sublimemediumRule

Attachment: ICS calendar invite with Google share.google URL shortener in google.<tld>/share.google path form

Detects inbound messages with ICS calendar invites containing links in the form google.<tld>/share.google?q=<code>, which inherits google.<tld> domain reputation to mask an attacker-controlled destination. This google.<tld>/share.google?q= path form is undocumented: Google publishes the share.google shortener feature, but not this on-Google-domain form or its 'q' parameter.

MITRE ATT&CK

T1566 T1566.001 T1566.002 T1598

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,
                any(.links,
                    .href_url.domain.sld == "google"
                    and strings.istarts_with(.href_url.path, '/share.google')
                    and 'q' in keys(.href_url.query_params_decoded)
                )
        )
)

Data Sources

Email MessagesEmail HeadersEmail Attachments

Platforms

Raw Content

name: "Attachment: ICS calendar invite with Google share.google URL shortener in google.<tld>/share.google path form"
description: "Detects inbound messages with ICS calendar invites containing links in the form google.<tld>/share.google?q=<code>, which inherits google.<tld> domain reputation to mask an attacker-controlled destination. This google.<tld>/share.google?q= path form is undocumented: Google publishes the share.google shortener feature, but not this on-Google-domain form or its 'q' parameter."
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,
                  any(.links,
                      .href_url.domain.sld == "google"
                      and strings.istarts_with(.href_url.path, '/share.google')
                      and 'q' in keys(.href_url.query_params_decoded)
                  )
          )
  )
attack_types:
  - "Spam"
  - "Credential Phishing"
tactics_and_techniques:
  - "Open redirect"
  - "Social engineering"
detection_methods:
  - "URL analysis"
  - "Content analysis"
  - "File analysis"
related_detections:
  - "link_google_share_google.yml"
  - "attachment_google_share_google_link.yml"
id: "49d95b8b-e2c6-5ac1-8a41-61afff9da2c2"