.account.local?) || mention.account.following?(@account) }
+  end
+
+  def solicited_reply?
+    !@status.thread.nil? && @status.thread.mentions.where(account: @account).exists?
+  end
+
+  def nilsimsa_compare_value(first, second)
+    first  = [first].pack('H*')
+    second = [second].pack('H*')
+    bits   = 0
+
+    0.upto(31) do |i|
+      bits += Nilsimsa::POPC[255 & (first[i].ord ^ second[i].ord)].ord
+    end
+
+    128 - bits # -128 <= Nilsimsa Compare Value <= 128
+  end
+
+  def nilsimsa?
+    hashable_text.size > NILSIMSA_MIN_SIZE
+  end
+
+  def other_digests
+    redis.zrange(redis_key, 0, -1)
+  end
+
+  def any_other_digest?(filter_algorithm)
+    other_digests.any? do |record|
+      algorithm, other_digest, status_id = record.split(':')
+
+      next unless algorithm == filter_algorithm
+
+      yield algorithm, other_digest, status_id
+    end
+  end
+
+  def matching_status_ids
+    if nilsimsa?
+      other_digests.select { |record| record.start_with?('nilsimsa') && nilsimsa_compare_value(digest, record.split(':')[1]) >= NILSIMSA_COMPARE_THRESHOLD }.map { |record| record.split(':')[2] }.compact
+    else
+      other_digests.select { |record| record.start_with?('md5') && record.split(':')[1] == digest }.map { |record| record.split(':')[2] }.compact
+    end
+  end
+
+  def redis_key
+    @redis_key ||= "spam_check:#{@account.id}"
+  end
+end