GHSA-5VP3-3CG6-2RQ3
Vulnerability from github – Published: 2026-03-24 19:22 – Updated: 2026-03-24 19:22Summary
to_markdown() is vulnerable when serializing attacker-controlled <pre> content. The <pre> handler emits a fixed three-backtick fenced code block, but writes decoded text content into that fence without choosing a delimiter longer than any backtick run inside the content.
An attacker can place backticks and HTML-like text inside a sanitized <pre> element so that the generated Markdown closes the fence early and leaves raw HTML outside the code block. When that Markdown is rendered by a CommonMark/GFM-style renderer that allows raw HTML, the HTML executes.
This is a bypass of the v1.12.0 Markdown hardening. That fix escaped HTML-significant characters for regular text nodes, but <pre> uses a separate serialization path and does not apply the same protection.
Details
The vulnerable <pre> Markdown path:
- extracts decoded text from the
<pre>subtree - opens a fenced block with a fixed delimiter of ``````
- writes the decoded text directly into the output
- closes with another fixed ``````
Because the fence length is fixed, attacker-controlled content containing a backtick run of length 3 or more can terminate the code block. If the content also contains decoded HTML-like text such as <img ...>, that text appears outside the fence in the resulting Markdown and is treated as raw HTML by downstream Markdown renderers.
The issue is not that HTML-like text appears inside code blocks. The issue is that the serializer allows attacker-controlled <pre> text to break out of the fixed fence.
Reproduction
from justhtml import JustHTML
payload = "<pre>```\n<img src=x onerror=alert(1)></pre>"
doc = JustHTML(payload, fragment=True) # default sanitize=True
print(doc.to_html(pretty=False))
# <pre>```
# <img src=x onerror=alert(1)></pre>
print(doc.to_markdown())
# ```
# ```
# <img src=x onerror=alert(1)>
# ```
Rendered as CommonMark/GFM-style Markdown, that output is interpreted as:
- Line 1 opens a fenced code block
- Line 2 closes it
- Line 3 is raw HTML outside the fence
- Line 4 opens a new fence
Impact
Applications that treat JustHTML(..., sanitize=True).to_markdown() output as safe for direct rendering in Markdown contexts may be exposed to XSS, depending on the downstream Markdown renderer's raw-HTML handling.
Root Cause
The <pre> Markdown serializer uses a fixed fence instead of selecting a delimiter longer than the longest backtick run in the content.
Fix
When serializing <pre> content to Markdown, choose a fence length longer than any backtick run present in the code block content, with a minimum length of 3.
{
"affected": [
{
"database_specific": {
"last_known_affected_version_range": "\u003c= 1.12.0"
},
"package": {
"ecosystem": "PyPI",
"name": "justhtml"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "1.13.0"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [],
"database_specific": {
"cwe_ids": [
"CWE-79",
"CWE-80"
],
"github_reviewed": true,
"github_reviewed_at": "2026-03-24T19:22:21Z",
"nvd_published_at": null,
"severity": "HIGH"
},
"details": "## Summary\n\n`to_markdown()` is vulnerable when serializing attacker-controlled `\u003cpre\u003e` content. The `\u003cpre\u003e` handler emits a fixed three-backtick fenced code block, but writes decoded text content into that fence without choosing a delimiter longer than any backtick run inside the content.\n\nAn attacker can place backticks and HTML-like text inside a sanitized `\u003cpre\u003e` element so that the generated Markdown closes the fence early and leaves raw HTML outside the code block. When that Markdown is rendered by a CommonMark/GFM-style renderer that allows raw HTML, the HTML executes.\n\nThis is a bypass of the v1.12.0 Markdown hardening. That fix escaped HTML-significant characters for regular text nodes, but `\u003cpre\u003e` uses a separate serialization path and does not apply the same protection.\n\n## Details\n\nThe vulnerable `\u003cpre\u003e` Markdown path:\n\n- extracts decoded text from the `\u003cpre\u003e` subtree\n- opens a fenced block with a fixed delimiter of ``````\n- writes the decoded text directly into the output\n- closes with another fixed ``````\n\nBecause the fence length is fixed, attacker-controlled content containing a backtick run of length 3 or more can terminate the code block. If the content also contains decoded HTML-like text such as `\u0026lt;img ...\u0026gt;`, that text appears outside the fence in the resulting Markdown and is treated as raw HTML by downstream Markdown renderers.\n\nThe issue is not that HTML-like text appears inside code blocks. The issue is that the serializer allows attacker-controlled `\u003cpre\u003e` text to break out of the fixed fence.\n\n## Reproduction\n\n```python\nfrom justhtml import JustHTML\n\npayload = \"\u003cpre\u003e\u0026#96;\u0026#96;\u0026#96;\\n\u0026lt;img src=x onerror=alert(1)\u0026gt;\u003c/pre\u003e\"\ndoc = JustHTML(payload, fragment=True) # default sanitize=True\n\nprint(doc.to_html(pretty=False))\n# \u003cpre\u003e```\n# \u0026lt;img src=x onerror=alert(1)\u0026gt;\u003c/pre\u003e\n\nprint(doc.to_markdown())\n# ```\n# ```\n# \u003cimg src=x onerror=alert(1)\u003e\n# ```\n\n```\n\nRendered as CommonMark/GFM-style Markdown, that output is interpreted as:\n\n1. Line 1 opens a fenced code block\n2. Line 2 closes it\n3. Line 3 is raw HTML outside the fence\n4. Line 4 opens a new fence\n\n## Impact\n\nApplications that treat `JustHTML(..., sanitize=True).to_markdown()` output as safe for direct rendering in Markdown contexts may be exposed to XSS, depending on the downstream Markdown renderer\u0027s raw-HTML handling.\n\n## Root Cause\n\nThe `\u003cpre\u003e` Markdown serializer uses a fixed fence instead of selecting a delimiter longer than the longest backtick run in the content.\n\n## Fix\n\nWhen serializing `\u003cpre\u003e` content to Markdown, choose a fence length longer than any backtick run present in the code block content, with a minimum length of 3.",
"id": "GHSA-5vp3-3cg6-2rq3",
"modified": "2026-03-24T19:22:21Z",
"published": "2026-03-24T19:22:21Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/EmilStenstrom/justhtml/security/advisories/GHSA-5vp3-3cg6-2rq3"
},
{
"type": "WEB",
"url": "https://github.com/EmilStenstrom/justhtml/commit/f35f8f723c713bd8f912d86e9ec6881275ff5af9"
},
{
"type": "PACKAGE",
"url": "https://github.com/EmilStenstrom/justhtml"
},
{
"type": "WEB",
"url": "https://github.com/EmilStenstrom/justhtml/releases/tag/v1.13.0"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N",
"type": "CVSS_V4"
}
],
"summary": "JustHTML is vulnerable to XSS via code fence breakout in \u003cpre\u003e content"
}
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or observed by the user.
- Confirmed: The vulnerability has been validated from an analyst's perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.
Related by attack behaviour
Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.