Body: Embedded email headers indicative of thread hijacking/abuse

name: "Body: Embedded email headers indicative of thread hijacking/abuse"
description: "Detects email headers embedded in the message body content, indicating forwarded phishing attempts, MIME boundary manipulation, delivery notification spoofing, or copy-paste phishing. This pattern is commonly seen when attackers forward legitimate emails and the headers get included in the body, or when spoofing system notifications."
type: "rule"
severity: "medium"
source: |
  type.inbound
  // Headers are in the body.plain.raw but not the body.html.raw
  and 2 of (
    strings.icontains(body.plain.raw, "Delivered-To:")
    and not strings.icontains(body.html.raw, "Delivered-To:"),
    strings.icontains(body.plain.raw, "X-Google-Smtp-Source:")
    and not strings.icontains(body.html.raw, "X-Google-Smtp-Source:"),
    strings.icontains(body.plain.raw, "ARC-Seal: i=")
    and not strings.icontains(body.html.raw, "ARC-Seal: i="),
  )
  and 1 of (
    regex.icontains(body.plain.raw, "Received: by .{10,80} with SMTP id")
    and not regex.icontains(body.html.raw, "Received: by .{10,80} with SMTP id"),
    regex.icontains(body.plain.raw, "X-Received: by .{10,80} with SMTP id")
    and not regex.icontains(body.html.raw, "X-Received: by .{10,80} with SMTP id"),
  )
  // Negate legitimate forwards where users intentionally include headers
  and not (
    (length(headers.references) > 0 or headers.in_reply_to is not null)
    and (subject.is_forward or subject.is_reply)
    and length(body.previous_threads) >= 1
  )
  // Sender is not from org or high trust domains
  and sender.email.domain.root_domain not in $org_domains
  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"
  - "BEC/Fraud"
  - "Spam"
tactics_and_techniques:
  - "Evasion"
  - "Social engineering"
  - "Spoofing"
detection_methods:
  - "Content analysis"
  - "Header analysis"
  - "Sender analysis"
id: "6e8eeebb-5ea2-53e0-b4a8-c07d1248d177"