Body: CSS zero-value calc() obfuscation

calendar Aug 5, 2026  ·
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Detects inbound messages containing HTML where a CSS calc() expression subtracts an identical value and unit from itself (e.g., calc(100px - 100px)), always resolving to zero. This pattern is commonly used to hide or collapse content from view while keeping it present in the underlying HTML, a technique often leveraged to evade text-based detection or conceal malicious content from recipients.

Sublime rule (View on GitHub)

 1name: "Body: CSS zero-value calc() obfuscation"
 2description: "Detects inbound messages containing HTML where a CSS calc() expression subtracts an identical value and unit from itself (e.g., calc(100px - 100px)), always resolving to zero. This pattern is commonly used to hide or collapse content from view while keeping it present in the underlying HTML, a technique often leveraged to evade text-based detection or conceal malicious content from recipients."
 3type: "rule"
 4severity: "medium"
 5source: |
 6  type.inbound
 7  and any(regex.iextract(body.html.raw,
 8                         'calc\(\s*(?P<n1>\d+(?:\.\d+)?)(?P<u1>px|vw|vh|em|rem|%)\s*-\s*(?P<n2>\d+(?:\.\d+)?)(?P<u2>px|vw|vh|em|rem|%)\s*\)'
 9          ),
10          .named_groups["n1"] == .named_groups["n2"]
11          and .named_groups["u1"] == .named_groups["u2"]
12  )  
13attack_types:
14  - "Credential Phishing"
15  - "Spam"
16tactics_and_techniques:
17  - "Evasion"
18detection_methods:
19  - "HTML analysis"
20id: "a9bd4f3f-3c8e-5807-9779-337498fc05e5"
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