Cross-Site Scripting (XSS) in justhtml (CVE-2026-8445)
Summary
Cross-site scripting in justhtml (CVE-2026-8445). Successful exploitation can lead to full system takeover. Mitigation: upgrade to 1.12.0 or later.
Original description
The description as published by the source.
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 , , or ``, 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
<script>...</script> - Text inside elements like
,,(when parsed as raw text), and
This is distinct from actual or elements in the DOM. Those are already dropped by default in to_markdown() unless html_passthrough=True.
Proof of Concept
General case
from justhtml import JustHTML
doc = JustHTML("<img src=x onerror=alert(1)>", fragment=True)
print(doc.to_html())
print()
print(doc.to_markdown())
Source: NIST National Vulnerability Database NVD GitHub Advisory
Auto-generated summary
Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.
A vulnerability tracked as CVE-2026-8445 has been found in justhtml.
Successful exploitation can lead to full system takeover. CVSS score: 9.8/10.
What to do: upgrade justhtml to 1.12.0 or later.
If unsure, ask your IT team or search "justhtml CVE-2026-8445" on the vendor's site.
CVE-2026-8445 (justhtml) — CWE-79 / CVSS v3 9.8
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Patched: 1.12.0 — apply immediately
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.
Cross-site scripting (XSS) (CWE-79) exists in justhtml.
Check the vendor advisory for the affected versions of justhtml.
Severity: Critical (CVSS 9.8/10). Successful exploitation can lead to full system takeover.
Update to 1.12.0.
Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.
Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'justhtml' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.
Response actions
Concrete steps and command examples for SOC/SRE teams to execute in order
-
1Identify exposure identify
grep -r 'justhtml' . | grep -v node_modulesリポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で
justhtmlを grep し、稼働しているサービス・バージョンを把握する。 -
2Apply patch patch
Upgrade justhtml to 1.12.0ステージング環境で 1.12.0 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。
-
3Post-deployment verification verify
Confirm patched version is live in productionパッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。
Affected packages
References
- package github.com
- web github.com
- web disclosure@vulncheck.com
- web nvd.nist.gov
- web github.com