GHSA-vrx2-77f2-ww34: Justhtml Justhtml Has Sanitization Bypass Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-vrx2-77f2-ww34",
  "modified" : "2026-04-22T21:25:46Z",
  "published" : "2026-04-22T21:25:46Z",
  "aliases" : [ ],
  "summary" : "justhtml has sanitization bypass in custom policies and programmatic DOM",
  "details" : "## Summary\n\n`justhtml` `1.17.0` fixes multiple security issues in sanitization, serialization, and programmatic DOM handling.\n\nMost of these issues affected advanced or custom configurations rather than the default safe path.\n\n## Affected versions\n\n- `justhtml` `<= 1.16.0`\n\n## Fixed version\n\n- `justhtml` `1.17.0` released on April 19, 2026\n\n## Impact\n\n### Custom SVG / MathML sanitization policies\nCustom policies that preserved foreign namespaces could allow dangerous content to survive sanitization, including:\n\n- active HTML integration points such as SVG `<foreignObject>`, MathML `<annotation-xml encoding=\"text/html\">`, SVG `<title>` / `<desc>`, and MathML text integration points\n- mutation-XSS parser-differential payloads that looked inert in memory but became active HTML after reparse\n- SVG `filter=\"url(...)\"` attributes that could trigger external fetches\n\nThese issues affected:\n- `JustHTML(..., sanitize=True)` with custom foreign-namespace policies\n- `sanitize()` / `sanitize_dom()`\n- low-level terminal `Sanitize(...)` transform execution\n\n### Preserved `<style>` handling\nConstructor-time sanitization and explicit `Sanitize(...)` transforms did not fully match `sanitize()` / `sanitize_dom()` when custom policies preserved `<style>`.\n\nThat could leave resource-loading CSS such as `@import` or `background-image:url(...)` in sanitized output from HTML string input.\n\n### Programmatic DOM serialization\nProgrammatic `script`, `style`, and `Comment(...)` nodes could still serialize into active markup in some edge cases.\n\nThis could affect applications that build or mutate DOM trees directly before calling `to_html()` or `to_markdown(html_passthrough=True)`.\n\n### Cache mutation and DOM cycle handling\nTwo lower-severity hardening fixes were included:\n\n- compiled sanitize-pipeline caches could be mutated after warming and weaken later sanitization\n- parent/child cycles in programmatic DOM trees could cause infinite loops in operations such as `to_html()` and `sanitize_dom()`\n\n## Default configuration\n\nMost of the issues above did **not** affect ordinary parsed HTML with the default `JustHTML(..., sanitize=True)` configuration.\n\nThe main risk areas were:\n\n- custom policies that preserve SVG or MathML\n- custom policies that preserve `<style>`\n- programmatic DOM construction or mutation\n- low-level direct sanitizer/transform APIs\n\n## Recommended action\n\nUpgrade to `justhtml` `1.17.0`.\n\nIf users cannot upgrade immediately:\n\n- avoid preserving SVG or MathML for untrusted input\n- avoid preserving `<style>` for untrusted input\n- avoid mutating programmatic DOM trees with untrusted `script`, `style`, or comment content\n- avoid mutating warmed policy internals or sanitizer caches\n\n## Credit\n\nDiscovered during an internal security review of `justhtml`.",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:N/VI:H/VA:N/SC:N/SI:L/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "justhtml"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "1.17.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/EmilStenstrom/justhtml/security/advisories/GHSA-vrx2-77f2-ww34"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/EmilStenstrom/justhtml"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-436", "CWE-471", "CWE-79", "CWE-835" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-04-22T21:25:46Z",
    "nvd_published_at" : null
  }
}