Spam: Default Microsoft Exchange Online sender domain (onmicrosoft.com)

 1name: "Spam: Default Microsoft Exchange Online sender domain (onmicrosoft.com)"
 2description: "The default Microsoft Exchange Online sender domain, onmicrosoft.com, is commonly used to send unwanted and malicious email. Enable this rule in your environment if receiving email from the onmicrosoft.com domain is unexpected behaviour."
 3type: "rule"
 4severity: "low"
 5source: |
 6  type.inbound
 7  and sender.email.domain.root_domain == "onmicrosoft.com"
 8  and length(recipients.to) < 2
 9  and length(body.links) > 0
10  // bounce-back negations
11  and not strings.like(sender.email.local_part, "*postmaster*", "*mailer-daemon*", "*administrator*")
12  and not any(attachments,
13              .content_type in (
14                "message/rfc822",
15                "message/delivery-status",
16                "text/calendar"
17              )
18  )
19  // negating legit replies
20  and not (
21    (
22      strings.istarts_with(subject.subject, "RE:")
23      or strings.istarts_with(subject.subject, "FW:")
24      or strings.istarts_with(subject.subject, "FWD:")
25      or regex.imatch(subject.subject, '[^:]?automat.[^:]{0,20}:.*$')
26    )
27    and (
28      length(headers.references) > 0
29      and any(headers.hops, any(.fields, strings.ilike(.name, "In-Reply-To")))
30    )
31  )
32  // construct the proper sender domain and check against known recipients
33  and not strings.concat(sender.email.domain.subdomain,  ".", sender.email.domain.tld) in $recipient_domains
34  and (
35    not profile.by_sender().solicited
36    or (
37      profile.by_sender().any_messages_malicious_or_spam
38      and not profile.by_sender().any_false_positives
39    )
40  )
41  and not sender.email.domain.domain in $org_domains  
42tags:
43 - "Attack surface reduction"
44attack_types:
45  - "Callback Phishing"
46  - "Credential Phishing"
47  - "Spam"
48tactics_and_techniques:
49  - "Free email provider"
50  - "Impersonation: Brand"
51  - "Social engineering"
52detection_methods:
53  - "Content analysis"
54  - "Sender analysis"
55id: "3f2a64ce-28bf-505e-a366-204241610ff9"