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.
- 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
- Fix
- None listed
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
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.
Escape output for the context it lands in and set a strict Content Security Policy. Upgrading the affected component is the real fix. More about Cross-site scripting.
Is CVE-2026-126365 being exploited?
There is no known exploitation of CVE-2026-126365 in the wild and no public exploit code.
Sources
Also known as GHSA-ff38-p3qj-4pmf.