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fix(security): escape mXSS-detection text fallback in sanitizeHtmlCore #182

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Merged
JosunLP merged 1 commit into from
Jul 6, 2026
Merged

fix(security): escape mXSS-detection text fallback in sanitizeHtmlCore #182

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23 changes: 21 additions & 2 deletions src/security/sanitize-core.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -41,6 +41,23 @@ const isAllowedAttribute = (
return allowedSet.has(lowerName);
};

/**
* Escape HTML entities so text is inert when assigned to an HTML sink.
* Local copy to avoid a circular import with `sanitize.ts`.
* @internal
*/
const escapeHtmlText = (text: string): string => {
const escapeMap: Record<string, string> = {
'&': '&amp;',
'<': '&lt;',
'>': '&gt;',
'"': '&quot;',
"'": '&#x27;',
'`': '&#x60;',
};
return text.replace(/[&<>"'`]/g, (char) => escapeMap[char]);
};

/**
* Check if an ID/name value could cause DOM clobbering.
* @internal
Expand Down Expand Up @@ -356,8 +373,10 @@ export const sanitizeHtmlCore = (html: string, options: SanitizeOptions = {}): s
// Verify stability: if content mutates between parses, it indicates mXSS attempt
if (firstPass !== secondPass) {
// Content mutated during re-parse - potential mXSS detected.
// Return safely escaped text content as fallback.
return fragment.textContent ?? '';
// Callers assign this return value to HTML sinks (innerHTML etc.), so the
// text fallback must be HTML-escaped: entity-decoded text nodes can contain
// live markup (e.g. `&lt;img onerror=...&gt;` decoded to `<img onerror=...>`).
return escapeHtmlText(fragment.textContent ?? '');
}

return secondPass;
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13 changes: 13 additions & 0 deletions tests/security.test.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -92,6 +92,19 @@ describe('security/sanitizeHtml', () => {
});
expect(result).not.toContain('data-secret');
});

it('escapes the mXSS-detection text fallback (entity-smuggled markup stays inert)', () => {
// Foster-parenting makes serialize→re-parse unstable, forcing the mXSS
// fallback branch; the entity-encoded payload decodes to live markup in
// textContent and must not survive as executable HTML.
const payload = '<a><table><a>&lt;img src=x onerror=alert(1)&gt;';
const result = String(sanitizeHtml(payload));
expect(result).not.toContain('<img');

const host = document.createElement('div');
host.innerHTML = result;
expect(host.querySelectorAll('img').length).toBe(0);
});
});

describe('security/escapeHtml', () => {
Expand Down