Attachment: RFC822 containing suspicious file sharing language with links from untrusted sender

name: "Attachment: RFC822 containing suspicious file sharing language with links from untrusted sender"
description: "This rule identifies messages with an RFC822 attachment contains language indicative of suspicious file-sharing activity. It checks both the original sender and the nested sender against highly trusted domains. The original message is unsolicited, and has not been previously flagged as a false positive."
type: "rule"
severity: "medium"
source: |
  type.inbound
  and any(attachments,
          .file_type == "unknown"
          and (.content_type == "message/rfc822" or .file_extension in ('eml'))
          and regex.contains(file.parse_eml(.).subject.subject,
                             '(shared.{0,30}with you|View Document)'
          )
          and 0 < length(file.parse_eml(.).body.links) < 10
          and file.parse_eml(.).sender.email.email not in $recipient_emails
  
          // exclude bounce backs & read receipts
          and not strings.like(file.parse_eml(.).sender.email.local_part,
                               "*postmaster*",
                               "*mailer-daemon*",
                               "*administrator*"
          )
          and not regex.imatch(file.parse_eml(.).subject.subject,
                               "(undeliverable|read:).*"
          )
          and not any(file.parse_eml(.).attachments,
                      .content_type == "message/delivery-status"
          )
  
          // negate highly trusted sender domains in the nested eml unless they fail DMARC
          and not (
            file.parse_eml(.).sender.email.domain.root_domain in $high_trust_sender_root_domains
            and coalesce(file.parse_eml(.).headers.auth_summary.dmarc.pass, false)
          )
          // negate org domains that passed dmarc
          and not file.parse_eml(.).sender.email.domain.root_domain in $org_domains
  )
  
  // negate highly trusted sender domains unless they fail DMARC authentication
  and not (
    sender.email.domain.root_domain in $high_trust_sender_root_domains
    and coalesce(headers.auth_summary.dmarc.pass, false)
  )
  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:
  - "Credential Phishing"
tactics_and_techniques:
  - "Evasion"
  - "Social engineering"
detection_methods:
  - "File analysis"
  - "Header analysis"
  - "Natural Language Understanding"
  - "Sender analysis"
id: "d96854d7-d0a2-5342-a363-cee1ad51e7c9"