• Sublime Core Feed
High Severity

Attachment: HTML smuggling with atob and high entropy via calendar invite

Labels

Credential Phishing
Malware/Ransomware
Evasion
HTML smuggling
Scripting
File analysis
HTML analysis
Javascript analysis
Sender analysis

Description

Scans calendar invites (.ics files) to detect HTML smuggling techniques.

References

No references.

Sublime Security
Created Jun 3rd, 2025 • Last updated Jun 3rd, 2025
Feed Source
Sublime Core Feed
Source
GitHub
type.inbound
and any(attachments,
        (
          .file_extension =~ "ics"
          or .content_type == "text/calendar"
        )
        and any(file.explode(.),
                .scan.entropy.entropy >= 5
                and (
                  length(filter(.scan.javascript.identifiers,
                                strings.like(., "document", "write", "atob")
                         )
                  ) == 3
                  // usage: document['write'](atob)
                  or any(.scan.strings.strings, regex.icontains(., "document.{0,10}write.{0,10}atob"))
                  // usage: some_var = atob();
                  or any(.scan.strings.strings, regex.icontains(., "=.?atob.*;"))
                  // usage: atob(atob
                  or any(.scan.strings.strings, strings.ilike(., "*atob?atob*"))
                  // usage: {src: atob
                  or any(.scan.strings.strings, strings.ilike(., "*{src: atob*"))
                  // usage: eval(atob)
                  or any(.scan.strings.strings, strings.ilike(., "*eval?atob*"))
                    // usage: atob(_0x)
                  or any(.scan.strings.strings, strings.ilike(., "*atob(?0x*"))
                  // usage : 'at'+'ob'
                  or any(.scan.strings.strings, strings.ilike(., "*'at'+'ob'*"))
                  // usage: obfuscating "atob"
                  or any(.scan.javascript.identifiers, strings.ilike(., '*ato\u0062*'))
                  // usage: document.head.insertAdjacentHTML("beforeend", atob(...
                  or any(.scan.strings.strings,
                         strings.ilike(.,
                                       "*document*insertAdjacentHTML*atob*"
                         )
                  )
                )
        )
)
// negate bouncebacks and undeliverables
and not any(attachments,
            .content_type in (
              "message/global-delivery-status",
              "message/delivery-status"
            )
)
// 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
)
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