GHSA-5vp3-3cg6-2rq3: Justhtml Cross-Site Scripting Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-5vp3-3cg6-2rq3",
  "modified" : "2026-03-24T19:22:21Z",
  "published" : "2026-03-24T19:22:21Z",
  "aliases" : [ ],
  "summary" : "JustHTML is vulnerable to XSS via code fence breakout in <pre> content",
  "details" : "## Summary\n\n`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.\n\nAn 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.\n\nThis 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.\n\n## Details\n\nThe vulnerable `<pre>` Markdown path:\n\n- extracts decoded text from the `<pre>` 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 `&lt;img ...&gt;`, 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 `<pre>` text to break out of the fixed fence.\n\n## Reproduction\n\n```python\nfrom justhtml import JustHTML\n\npayload = \"<pre>&#96;&#96;&#96;\\n&lt;img src=x onerror=alert(1)&gt;</pre>\"\ndoc = JustHTML(payload, fragment=True)  # default sanitize=True\n\nprint(doc.to_html(pretty=False))\n# <pre>```\n# &lt;img src=x onerror=alert(1)&gt;</pre>\n\nprint(doc.to_markdown())\n# ```\n# ```\n# <img src=x onerror=alert(1)>\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's raw-HTML handling.\n\n## Root Cause\n\nThe `<pre>` 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 `<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.",
  "severity" : [ {
    "type" : "CVSS_V4",
    "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"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "justhtml"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "1.13.0"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 1.12.0"
    }
  } ],
  "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"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-79", "CWE-80" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-03-24T19:22:21Z",
    "nvd_published_at" : null
  }
}