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Create headers_mailer_contains_hidden_content.yml #3474

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Create headers_mailer_contains_hidden_content.yml #3474

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20c5d57
new rule
D-Bolton Nov 4, 2025
e3101ec
Auto add rule ID
Nov 4, 2025
6544e8b
fix formatting
D-Bolton Nov 5, 2025
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28 changes: 28 additions & 0 deletions detection-rules/headers_mailer_contains_hidden_content.yml
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Original file line number Diff line number Diff line change
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name: "Headers: Microsoft CDO or PHPMailer with hidden HTML content"
description: "Detects messages sent via Microsoft CDO for Windows 2000 or PHPMailer that contain HTML paragraph elements with transparent text or hidden content styling, commonly used to evade content analysis."
type: "rule"
severity: "low"
source: |
type.inbound
and (
headers.mailer == 'Microsoft CDO for Windows 2000'
or strings.ilike(headers.mailer, "PHPMailer*")
)
and any(html.xpath(body.html, '//p').nodes,
.display_text == ''
and .inner_text == ''
and regex.icontains(.raw,
'[^-]color\s*:\s*transparent|mso-hide\s*:\s*all'
)
)

attack_types:
- "Credential Phishing"
- "Spam"
tactics_and_techniques:
- "Evasion"
detection_methods:
- "Content analysis"
- "Header analysis"
- "HTML analysis"
id: "550f1cff-6203-543a-9aa4-54717148bca9"
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