Federated social network node, running on kosmos.social https://kosmos.social
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# frozen_string_literal: true
class SpamCheck
include Redisable
include ActionView::Helpers::TextHelper
# Threshold over which two Nilsimsa values are considered
# to refer to the same text
NILSIMSA_COMPARE_THRESHOLD = 95
# Nilsimsa doesn't work well on small inputs, so below
# this size, we check only for exact matches with MD5
NILSIMSA_MIN_SIZE = 10
# How long to keep the trail of digests between updates,
# there is no reason to store it forever
EXPIRE_SET_AFTER = 1.week.seconds
# How many digests to keep in an account's trail. If it's
# too small, spam could rotate around different message templates
MAX_TRAIL_SIZE = 10
# How many detected duplicates to allow through before
# considering the message as spam
THRESHOLD = 5
def initialize(status)
@account = status.account
@status = status
end
def skip?
disabled? || already_flagged? || trusted? || no_unsolicited_mentions? || solicited_reply?
end
def spam?
if insufficient_data?
false
elsif nilsimsa?
digests_over_threshold?('nilsimsa') { |_, other_digest| nilsimsa_compare_value(digest, other_digest) >= NILSIMSA_COMPARE_THRESHOLD }
else
digests_over_threshold?('md5') { |_, other_digest| other_digest == digest }
end
end
def flag!
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, -(MAX_TRAIL_SIZE + 1))
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
class << self
def perform(status)
spam_check = new(status)
return if spam_check.skip?
if spam_check.spam?
spam_check.flag!
else
spam_check.remember!
end
end
end
private
def disabled?
!Setting.spam_check_enabled
end
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_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'))
end
def already_flagged?
@account.silenced? || @account.targeted_reports.unresolved.where(account_id: -99).exists?
end
def trusted?
@account.trust_level > Account::TRUST_LEVELS[:untrusted] || (@account.local? && @account.user_staff?)
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 digests_over_threshold?(filter_algorithm)
other_digests.select do |record|
algorithm, other_digest, status_id = record.split(':')
next unless algorithm == filter_algorithm
yield algorithm, other_digest, status_id
end.size >= THRESHOLD
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 }.filter_map { |record| record.split(':')[2] }
else
other_digests.select { |record| record.start_with?('md5') && record.split(':')[1] == digest }.filter_map { |record| record.split(':')[2] }
end
end
def redis_key
@redis_key ||= "spam_check:#{@account.id}"
end
end