Free subdomain link with login or captcha (untrusted sender)

name: "Free subdomain link with login or captcha (untrusted sender)"
description: |
  Message contains a link that uses a free subdomain provider, and has a login or captcha on the page.
type: "rule"
severity: "medium"
source: |
  type.inbound
  and 0 < length(body.links) < 10
  and any(body.links,
  
          // contains login or captcha
          (
            ml.link_analysis(.).credphish.contains_login
            or ml.link_analysis(.).credphish.contains_captcha
          )
  
          // either the original or the redirect is a free subdomain
          and (
            ml.link_analysis(.).effective_url.domain.root_domain in $free_subdomain_hosts
            or .href_url.domain.root_domain in $free_subdomain_hosts
          )
  
          // exclude FP prone free subdomain hosts
          // if it's a known brand impersonation, we'll detect it in other rules
          and .href_url.domain.root_domain not in ("zendesk.com")
          and ml.link_analysis(.).effective_url.domain.root_domain not in (
            "zendesk.com"
          )
          and ml.link_analysis(.).effective_url.domain.domain not in (
            "login.squarespace.com"
          )
  )
  
  // exclude all freesubdomain links are images
  and length(filter(body.links,
                    .href_url.domain.root_domain in $free_subdomain_hosts
                    and .href_url.domain.subdomain is not null
                    and .href_url.domain.subdomain != "www"
             )
  ) > 0
  and not all(filter(body.links,
                     .href_url.domain.root_domain in $free_subdomain_hosts
                     and .href_url.domain.subdomain is not null
                     and .href_url.domain.subdomain != "www"
              ),
              (
                strings.ends_with(.href_url.url, "jpeg")
                or strings.ends_with(.href_url.url, "png")
              )
  )
  
  // exclude FP prone senders
  and sender.email.domain.root_domain not in ("sharepointonline.com")
  // negate highly trusted sender domains unless they fail DMARC authentication
  and (
    (
      sender.email.domain.root_domain in $high_trust_sender_root_domains
      and not headers.auth_summary.dmarc.pass
    )
    or sender.email.domain.root_domain not in $high_trust_sender_root_domains
  )
  and (
    (
      profile.by_sender().prevalence in ("new", "outlier")
      and not profile.by_sender().solicited
    )
    or (
      profile.by_sender().any_messages_malicious_or_spam
      and not profile.by_sender().any_messages_benign
    )
  )
  and not profile.by_sender().any_messages_benign
tags:
  - "Attack surface reduction"
attack_types:
  - "Credential Phishing"
tactics_and_techniques:
  - "Free subdomain host"
  - "Social engineering"
detection_methods:
  - "Computer Vision"
  - "File analysis"
  - "Sender analysis"
  - "URL screenshot"
id: "93288f82-fbd2-5e88-b207-7f06386495a6"