sublimemediumRule

Callback phishing via calendar invite

Detects calendar invites containing callback phishing language in the DESCRIPTION of the invite.

MITRE ATT&CK

initial-accessdefense-evasion

Detection Query

type.inbound
and length(attachments) > 0
and all(attachments, .content_type in ("text/calendar", "application/ics"))
and any(attachments,
        // extract the calendar invite description and use NLU against it
        any(file.explode(.),
            any(.scan.ics.calendars,
                any(.components,
                    any(ml.nlu_classifier(.description).intents,
                        .name == "callback_scam"
                    )
                )
            )
        )
)
and (
  not profile.by_sender_email().solicited
  and not profile.by_sender_email().any_messages_benign
)

// 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
)

Data Sources

Email MessagesEmail HeadersEmail Attachments

Platforms

Raw Content

name: "Callback phishing via calendar invite"
description: "Detects calendar invites containing callback phishing language in the DESCRIPTION of the invite."
type: "rule"
severity: "medium"
source: |
  type.inbound
  and length(attachments) > 0
  and all(attachments, .content_type in ("text/calendar", "application/ics"))
  and any(attachments,
          // extract the calendar invite description and use NLU against it
          any(file.explode(.),
              any(.scan.ics.calendars,
                  any(.components,
                      any(ml.nlu_classifier(.description).intents,
                          .name == "callback_scam"
                      )
                  )
              )
          )
  )
  and (
    not profile.by_sender_email().solicited
    and not profile.by_sender_email().any_messages_benign
  )
  
  // 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
  )
attack_types:
  - "Callback Phishing"
tactics_and_techniques:
  - "Social engineering"
  - "Evasion"
detection_methods:
  - "File analysis"
  - "Header analysis"
  - "Natural Language Understanding"
  - "Sender analysis"
id: "95c84360-d5a5-5396-b9ce-c61016cb178f"