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Fencer Intel
medium 5.4

CVE-2026-126365 json

Astro: XSS via unescaped code for unsupported languages in @astrojs/prism

published 17d ago modified 2d ago also EUVD-2026-124749 , GHSA-ff38-p3qj-4pmf

Unreviewed: the maintainers' own advisory

Published by the maintainers on their GitHub repository withastro/astro. GitHub has not reviewed it into the GitHub Advisory Database, so the affected packages and versions are the maintainers' word. Read the full advisory on GitHub.

cvss
5.4
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.2%
EPSS 0.2% chance of exploitation in 30 days, percentile 6%
public exploits
0
none known

Summary

Astro is a web framework for content-driven websites. Prior to @astrojs/prism 4.1.2, and from @astrojs/markdoc 0.3.0 until 2.1.2, runHighlighterWithAstro() returned code unchanged when Prism had no grammar for the requested language, and callers inserted that result as raw HTML. A code block labeled with an unsupported language could therefore render embedded HTML and script instead of displaying it as text. The Prism component was affected when applications supplied untrusted code with an unsupported or attacker-controlled lang, and the @astrojs/markdoc prism() extension was affected for less-trusted content authors even when Markdoc's ordinary HTML rendering was disabled. This issue is fixed in @astrojs/prism 4.1.2 and @astrojs/markdoc 2.1.2.

Patch / fix

  • npm @astrojs/prism 4.1.2
  • npm @astrojs/markdoc 2.1.2

Is CVE-2026-126365 in your dependencies? Fencer shows which of your repositories are affected as soon as an advisory lands, and opens the fix as a pull request.

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Exploit prediction

0.17% chance of exploitation in the next 30 days, higher than 6% of all scored CVEs (FIRST EPSS, 2026-11-14).

CVSS scores

assigned by version score vector
CNA 3.1 5.4 CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N

Affected packages

ecosystem package affected fixed in
npm @astrojs/prism
<=4.1.1
4.1.2
npm @astrojs/markdoc
>=0.3.0, <=2.1.1
2.1.2

Affected products

  • withastro astro
  • @astrojs prism
  • @astrojs markdoc

Classification

Weakness
  • CWE-79 Improper Neutralization of Input During Web Page Generation

References

Sources

Fencer Intel keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

The maintainers of withastro/astro published their own advisory on GitHub. Reviewed sources take precedence over it.