Attachment: ICS link with valueless base64 query parameter

name: "Attachment: ICS link with valueless base64 query parameter"
description: "Detects inbound emails containing ICS calendar attachments where embedded event links include a randomized 8-character base64 query parameter with no assigned value. The rule excludes messages from high-trust sender domains that pass DMARC authentication."
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.path == '/'
                        and regex.imatch(.href_url.query_params,
                                         '[A-Za-z0-9+/]{8}'
                        )
                        and length(.href_url.query_params_decoded) == 1
                        and all(flatten(values(.href_url.query_params_decoded)),
                                . == ''
                        )
                      )
                      or (
                        ml.link_analysis(., mode="aggressive").effective_url.path == '/'
                        and regex.imatch(ml.link_analysis(., mode="aggressive").effective_url.query_params,
                                         '[A-Za-z0-9+/]{8}'
                        )
                        and length(ml.link_analysis(., mode="aggressive").effective_url.query_params_decoded
                        ) == 1
                        and all(flatten(values(ml.link_analysis(.,
                                                                mode="aggressive"
                                               ).effective_url.query_params_decoded
                                        )
                                ),
                                . == ''
                        )
                      )
                      or (
                        any(ml.link_analysis(., mode="aggressive").final_dom.links,
                            .href_url.path == '/'
                            and regex.imatch(.href_url.query_params,
                                             '[A-Za-z0-9+/]{8}'
                            )
                            and length(.href_url.query_params_decoded) == 1
                            and all(flatten(values(.href_url.query_params_decoded)
                                    ),
                                    . == ''
                            )
                        )
                      )
                  )
          )
  )
  and not (
    sender.email.domain.root_domain in $high_trust_sender_root_domains
    and coalesce(headers.auth_summary.dmarc.pass, false)
  )
attack_types:
  - "Credential Phishing"
tactics_and_techniques:
  - "Evasion"
  - "Social engineering"
  - "ICS Phishing"
detection_methods:
  - "File analysis"
  - "URL analysis"
  - "Header analysis"
id: "d5ce3bc3-9771-541c-a5e5-aedf94cc9e8f"