Cross-Site Scripting (XSS) in justhtml (CVE-2026-5751)
Summary
Cross-site scripting in justhtml (CVE-2026-5751). Risk of unauthorized operations or information disclosure. Mitigation: upgrade to 1.14.0 or later.
Original description
The description as published by the source.
Summary
A parser-differential / mutation XSS issue was found in justhtml when using a custom sanitization policy that preserves foreign namespaces such as SVG or MathML.
Under these custom settings, specially crafted input could sanitize into HTML that looked safe at first, but became unsafe when parsed again by a browser or another HTML parser.
Impact
This issue does not affect the default safe configuration.
You may be affected if you use a custom SanitizationPolicy with settings like:
drop_foreign_namespaces=False- allowlisted foreign elements such as MathML or SVG
- allowlisted raw-text containers such as ``
In that case, an attacker could inject markup that survives sanitization and turns into active HTML after re-parsing.
Affected versions
justhtml<= 1.13.0
Fixed version
- Fixed in
1.14.0
Workarounds
Until you upgrade:
- keep
drop_foreign_namespaces=True - avoid allowlisting foreign namespaces for untrusted input
- avoid allowlisting raw-text containers such as `` in custom policies
Notes
The default JustHTML(..., sanitize=True) behavior was not found to be vulnerable in this issue.
Credit
Discovered by JustHTML author during a LLM-based security review of justhtml.
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-2026-5751 has been found in justhtml.
Risk of unauthorized operations or information disclosure. CVSS score: 6.1/10.
What to do: upgrade justhtml to 1.14.0 or later.
If unsure, ask your IT team or search "justhtml CVE-2026-5751" on the vendor's site.
CVE-2026-5751 (justhtml) — CWE-79 / CVSS v3 6.1
Attack vector: remote (network-reachable) / unauthenticated
Patched: 1.14.0 — 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: Medium (CVSS 6.1/10). Risk of unauthorized operations or information disclosure.
Update to 1.14.0.
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 1.14.0ステージング環境で 1.14.0 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
- web github.com
- web github.com