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

name: "Spam: Default Microsoft Exchange Online sender domain (onmicrosoft.com)"
description: "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."
type: "rule"
severity: "low"
source: |
  type.inbound
  and (
    sender.email.domain.root_domain == "onmicrosoft.com"
    or (
      strings.icontains(sender.display_name, "onmicrosoft.com")
      and sender.email.domain.valid == false
    )
  )
  and length(recipients.to) < 2
  and length(body.links) > 0
  
  // bounce-back negations
  and not strings.like(sender.email.local_part,
                       "*postmaster*",
                       "*mailer-daemon*",
                       "*administrator*"
  )
  and not any(attachments,
              .content_type in (
                "message/rfc822",
                "message/delivery-status",
                "text/calendar"
              )
              or (.content_type == "text/plain" and .file_extension == "ics")
  )
  
  // negating legit replies
  and not (
    (
      strings.istarts_with(subject.subject, "RE:")
      or strings.istarts_with(subject.subject, "FW:")
      or strings.istarts_with(subject.subject, "FWD:")
      or regex.imatch(subject.subject,
                      '(\[[^\]]+\]\s?){0,3}(re|fwd?|automat.*)\s?:.*'
      )
      or strings.istarts_with(subject.subject, "Réponse automatique")
    )
    and (length(headers.references) > 0 and headers.in_reply_to is not null)
  )
  
  // negate auto-replies
  and not (
    any(headers.hops,
        any(.fields, .name =~ "auto-submitted" and .value =~ "auto-generated")
        and any(.fields,
                .name =~ "X-MS-Exchange-Generated-Message-Source"
                and .value not in ("Antispam Quarantine Agent")
        )
    )
  )
  
  // Microsoft has some legit onmicrosoft domains...
  and not (
    sender.email.domain.domain == "microsoft.onmicrosoft.com"
    and headers.auth_summary.spf.pass
    and all(body.links, .href_url.domain.root_domain == "microsoft.com")
  )
  
  // negate legitimate microsoft b2b applications invitations
  and not (
    length(body.links) > 0
    and (
      sender.email.local_part == "invites"
      and sender.email.domain.root_domain == "onmicrosoft.com"
      // infra validated message id
      and strings.icontains(headers.message_id, "pepf")
    )
  )
  
  // construct the proper sender domain and check against known recipients
  and not strings.concat(sender.email.domain.subdomain,
                         ".",
                         sender.email.domain.tld
  ) in $recipient_domains
  
  // sender profiles
  and (
    not profile.by_sender().solicited
    or (
      profile.by_sender().any_messages_malicious_or_spam
      and not profile.by_sender().any_messages_benign
    )
  )
  
  // negate org domains
  and not (
    sender.email.domain.domain in $org_domains
    and coalesce(headers.auth_summary.dmarc.pass, false)
  )

tags:
 - "Attack surface reduction"
attack_types:
  - "Callback Phishing"
  - "Credential Phishing"
  - "Spam"
tactics_and_techniques:
  - "Free email provider"
  - "Impersonation: Brand"
  - "Social engineering"
detection_methods:
  - "Content analysis"
  - "Sender analysis"
id: "3f2a64ce-28bf-505e-a366-204241610ff9"