sublimemediumRule

Attachment: ICS calendar file with suspicious UID domain

Detects inbound messages containing ICS calendar attachments whose event UID ends with a known-malicious domain (@example.com) or contains a known bulk-invite automation toolkit identifier (mr-calendar-automation). Because the UID is client-supplied and passed through unmodified by calendar providers, it exposes attacker tooling even on invites relayed by legitimate, authenticated infrastructure.

MITRE ATT&CK

T1566 T1566.001 T1566.002 T1598 T1534 T1656

initial-access

Detection Query

type.inbound
and any(attachments,
        (
          .file_type == "ics"
          or .file_extension == "ics"
          or .content_type in ("application/ics", "text/calendar")
        )
        //
        // This rule makes use of a beta feature and is subject to change without notice
        // using the beta feature in custom rules is not suggested until it has been formally released
        //
        and any(beta.file.parse_ics(.).events,
                strings.iends_with(.uid,
                                   "@example.com",
                                   "mr-calendar-automation"
                )
        )
)

Data Sources

Email MessagesEmail HeadersEmail Attachments

Platforms

Tags

Attack surface reduction

Raw Content

name: "Attachment: ICS calendar file with suspicious UID domain"
description: "Detects inbound messages containing ICS calendar attachments whose event UID ends with a known-malicious domain (@example.com) or contains a known bulk-invite automation toolkit identifier (mr-calendar-automation). Because the UID is client-supplied and passed through unmodified by calendar providers, it exposes attacker tooling even on invites relayed by legitimate, authenticated infrastructure."
type: "rule"
severity: "medium"
source: |
  type.inbound
  and any(attachments,
          (
            .file_type == "ics"
            or .file_extension == "ics"
            or .content_type in ("application/ics", "text/calendar")
          )
          //
          // This rule makes use of a beta feature and is subject to change without notice
          // using the beta feature in custom rules is not suggested until it has been formally released
          //
          and any(beta.file.parse_ics(.).events,
                  strings.iends_with(.uid,
                                     "@example.com",
                                     "mr-calendar-automation"
                  )
          )
  )
tags:
  - "Attack surface reduction"
attack_types:
  - "Credential Phishing"
  - "BEC/Fraud"
tactics_and_techniques:
  - "Social engineering"
  - "ICS Phishing"
detection_methods:
  - "File analysis"
  - "Content analysis"
id: "cbe06f4f-f041-501b-bfc0-957cd545cd83"