JustHTML has a Sanitizer Bypass (in Markdown)
Date: 2026-03
Affected: pip/justhtml
Attack vector: xss
Category: vulnerability-disclosure
Severity: Medium
CWE: CWE-79

## Summary `to_markdown()` does not sufficiently escape text content that looks like HTML. As a result, untrusted input that is safe in `to_html()` can become raw HTML in Markdown output. This is not specific to tokenizer raw-text states like `<title>`, `<noscript>`, or `<plaintext>`, although those states can trigger the behavior. The root cause is broader: Markdown text serialization leaves angle brackets unescaped in text nodes. ## Details When converting a parsed document to Markdown, text nodes are escaped for a small set of Markdown metacharacters, but HTML-significant characters such as `<` and `>` are preserved. That means content parsed as text, including entity-decoded text or text produced by RCDATA/RAWTEXT-style parsing, can be emitted into Markdown as raw HTML. Examples of affected input include: - Text produced from entity-decoded input such as `&lt;script&gt;...&lt;/script&gt;` - Text inside elements like `<title>`, `<textarea>`, `<noscript>` (when parsed as raw text), and `<plaintext>` This is distinct from actual `<script>` or `<style>` elements in the DOM. Those are already dropped by default in `to_markdown()` unless `html_passthrough=True`. ## Proof of Concept ### General case ```python from justhtml import JustHTML doc = JustHTML("<p>&lt;img src=x onerror=alert(1)&gt;</p>", fragment=True) print(doc.to_html()) print() print(doc.to_markdown())


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