JustHTML is vulnerable to XSS via code fence breakout in content

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

7.1 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N · score source

EPSS

• CVE-2026-5389: 0.23% estimated exploitation activity in the next 30 days; percentile 13.89 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary tomarkdown() is vulnerable when serializing attacker-controlled <pre content. The <pre 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 <pre 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 <pre uses a separate serialization path and does not apply the same protection. Details The vulnerable <pre Markdown path: - extracts decoded text from the <pre subtree - opens a fenced block with a fixed delimiter of - writes the decoded text directly into the output - closes with another fixed Because the…

Published by the source

24 Mar 2026, 19:22 UTC

Last modified by the source

24 Aug 2026, 03:56 UTC

Source last checked

26 Oct 2026, 13:37 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-5vp3-3cg6-2rq3

2. Revision 2 · amended · 25 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-5vp3-3cg6-2rq3

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# Patch Next advisory brief: CVE-2026-5389

Advisory: JustHTML is vulnerable to XSS via code fence breakout in content
IDs: CVE-2026-5389, GHSA-5vp3-3cg6-2rq3
Record: https://patchnext.com/advisories/cve-2026-5389 (revision 3, source checked 26 Oct 2026, 13:37 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.1)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: justhtml 1.13.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI justhtml: affected <= 1.12.0; fixed in 1.13.0

## Suggested inspection (read-only)
- `python -m pip show justhtml`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on justhtml.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-5vp3-3cg6-2rq3 (OSV record GHSA-5vp3-3cg6-2rq3 lists affected ranges: PyPI justhtml >=0 <1.13.0 (fixed))
- Verified: https://github.com/advisories/GHSA-5vp3-3cg6-2rq3 (GitHub-reviewed advisory GHSA-5vp3-3cg6-2rq3 lists affected ranges: PyPI justhtml >=0 <=1.12.0)

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