Attachment: Fake secure message and suspicious indicators

name: "Attachment: Fake secure message and suspicious indicators"
description: 'Body contains language resembling credential theft, and an attached "secure message" from an untrusted sender.'
type: "rule"
severity: "medium"
source: |
  type.inbound
  and any(ml.nlu_classifier(body.current_thread.text).intents,
          .name == "cred_theft" and .confidence == "high"
  )
  
  // ----- other suspicious signals here -----
  and any(attachments,
          any(file.explode(.),
              any(.scan.strings.strings, strings.icontains(., "secure message"))
              and (
                any(.scan.url.urls, .domain.tld in $suspicious_tlds)
                or any(.scan.url.urls,
                       any(.rewrite.encoders,
                           strings.icontains(., "open_redirect")
                       )
                )
              )
              and (
                any(.scan.url.urls,
                    .domain.root_domain != sender.email.domain.root_domain
                )
                or not sender.email.domain.valid
              )
          )
  )
  
  // negate legitimate message senders
  and (
    (
      sender.email.domain.root_domain not in ("protectedtrust.com")
      or not sender.email.domain.valid
    )
    and any(headers.hops,
            .index == 0
            and not any(.fields,
                        strings.contains(.value,
                                         'multipart/mixed; boundary="PROOFPOINT_BOUNDARY_1"'
                        )
            )
    )
    and not (
      any(headers.hops, any(.fields, .name == 'X-ZixNet'))
      and any(headers.domains,
              .root_domain in ("zixport.com", "zixcorp.com", "zixmail.net")
      )
    )
    and not all(body.links,
                .href_url.domain.root_domain in ("mimecast.com", "cisco.com")
    )
  )
  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
  
  // 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
  )
attack_types:
  - "Credential Phishing"
tactics_and_techniques:
  - "Image as content"
  - "Impersonation: Brand"
  - "Social engineering"
detection_methods:
  - "Content analysis"
  - "File analysis"
  - "Header analysis"
  - "Natural Language Understanding"
  - "Sender analysis"
id: "20a34d94-61b7-5f8c-8070-47edce70e5de"