sublimemediumRule

Attachment: ICS calendar with suspicious link Leading to minimal JS landing page

Detects inbound emails with .ics calendar attachments whose parsed events contain links pointing to free file hosts, self-service creation platforms, suspicious TLDs, URL shorteners, or recently registered domains. The rule further confirms suspicion by identifying minimal JavaScript landing pages that load only a script and favicon, a common pattern for redirecting victims to malicious content while evading detection. Emails from highly trusted, DMARC-passing senders are excluded.

MITRE ATT&CK

T1566 T1566.001 T1566.002 T1598 T1036 T1027 T1059

defense-evasionexecutioninitial-access

Detection Query

type.inbound
and any(attachments,
        (
          .file_type == "ics"
          or .file_extension == "ics"
          or .content_type in ("application/ics", "text/calendar")
        )
        and any(beta.file.parse_ics(.).events,
                // sus link
                any(.links,
                    (
                      .href_url.domain.root_domain in $free_file_hosts
                      or .href_url.domain.domain in $free_file_hosts
                      or .href_url.domain.root_domain in $self_service_creation_platform_domains
                      or .href_url.domain.domain in $self_service_creation_platform_domains
                      or .href_url.domain.tld in $suspicious_tlds
                      or .href_url.domain.domain in $url_shorteners
                      or .href_url.domain.root_domain in $url_shorteners
                      or network.whois(.href_url.domain).days_old < 90
                    )
                    // minimal js landing page
                    and length(filter(ml.link_analysis(., mode="aggressive").unique_urls_accessed,
                                      .url == ..href_url.url
                                      or (
                                        strings.starts_with(.url,
                                                            ..href_url.url
                                        )
                                        and regex.icontains(.url,
                                                            '[a-z]{3,}\.[0-9a-f]{20}\.js'
                                        )
                                      )
                                      or .url == strings.concat(..href_url.url,
                                                                'favicon.png'
                                      )
                               )
                    ) == 3
                )
        )
)
and not (
  sender.email.domain.root_domain in $high_trust_sender_root_domains
  and coalesce(headers.auth_summary.dmarc.pass, false)
)

Data Sources

Email MessagesEmail HeadersEmail Attachments

Platforms

Raw Content

name: "Attachment: ICS calendar with suspicious link Leading to minimal JS landing page"
description: "Detects inbound emails with .ics calendar attachments whose parsed events contain links pointing to free file hosts, self-service creation platforms, suspicious TLDs, URL shorteners, or recently registered domains. The rule further confirms suspicion by identifying minimal JavaScript landing pages that load only a script and favicon, a common pattern for redirecting victims to malicious content while evading detection. Emails from highly trusted, DMARC-passing senders are excluded."
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")
          )
          and any(beta.file.parse_ics(.).events,
                  // sus link
                  any(.links,
                      (
                        .href_url.domain.root_domain in $free_file_hosts
                        or .href_url.domain.domain in $free_file_hosts
                        or .href_url.domain.root_domain in $self_service_creation_platform_domains
                        or .href_url.domain.domain in $self_service_creation_platform_domains
                        or .href_url.domain.tld in $suspicious_tlds
                        or .href_url.domain.domain in $url_shorteners
                        or .href_url.domain.root_domain in $url_shorteners
                        or network.whois(.href_url.domain).days_old < 90
                      )
                      // minimal js landing page
                      and length(filter(ml.link_analysis(., mode="aggressive").unique_urls_accessed,
                                        .url == ..href_url.url
                                        or (
                                          strings.starts_with(.url,
                                                              ..href_url.url
                                          )
                                          and regex.icontains(.url,
                                                              '[a-z]{3,}\.[0-9a-f]{20}\.js'
                                          )
                                        )
                                        or .url == strings.concat(..href_url.url,
                                                                  'favicon.png'
                                        )
                                 )
                      ) == 3
                  )
          )
  )
  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:
  - "Free file host"
  - "Free subdomain host"
  - "Evasion"
  - "Scripting"
  - "Social engineering"
  - "ICS Phishing"
detection_methods:
  - "File analysis"
  - "URL analysis"
  - "Whois"
  - "Javascript analysis"
  - "Content analysis"
id: "506490a6-550c-5d1e-a3ea-e2feb58d5cbf"