Description

Detects inbound messages where the HTML body (including HTML content within .eml attachments) uses CSS clamp() with a font-size or line-height minimum bound of zero or a negative value. This technique renders text invisible or unreadable to recipients while remaining present in the underlying markup, a method commonly used to hide malicious content or evade text-based detection engines.

References

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Sublime Security
Created Jul 29th, 2026 • Last updated Sep 1st, 2026
Source
type.inbound
and (
  (
    regex.icontains(body.html.raw,
                    '(?:font-size|line-height):\s*clamp\s*\(\s*(?:-\d+|0)(?:px|em|rem|pt)?,'
    )
  )
  or any(attachments,
         (.content_type == "message/rfc822" or .file_extension =~ "eml")
         and regex.icontains(file.parse_eml(.).body.html.raw,
                             '(?:font-size|line-height):\s*clamp\s*\(\s*(?:-\d+|0)(?:px|em|rem|pt)?,'
         )
  )
)
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