# CVE-2026-126365: Astro: XSS via unescaped code for unsupported languages in @astrojs/prism

> Apps passing untrusted code with unsupported languages to Astro's Prism highlighter render attacker HTML and script; Astro and Markdoc users should care.

- **Severity:** Medium 5.4 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Harmful if only installed:** No
- **Needs a specific call:** Yes (`runHighlighterWithAstro()`)
- **Fix available:** No
- **Published:** 12 Nov 2026
- **Updated:** 12 Nov 2026
- **Also known as:** GHSA-ff38-p3qj-4pmf

## What is CVE-2026-126365?

CVE-2026-126365 is a medium-severity cross-site scripting vulnerability in @astrojs astro, published on 12 Nov 2026. Any signed-in user can exploit it over the network, and only if a user opens a file or follows a link.

### Official description

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.

Weakness: CWE-79 (Improper Neutralization of Input During Web Page Generation)

## Which versions are affected by CVE-2026-126365?

An affected version is only exploitable if your software calls runHighlighterWithAstro(). Many projects that include it are not exposed.


## How do I fix CVE-2026-126365?

No fix has been published for CVE-2026-126365 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-126365 being exploited?

There is no known exploitation of CVE-2026-126365 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-126365 and fix it if it is affected.

What it is: CVE-2026-126365 is a medium-severity cross-site scripting vulnerability in @astrojs astro, published on 12 Nov 2026. Any signed-in user can exploit it over the network, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2026-126365 in the wild and no public exploit code.
Exposure: An affected version is only exploitable if your software calls runHighlighterWithAstro(). Many projects that include it are not exposed.

Affected product: @astrojs astro. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
   If an affected version is present, also search the code for calls to runHighlighterWithAstro() and report whether any exist.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-126365/ (Markdown: https://insights.sbomify.com/CVE-2026-126365/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-126365
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-126365
- OSV: https://osv.dev/vulnerability/CVE-2026-126365

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From sbomify Insights: https://insights.sbomify.com/CVE-2026-126365/
Summaries are generated and may contain mistakes; the official description above is the record.