Attachment: Calendar invite with suspicious link leading to an open redirect

name: "Attachment: Calendar invite with suspicious link leading to an open redirect"
description: "Calendar invite contains a link to either a free file host or free subdomain host, and the resulting webpage contains another link to an open redirect."
type: "rule"
severity: "high"
source: |
  type.inbound
  and any(attachments,
          (
            .file_extension == "ics"
            or .content_type in ("application/ics", "text/calendar")
          )
          and any(file.explode(.),
                  any(.scan.url.urls,
                      (
                        .domain.domain in $free_file_hosts
                        or (
                          .domain.root_domain in $free_subdomain_hosts
                          and .domain.subdomain is not null
                          and not (
                            strings.ends_with(.url, "jpeg")
                            or strings.ends_with(.url, "png")
                          )
                          and .domain.subdomain != "www"
                        )
                      )
                      and any(ml.link_analysis(.).final_dom.links,
                              any(.href_url.rewrite.encoders,
                                  strings.icontains(., "open_redirect")
                              )
                      )
                  )
          )
  )
  // 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
    )
  )
  and not profile.by_sender().any_messages_benign

attack_types:
  - "Spam"
tactics_and_techniques:
  - "Free email provider"
  - "Free file host"
  - "Free subdomain host"
  - "Open redirect"
detection_methods:
  - "Content analysis"
  - "URL analysis"
id: "5d6294c7-0b75-5722-851e-27015170bd42"