GHSA-3RCM-VJRC-P45J

Vulnerability from github – Published: 2026-03-18 20:19 – Updated: 2026-03-18 20:19
VLAI
Summary
JustHTML has a Sanitizer Bypass (in Markdown)
Details

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("

<img src=x onerror=alert(1)>

", fragment=True)

print(doc.to_html()) print() print(doc.to_markdown())

Severity
5.3 (Medium)
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N
Show details on source website
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{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 1.11.0"
      },
      "package": {
        "ecosystem": "PyPI",
        "name": "justhtml"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "1.12.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [],
  "database_specific": {
    "cwe_ids": [
      "CWE-79"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-03-18T20:19:56Z",
    "nvd_published_at": null,
    "severity": "MODERATE"
  },
  "details": "## Summary\n\n`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.\n\nThis is not specific to tokenizer raw-text states like `\u003ctitle\u003e`, `\u003cnoscript\u003e`, or `\u003cplaintext\u003e`, although those states can trigger the behavior. The root cause is broader: Markdown text serialization leaves angle brackets unescaped in text nodes.\n\n## Details\n\nWhen converting a parsed document to Markdown, text nodes are escaped for a small set of Markdown metacharacters, but HTML-significant characters such as `\u003c` and `\u003e` 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.\n\nExamples of affected input include:\n\n- Text produced from entity-decoded input such as `\u0026lt;script\u0026gt;...\u0026lt;/script\u0026gt;`\n- Text inside elements like `\u003ctitle\u003e`, `\u003ctextarea\u003e`, `\u003cnoscript\u003e` (when parsed as raw text), and `\u003cplaintext\u003e`\n\nThis is distinct from actual `\u003cscript\u003e` or `\u003cstyle\u003e` elements in the DOM. Those are already dropped by default in `to_markdown()` unless `html_passthrough=True`.\n\n## Proof of Concept\n\n### General case\n\n```python\nfrom justhtml import JustHTML\n\ndoc = JustHTML(\"\u003cp\u003e\u0026lt;img src=x onerror=alert(1)\u0026gt;\u003c/p\u003e\", fragment=True)\n\nprint(doc.to_html())\nprint()\nprint(doc.to_markdown())",
  "id": "GHSA-3rcm-vjrc-p45j",
  "modified": "2026-03-18T20:19:56Z",
  "published": "2026-03-18T20:19:56Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/EmilStenstrom/justhtml/security/advisories/GHSA-3rcm-vjrc-p45j"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/EmilStenstrom/justhtml"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N",
      "type": "CVSS_V4"
    }
  ],
  "summary": "JustHTML has a Sanitizer Bypass (in Markdown)"
}



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Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.

Sightings

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Nomenclature

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  • Confirmed: The vulnerability has been validated from an analyst's perspective.
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