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Detection rules › Sublime MQL

Link: Credential harvesting with excess padding evasion

Severity
high
Type
rule
Source
github.com/sublime-security/sublime-rules

Detects inbound messages containing credential-related action links with tall screenshot images and HTML padding techniques used to evade detection. The rule identifies messages with excessive empty div tags, non-breaking spaces, or large margin-top values that artificially increase content height while hiding malicious intent.

Threat classification

Sublime's own taxonomy (not MITRE ATT&CK).

CategoryValues
Attack typesCredential Phishing
Tactics and techniquesEvasion, Social engineering

Event coverage

Message attribute
body.current_thread
body.html
sender.email
type

Rule body MQL

type.inbound
// CTA link with action-oriented display text pointing to a different domain than the sender
and any(body.current_thread.links,
        regex.icontains(.display_text,
                        '(?:open|sign.?in|log.?in|retain|credential|secure|confirm|accept|release|document)'
        )
        and .href_url.domain.root_domain != sender.email.domain.root_domain
)
// tall rendered email with low word density
and beta.parse_exif(file.message_screenshot()).image_height > 1500
and beta.parse_exif(file.message_screenshot()).image_height * 100 / regex.count(body.html.display_text,
                                                                                '\S+'
) > 500
// html whitespace stuffing patterns
and (
  // bare div-br blocks repeated 30+ times
  regex.icontains(body.html.raw, '(?:<div>\s*<br\s*/?\s*>\s*</div>\s*){30,}')
  // style div-br blocks repeated 20+ times
  or regex.icontains(body.html.raw,
                     '(?:<div\s+style="[^"]+"\s*[^>]*>\s*<br\s*/?\s*>\s*</div>\s*){20,}'
  )
  // class-attributed div-br blocks repeated 20+ times (Outlook elementToProof pattern)
  or regex.icontains(body.html.raw,
                     '(?:<div\s+class="[^"]*"[^>]*>\s*<br\s*/?\s*>\s*</div>\s*){20,}'
  )
  // p-nbsp blocks repeated 25+ times
  or regex.icontains(body.html.raw,
                     '(?:<p>\s*(?:&nbsp;|&#160;)\s*</p>\s*){25,}'
  )
  // css margin-top pushdown >= 1500px
  or (
    regex.icontains(body.html.raw,
                    'margin-top\s*:\s*(?:1[5-9]\d{2}|[2-9]\d{3}|\d{5,})px'
    )
    and not regex.icontains(body.html.raw,
                            'position\s*:\s*absolute[^"]*margin-top\s*:\s*(?:1[5-9]\d{2}|[2-9]\d{3}|\d{5,})px'
    )
    and not regex.icontains(body.html.raw,
                            'margin-left\s*:\s*\d{3,}px[^"]*margin-top\s*:\s*(?:1[5-9]\d{2}|[2-9]\d{3}|\d{5,})px'
    )
  )
)

Detection logic

Scope: inbound message.

Detects inbound messages containing credential-related action links with tall screenshot images and HTML padding techniques used to evade detection. The rule identifies messages with excessive empty div tags, non-breaking spaces, or large margin-top values that artificially increase content height while hiding malicious intent.

  1. inbound message
  2. any of body.current_thread.links where all hold:
    • .display_text matches '(?:open|sign.?in|log.?in|retain|credential|secure|confirm|accept|release|document)'
    • .href_url.domain.root_domain is not sender.email.domain.root_domain
  3. beta.parse_exif(file.message_screenshot()).image_height > 1500
  4. beta.parse_exif(file.message_screenshot()).image_height * 100 / regex.count(body.html.display_text) > 500
  5. any of:
    • body.html.raw matches '(?:<div>\\s*<br\\s*/?\\s*>\\s*</div>\\s*){30,}'
    • body.html.raw matches '(?:<div\\s+style="[^"]+"\\s*[^>]*>\\s*<br\\s*/?\\s*>\\s*</div>\\s*){20,}'
    • body.html.raw matches '(?:<div\\s+class="[^"]*"[^>]*>\\s*<br\\s*/?\\s*>\\s*</div>\\s*){20,}'
    • body.html.raw matches '(?:<p>\\s*(?:&nbsp;|&#160;)\\s*</p>\\s*){25,}'
    • all of:
      • body.html.raw matches 'margin-top\\s*:\\s*(?:1[5-9]\\d{2}|[2-9]\\d{3}|\\d{5,})px'
      • not:
        • body.html.raw matches 'position\\s*:\\s*absolute[^"]*margin-top\\s*:\\s*(?:1[5-9]\\d{2}|[2-9]\\d{3}|\\d{5,})px'
      • not:
        • body.html.raw matches 'margin-left\\s*:\\s*\\d{3,}px[^"]*margin-top\\s*:\\s*(?:1[5-9]\\d{2}|[2-9]\\d{3}|\\d{5,})px'

Inspects: body.current_thread.links, body.current_thread.links[].display_text, body.current_thread.links[].href_url.domain.root_domain, body.html.raw, sender.email.domain.root_domain, type.inbound. Sensors: beta.parse_exif, file.message_screenshot, regex.icontains.

Indicators matched (8)

FieldMatchValue
regex.icontainsregex(?:open|sign.?in|log.?in|retain|credential|secure|confirm|accept|release|document)
regex.icontainsregex(?:<div>\s*<br\s*/?\s*>\s*</div>\s*){30,}
regex.icontainsregex(?:<div\s+style="[^"]+"\s*[^>]*>\s*<br\s*/?\s*>\s*</div>\s*){20,}
regex.icontainsregex(?:<div\s+class="[^"]*"[^>]*>\s*<br\s*/?\s*>\s*</div>\s*){20,}
regex.icontainsregex(?:<p>\s*(?:&nbsp;|&#160;)\s*</p>\s*){25,}
regex.icontainsregexmargin-top\s*:\s*(?:1[5-9]\d{2}|[2-9]\d{3}|\d{5,})px
regex.icontainsregexposition\s*:\s*absolute[^"]*margin-top\s*:\s*(?:1[5-9]\d{2}|[2-9]\d{3}|\d{5,})px
regex.icontainsregexmargin-left\s*:\s*\d{3,}px[^"]*margin-top\s*:\s*(?:1[5-9]\d{2}|[2-9]\d{3}|\d{5,})px