Medium Severity

Attachment: HTML smuggling with dynamically constructed redirect URL

Labels

Credential Phishing

HTML smuggling

Description

Detects HTML attachments, including those nested inside forwarded EML files, that contain obfuscated JavaScript designed to dynamically assemble a redirect URL at runtime rather than embedding it directly in the markup. The script typically builds an empty route via route.join, sets an image or iframe src from a url/protocol variable, and defines a host string separately, a pattern used to evade static URL scanning before sending the recipient to a credential phishing page.

References

No references.

Sublime Security

Created Sep 29th, 2026 • Last updated Sep 29th, 2026

Feed Source

Sublime Core Feed

Source

GitHub

type.inbound
and any(attachments,
        (
          .file_type == "html"
          and 2 of (
            regex.icontains(file.parse_html(.).raw,
                            'route\.join\(\s*["'']{2}\s*\)'
            ),
            regex.icontains(file.parse_html(.).raw,
                            '\.src\s*=\s*(?:url|protocol)'
            ),
            regex.icontains(file.parse_html(.).raw, 'const\s+host\s*=\s*["'']')
          )
        )
        or (
          (.file_extension == "eml" or .content_type == "message/rfc822")
          and any(file.parse_eml(.).attachments,
                  .file_type == "html"
                  and 2 of (
                    regex.icontains(file.parse_html(.).raw,
                                    'route\.join\(\s*["'']{2}\s*\)'
                    ),
                    regex.icontains(file.parse_html(.).raw,
                                    '\.src\s*=\s*(?:url|protocol)'
                    ),
                    regex.icontains(file.parse_html(.).raw,
                                    'const\s+host\s*=\s*["'']'
                    )
                  )
          )
        )
)

MQL Rule Console

•Docs•Learning Labs

Playground

Test against your own EMLs or sample data.

Post about this on your socials.

Get Started. Today.

Managed or self-managed. No MX changes.