Body: Embedded email headers indicative of thread hijacking/abuse

 1name: "Body: Embedded email headers indicative of thread hijacking/abuse"
 2description: "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."
 3type: "rule"
 4severity: "medium"
 5source: |
 6  type.inbound
 7  // Headers are in the body.plain.raw but not the body.html.raw
 8  and 2 of (
 9    strings.icontains(body.plain.raw, "Delivered-To:")
10    and not strings.icontains(body.html.raw, "Delivered-To:"),
11    strings.icontains(body.plain.raw, "X-Google-Smtp-Source:")
12    and not strings.icontains(body.html.raw, "X-Google-Smtp-Source:"),
13    strings.icontains(body.plain.raw, "ARC-Seal: i=")
14    and not strings.icontains(body.html.raw, "ARC-Seal: i="),
15  )
16  and 1 of (
17    regex.icontains(body.plain.raw, "Received: by .{10,80} with SMTP id")
18    and not regex.icontains(body.html.raw, "Received: by .{10,80} with SMTP id"),
19    regex.icontains(body.plain.raw, "X-Received: by .{10,80} with SMTP id")
20    and not regex.icontains(body.html.raw, "X-Received: by .{10,80} with SMTP id"),
21  )
22  // Negate legitimate forwards where users intentionally include headers
23  and not (
24    (length(headers.references) > 0 or headers.in_reply_to is not null)
25    and (subject.is_forward or subject.is_reply)
26    and length(body.previous_threads) >= 1
27  )
28  // Sender is not from org or high trust domains
29  and sender.email.domain.root_domain not in $org_domains
30  and (
31    (
32      sender.email.domain.root_domain in $high_trust_sender_root_domains
33      and not headers.auth_summary.dmarc.pass
34    )
35    or sender.email.domain.root_domain not in $high_trust_sender_root_domains
36  )  
37
38attack_types:
39  - "Credential Phishing"
40  - "BEC/Fraud"
41  - "Spam"
42tactics_and_techniques:
43  - "Evasion"
44  - "Social engineering"
45  - "Spoofing"
46detection_methods:
47  - "Content analysis"
48  - "Header analysis"
49  - "Sender analysis"
50id: "6e8eeebb-5ea2-53e0-b4a8-c07d1248d177"