Cross-Site Scripting (XSS) in justhtml (CVE-2026-77088)
Summary
Cross-site scripting in justhtml (CVE-2026-77088). Risk of unauthorized operations or information disclosure.
Original description
The description as published by the source.
justhtml: to_markdown() code-span blank-line breakout enables XSS
Summary
In justhtml 0.9.0 through 1.21.0, to_markdown() renders text (and text inside a link) as an inline Markdown code span whose only protection is backtick-fence length. A blank line (\n\n) in that text terminates the inline span in any compliant Markdown renderer, so attacker-controlled text that survived HTML sanitization is emitted unescaped after the blank line and is re-parsed as live raw HTML/Markdown — yielding XSS in the default configuration. Likely CWE-79 (Cross-site Scripting) arising from CWE-116 (Improper Encoding/Escaping of Output).
Details
to_markdown() is documented as a safety surface. docs/text.md states the guarantee applies "to the HTML produced by rendering that Markdown with a compliant Markdown renderer," and SECURITY.md promises to_markdown() "escapes line-start Markdown markers that could change block structure" and "uses code fences long enough to contain backticks safely."
The inline code-span helper only sizes the backtick fence; it never accounts for block boundaries:
src/justhtml/node.py:32-41 (tag v1.21.0):
def _markdown_code_span(s: str | None) -> str:
if s is None:
s = ""
# Use a backtick fence longer than any run of backticks inside.
fence = _markdown_backtick_fence(s, minimum=1)
# CommonMark requires a space if the content starts/ends with backticks.
needs_space = s.startswith("`")…
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-2026-77088 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-2026-77088" on the vendor's site.
CVE-2026-77088 (justhtml) — CWE-79 / 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.
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 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