Display name and subject impersonation using recipient SLD (new sender)

name: "Display name and subject impersonation using recipient SLD (new sender)"
description: |
  The recipient domain's SLD is used in the sender's display name
  and in the subject to impersonate the organization. 
type: "rule"
severity: "medium"
source: |
  type.inbound
  and (
    // recipient SLD is being impersonated in the subject + display name
    (
      // these are usually targeted with just 1 recipient,
      // but sometimes they CC themselves or have a blank CC
      (
        length(recipients.to) + length(recipients.cc) + length(recipients.bcc) <= 2
      )
      and any(recipients.to,
              length(.email.domain.sld) >= 4
              // ensure that we're checking the org SLD
              and .email.domain.sld in $org_slds
              and strings.icontains(subject.subject, .email.domain.sld)
              and strings.icontains(sender.display_name, .email.domain.sld)
      )
    )
    or (
      // accounts for BCC'd messages where the recipients are empty
      // if BCC, sometimes the recipient will be the attacker's email
      (
        length(recipients.to) + length(recipients.cc) + length(recipients.bcc) <= 2
      )
      and length(mailbox.email.domain.sld) >= 4
      and strings.icontains(subject.subject, mailbox.email.domain.sld)
      and strings.icontains(sender.display_name, mailbox.email.domain.sld)
    )
  )
  and (
    // at least 1 link or non-image attachment
    (
      length(body.links) > 0
      // these attacks all use compromosed senders, so we look for a domain
      // that doesn't match the sender's domain to weed out legit messages
      and any(body.links,
              .href_url.domain.root_domain != sender.email.domain.root_domain
      )
    )
    or length(filter(attachments, .file_type not in $file_types_images)) > 0
  )
  and not (
    strings.contains(sender.display_name, "on behalf of")
    and sender.email.domain.root_domain == "microsoftonline.com"
  )
  and all(recipients.to, .email.email != sender.email.email)
  
  // negate org domain senders, which can often be misconfigured and fail
  // authentication, causing them to be type.inbound instead of type.internal.
  // this is fine because we should catch spoofs in other ways.
  // also, we use root_domain here to account for subdomains used by internal tools that aren't connected to the tenant.
  // this should also be safe because domains like onmicrosoft[.]com are tracked as FQDNs in $org_domains, so they won't match
  and sender.email.domain.root_domain not in $org_domains
  
  // negate highly trusted sender domains unless they fail DMARC authentication
  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
  )
  and (
    not profile.by_sender().solicited
    or (
      profile.by_sender().any_messages_malicious_or_spam
      and not profile.by_sender().any_messages_benign
    )
  )
  and not profile.by_sender().any_messages_benign

attack_types:
  - "Credential Phishing"
tactics_and_techniques:
  - "Social engineering"
detection_methods:
  - "Header analysis"
  - "Sender analysis"
id: "cb2b3ed3-268f-5753-9d2b-194d2ee1ed2e"