Link: Google Calendar invite linking to an open redirect from an untrusted freemail sender

name: "Link: Google Calendar invite linking to an open redirect from an untrusted freemail sender"
description: "Attackers invite users to view a Google Calendar whose name contains a suspicious link, generally linking to spam content such as crypto giveaways, using open redirects to mask the true destination."
type: "rule"
severity: "high"
source: |
  type.inbound
  and strings.iends_with(subject.subject, "has shared a calendar with you")
  and strings.ilike(headers.message_id, "<calendar*@google.com>")
  and sender.email.domain.domain in $free_email_providers
  and any(body.links,
          any(.href_url.rewrite.encoders, strings.icontains(., "open_redirect"))
          or (
            any(ml.link_analysis(.).final_dom.links,
                any(.href_url.rewrite.encoders,
                    strings.icontains(., "open_redirect")
                )
            )
          )
  )
  and profile.by_sender().prevalence in ("new", "outlier")
  // negate highly trusted sender domains unless they fail DMARC authentication
  and (
    (
      sender.email.domain.root_domain in $high_trust_sender_root_domains
      and (
        any(distinct(headers.hops, .authentication_results.dmarc is not null),
            strings.ilike(.authentication_results.dmarc, "*fail")
        )
      )
    )
    or sender.email.domain.root_domain not in $high_trust_sender_root_domains
  )
  and (
    not profile.by_sender().solicited
    or (
      profile.by_sender().any_messages_malicious_or_spam
      and not profile.by_sender().any_messages_benign
    )
  )

attack_types:
  - "Spam"
tactics_and_techniques:
  - "Free email provider"
  - "Free file host"
  - "Open redirect"
  - "Social engineering"
detection_methods:
  - "Content analysis"
  - "Sender analysis"
  - "URL analysis"
id: "bb4f1ea9-0888-5f93-bbfe-fcb7f08ba500"