Vulnerability in justhtml (CVE-2026-5389)
Summary
Vulnerability in justhtml (CVE-2026-5389). Risk of unauthorized operations or information disclosure.
Original description
The description as published by the source.
Summary
to_markdown() is vulnerable when serializing attacker-controlled content. The 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 `` 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 `` uses a separate serialization path and does not apply the same protection.
Details
The vulnerable `` Markdown path:
- extracts decoded text from the `` 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 `` text to break out of the…
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-2026-5389 has been found in justhtml.
Risk of unauthorized operations or information disclosure. CVSS score: 6.1/10.
What to do: apply the vendor's official patched release.
If unsure, ask your IT team or search "justhtml CVE-2026-5389" on the vendor's site.
CVE-2026-5389 (justhtml) — CWE-80 / CVSS v3 6.1
Attack vector: remote (network-reachable) / unauthenticated
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.
A vulnerability (CWE-80) 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 the patched release per vendor advisory.
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。 -
2Post-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