ruby/rails user signup email screening with google safebrowsing api -- catch phishing domain typos in user email addresses

email-screener-google-safebrowsing.yml

---
http_interactions:
- request:
    method: get
    uri: https://sb-ssl.google.com/safebrowsing/api/lookup?appver=1&client=your-app-name-here&key=abc123fakekey&pver=3.1&url=gmai.com
    body:
      encoding: US-ASCII
      string: ''
    headers:
      Accept:
      - "*/*; q=0.5, application/xml"
      Accept-Encoding:
      - gzip, deflate
      Timeout:
      - '4'
      User-Agent:
      - Ruby
  response:
    status:
      code: 200
      message: OK
    headers:
      Content-Type:
      - application/octet-stream
      P3p:
      - CP="This is not a P3P policy! See http://www.google.com/support/accounts/bin/answer.py?hl=en&answer=151657
        for more info."
      X-Content-Type-Options:
      - nosniff
      Date:
      - Wed, 05 Aug 2015 22:59:16 GMT
      Server:
      - HTTP server (unknown)
      Content-Length:
      - '7'
      X-Xss-Protection:
      - 1; mode=block
      X-Frame-Options:
      - SAMEORIGIN
      Set-Cookie:
      - NID=70=RqpUA10yjTPamIiNtmDoFuI7HMSg0TNISCuaU1SgRBpKY9ScNcBL6fHN7jaeynrEXevBm9WFlAgO3DRUQ1BOABzUAu6fZ-A-aGnzzmw8OAqdh-mi9ZlhmnqR8_XE7pF4;
        expires=Thu, 04-Feb-2016 22:59:16 GMT; path=/; domain=.google.com; HttpOnly
      - PREF=ID=1111111111111111:TM=1438815556:LM=1438815556:V=1:S=QbXwVGLqYBhLAblQ;
        expires=Fri, 04-Aug-2017 22:59:16 GMT; path=/; domain=.google.com
      Alternate-Protocol:
      - 443:quic,p=1
      Expires:
      - Wed, 05 Aug 2015 22:59:16 GMT
      Cache-Control:
      - private
    body:
      encoding: UTF-8
      string: malware
    http_version:
  recorded_at: Wed, 05 Aug 2015 22:59:16 GMT
recorded_with: VCR 2.9.3

email_screener.rb

require 'rest-client'

# sometimes users give the wrong email address
# moreso since smartphone keyboards, errant auto-correct, and big players mixing their domains inconsistently by country.
# even worse for some apps, they might type in a domain that's incorrect, but has a phishing operator sat on it running an MX server to intercept any email
# help your users out: warn them if they've done something that looks supicious
# e.g. if they registered with someguy@gmai.com instead of gmail.com, chances are it's an error
# You may find confirmation emails solve this problem, but your users may get pretty confused waiting for ones that will never arrive, losing you sales.
class EmailScreener
  attr_reader :email
  def initialize(email)
    @email = email
  end

  # is the given email address probably a data-entry error
  # e.g. algo@gmai.com instead of algo@gmail.com
  def suspects_domain_error?
    !domain_exists? || phishing?
  end

  # return the address if it could be understood
  def parseable?
    begin
      Mail::Address.new(email).address
    rescue Mail::Field::ParseError
      false
    end
  end

  def username
    @username ||= Mail::Address.new(email).local
  end

  def domain
    @domain ||= Mail::Address.new(email).domain
  end

  # is there a mail exchange (MX) record (DNS) for this domain?
  def domain_exists?
    ValidateEmail.mx_valid?(email)
  end

  def warnings
    @warnings ||= if %w(hotmail.com hotmail.co.uk).include?(domain)
      'Please double-check whether your address is with hotmail.com or hotmail.co.uk'
    end
  end

  # how does google safebrowsing consider this domain?
  # Is it a gmail-typo-phishing-site? e.g. gmai.com
  def phishing?
    unless safebrowsing_api_key.present?
      Rails.logger.error "Safebrowsing API key missing"
      return false
    end

    begin
      response = RestClient.get(
        'https://sb-ssl.google.com/safebrowsing/api/lookup',
        timeout: safebrowsing_timeout,
        params: {
          key: safebrowsing_api_key,
          appver: '1',
          client: 'your-app-name-here',
          pver:'3.1',
          url: domain
        }
      )
      # 200 for malware or phishing, 204 not malware
      # https://developers.google.com/safe-browsing/lookup_guide
      response.code == 200
    rescue RestClient::Exception
      Rails.logger.info "SafeBrowsing email check returned error #{e.code} for #{email}"
      # can't tell, so just approve
      false
    rescue
      Rails.logger.error "SafeBrowsing lookup unknown error #{e.message}"
      # can't tell, so just approve
      false
    end
  end

  private

  def safebrowsing_timeout
    ENV['GOOGLE_SAFEBROWSING_TIMEOUT'] || 4
  end

  def safebrowsing_api_key
    ENV['GOOGLE_SAFEBROWSING_API_KEY']
  end
end

email_screener_spec.rb

# -*- encoding : utf-8 -*-
require 'spec_helper'

describe "EmailScreener" do
  describe 'phishing' do
    it 'returns false when no API key present' do
      expect(EmailScreener.new('foo@gmai.com')).not_to be_phishing
    end

    it 'suspects gmai.com when response says so (vcr/webmock)' do
      original_key = ENV['GOOGLE_SAFEBROWSING_API_KEY']
      begin
        ENV['GOOGLE_SAFEBROWSING_API_KEY'] = 'abc123fakekey'
        VCR.use_cassette('email-screener-google-safebrowsing', record: :once) do
          screener = EmailScreener.new('foo@gmai.com')
          expect(screener).to be_phishing
        end
      ensure
        ENV['GOOGLE_SAFEBROWSING_API_KEY'] = original_key
      end
    end
  end

  describe 'suspects_domain_error?' do
    it 'true for phishing' do
      screener = EmailScreener.new('foo@gmai.com')
      allow(screener).to receive(:phishing?).and_return true
      expect(screener.suspects_domain_error?).to be true
    end

    it 'true for mx not found' do
      expect(EmailScreener.new('foo@gmaix2kdlkjf.co.uk').suspects_domain_error?).to be true
    end
  end

  describe 'parseable?' do
    it 'returns the address if it could be read' do
      expect(EmailScreener.new('foo@bar.net').parseable?).to eq('foo@bar.net')
    end
    it 'returns false if it couldnt understand the address' do
      expect(EmailScreener.new('foo@bar.').parseable?).to be false
    end
  end

  describe 'warnings' do
    it 'warns hotmail users to check .com or .co.uk' do
      hotmail_warning = 'Please double-check whether your address is with hotmail.com or hotmail.co.uk'
      expect(EmailScreener.new('foo@hotmail.com').warnings).to eq(hotmail_warning)
      expect(EmailScreener.new('foo@hotmail.co.uk').warnings).to eq(hotmail_warning)
    end
  end

  describe 'username' do
    it 'returns the username before @' do
      expect(EmailScreener.new('foo@hotmail.com').username).to eq('foo')
    end
    it 'fails when address not parsable' do
    end
  end

  describe 'domain' do
    it 'returns the mailhost after @' do
      expect(EmailScreener.new('foo@hotmail.com').domain).to eq('hotmail.com')
    end
  end
end