CVE-2025-64764 is a high-severity (CVSS 7.1) CWE-80 vulnerability in astro. A fix is available for astro — see the affected versions and patch details below.
Astro is vulnerable to Reflected XSS via the server islands feature
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
Exploitation and automatability from CISA’s SSVC triage for CVE-2025-64764.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2025-64764 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 378,567 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
astronpmDescription
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 exportp: the transmitted properties, encrypteds: 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:
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 thesparameter, 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
Access the following URL and note the opening of the popup, demonstrating the reflected XSS:
<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 :
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;)
- Allam Yasser (inzo)
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | astro | all versions | 5.15.8npm install astro@5.15.8 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for astro, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update astro to 5.15.8 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2025-64764 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2025-64764 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2025-64764. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
How to detect CVE-2025-64764
A community-maintained Nuclei template exists for this CVE. You can scan for it directly:
nuclei -id cve-2025-64764 -u https://target- Template
- Astro - Reflected XSS via server islands feature
- Severity
- high
- Impact
- Remote attackers can execute scripts in users' browsers, potentially leading to session hijacking or data theft.
- Remediation
- Update to version 5.15.8 or later.
Template by ProjectDiscovery nuclei-templates (DhiyaneshDk, zhero___), MIT licensed. View the full template. Scan only systems you are authorised to test.
Frequently Asked Questions
Is CVE-2025-64764 in your dependencies?
O3 Security finds CVE-2025-64764 across npm dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.