Astro: Reflected/Stored XSS via unescaped <script>/<style> tag content in JSX/MDX rendering

## Summary Astro's JSX and MDX rendering pipeline treated string children of `<script>` and `<style>` elements as trusted HTML. When a dynamic value was passed through a JSX expression, such as `<script>{value}</script>`, an attacker-controlled value containing `</script>` or `</style>` could terminate the element and inject arbitrary HTML and JavaScript. Other JSX expression contexts escaped dynamic strings automatically. This behavior was limited to `<script>` and `<style>` children rendered through Astro's `astro:jsx` renderer, including MDX components. ## Details The affected rendering paths marked all string children of `<script>` and `<style>` elements as safe HTML, without distinguishing literal content generated by the compiler from dynamic expression values. An application was vulnerable when attacker-controlled data, such as a query parameter or CMS field, was interpolated directly into one of these elements. For example, a value containing `</script><script>alert(1)</script>` passed to `<script>{value}</script>` was emitted without escaping. The browser interpreted the first closing tag as markup and executed the injected script. The fix preserves raw rendering for literal script and style content while escaping dynamic string children like other JSX expression values. ## Impact Affected applications could be vulnerable to reflected or stored cross-site scripting. Exploitation required an application to pass attacker-controlled data directly as a dynamic child of a `<script>` or `<style>` element in MDX or through the `astro:jsx` runtime. Successful exploitation allowed arbitrary JavaScript execution in the victim's browser under the application's origin. ## Patches This issue is patched in `[email protected]`. ## Workarounds Do not interpolate untrusted dynamic values directly into `<script>` or `<style>` element children. Applications that cannot upgrade should validate and contextually encode these values before rendering them. ## Credit Reported by [@RKasai127](https://github.com/RKasai127).

Source published

22 Oct 2026 16:30:26 UTC

Source modified

22 Oct 2026 16:45:03 UTC

Observed here

24 Oct 2026 13:17:17 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N

Publisher rating · MODERATE