Astro is vulnerable to Reflected XSS via the server islands feature

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

Astro is a web framework. Prior to version 5.15.8, a reflected XSS vulnerability is present when the server islands feature is used in the targeted application, regardless of what was intended by the component template(s). This issue has been patched in version 5.15.8.

Source published

19 Nov 2025 16:41:03 UTC

Source modified

19 Nov 2025 21:07:23 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

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CVSS 3.1 · 7.1 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:H/A:N

Astro is a web framework. Prior to version 5.15.8, a reflected XSS vulnerability is present when the server islands feature is used in the targeted application, regardless of what was intended by the component template(s). This issue has been patched in version 5.15.8.

Source published

Nov 19, 2025, 4:41:03 PM (timezone not stated)

Source modified

Nov 19, 2025, 9:07:23 PM (timezone not stated)

Observed here

9 Nov 2026 04:07:50 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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EUVD CVSS 3.1 · 7.1 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:H/A:N

Astro is a web framework. Prior to version 5.15.8, a reflected XSS vulnerability is present when the server islands feature is used in the targeted application, regardless of what was intended by the component template(s). This issue has been patched in version 5.15.8.

Source published

19 Nov 2025 17:15:52 UTC

Source modified

18 Oct 2026 20:10:00 UTC

Observed here

25 Oct 2026 03:17:36 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 7.1 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:H/A:N

CVSS 3.1 · [email protected] · Primary · 5.4 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N

## Summary After some research it appears that it is possible to obtain a reflected XSS when the server islands feature is used in the targeted application, **regardless of what was intended by the component template(s)**. ## Details Server islands run in their own isolated context outside of the page request and use the following pattern path to hydrate the page: `/_server-islands/[name]`. These paths can be called via GET or POST and use three parameters: - `e`: component to export - `p`: the transmitted properties, encrypted - `s`: for the slots Slots are placeholders for external HTML content, and therefore allow, by default, the injection of code if the component template supports it, nothing exceptional in principle, just a feature. This is where it becomes problematic: it is possible, independently of the component template used, even if it is completely empty, to inject a slot containing an XSS payload, whose parent is a tag whose name is is the absolute path of the island file. Enabling reflected XSS on any application, regardless of the component templates used, provided that the server islands is used at least once. **How ?** By default, when a call is made to the endpoint `/_server-islands/[name]`, the value of the parameter `e` is `default`, pointing to a function exported by the component's module. Upon further investigation, we find that two other values ​​are possible for the component export (param `e`) in a typical configuration: `url` and `file`. `file` returns a string value corresponding to the absolute path of the island file. Since the value is of type `string`, it fulfills the following condition and leads to [this code block](https://github.com/withastro/astro/blob/190106149908ef6826899459146ef9f0ead602ab/packages/astro/src/runtime/server/render/component.ts#L279): <img width="804" height="571" alt="image" src="https://github.com/user-attachments/assets/25ea6c16-fc27-477a-a1ad-e5edf0819b31" /> An entire template is created, completely independently, and then returned: - the absolute path name is sanitized and then injected as the tag name - `childSlots`, the value provided to the `s` parameter, is injected as a child All of this is done using `markHTMLString`. This allows the injection of any XSS payload, **even if the component template intended by the application is initially empty or does not provide for the use of slots.** ## Proof of concept For our Proof of Concept (PoC), we will use a minimal repository: - Latest Astro version at the time (5.15.6) - Use of Island servers, with a completely empty component, to demonstrate what we explained previously [Download the PoC repository](https://github.com/zhero-web-sec/astro-app-2) Access the following URL and note the opening of the popup, demonstrating the reflected XSS: http://localhost:4321/_server-islands/ServerTime?e=file&p=&s={%22zhero%22:%22%3Cimg%20src=x%20onerror=alert(0)%3E%22} <img width="1781" height="529" alt="image" src="https://github.com/user-attachments/assets/92f8134a-d1c7-4d3f-818e-214842c239c8" /> The value of the parameter `s` must be in JSON format and the payload must be injected at the value level, not the key level : <img width="3273" height="1840" alt="for_respected_patron" src="https://github.com/user-attachments/assets/8ac0079a-3dee-49e8-b639-322f77c84b83" /> Despite the initial template being empty, it is created because the value of the URL parameter `e` is set to `file`, as explained earlier. The parent tag is the name of the component's internal route, and its child is the value of the key "zhero" (*the name doesn't matter*) of the URL parameter `s`. ## Credits - Allam Rachid ([zhero;](https://zhero-web-sec.github.io/research-and-things/)) - Allam Yasser (inzo)

Source published

19 Nov 2025 20:00:14 UTC

Source modified

20 Nov 2025 14:43:59 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:L/I:H/A:N

Publisher rating · HIGH