Link: Free subdomain host with undisclosed recipients

 1name: "Link: Free subdomain host with undisclosed recipients"
 2description: |
 3    Detects messages with undisclosed recipients, containing links to free subdomain hosts
 4type: "rule"
 5severity: "medium"
 6source: |
 7  type.inbound
 8  and any(body.links,
 9          .href_url.domain.root_domain in $free_subdomain_hosts
10          and .href_url.domain.subdomain is not null
11          and .href_url.domain.subdomain != "www"
12          and not (
13            .href_url.domain.root_domain == "googleusercontent.com"
14            and strings.istarts_with(.href_url.path, "/mail-sig")
15          )
16  )
17  and (
18    length(recipients.to) == 0
19    or all(recipients.to, .display_name == "Undisclosed recipients")
20  )
21  and length(recipients.cc) == 0
22  and length(recipients.bcc) == 0
23  
24  // negate listmailers & benign threads
25  and not (
26    any(headers.hops, any(.fields, .name == "List-Unsubscribe"))
27    or any(ml.nlu_classifier(body.current_thread.text).intents,
28           .name == "benign" and .confidence == "high"
29    )
30  )
31  and (
32    profile.by_sender().prevalence in ("new", "outlier")
33    or profile.by_sender().any_messages_malicious_or_spam
34  )
35  and not profile.by_sender().any_messages_benign  
36tags:
37  - "Attack surface reduction"
38tactics_and_techniques:
39  - "Free subdomain host"
40detection_methods:
41  - "Header analysis"
42  - "URL analysis"
43id: "c23d979d-de80-599d-b0ca-c7fef217885e"