CVE-2025-65019: Astro Cloudflare adapter has Stored Cross-site Scripting vulnerability in /_image endpoint

Summary A Cross-Site Scripting (XSS) vulnerability exists in Astro when using the @astrojs/cloudflare adapter with output: 'server'. The built-in image optimization endpoint (/_image) uses isRemoteAllowed() from Astro’s internal helpers, which unconditionally allows data: URLs. When the endpoint receives a valid data: URL pointing to a malicious SVG containing JavaScript, and the Cloudflare-specific implementation performs a 302 redirect back to the original data: URL, the browser directly executes the embedded JavaScript. This completely bypasses any domain allow-listing (image.domains / image.remotePatterns) and typical Content Security Policy mitigations.

Affected Versions

• @astrojs/cloudflare ≤ 12.6.10 (and likely all previous versions)
• Astro ≥ 4.x when used with output: 'server' and the Cloudflare adapter

Root Cause – Vulnerable Code File: node_modules/@astrojs/internal-helpers/src/remote.ts

export function isRemoteAllowed(src: string, ...): boolean {
if (!URL.canParse(src)) {
return false;
}
const url = new URL(src);

// Data URLs are always allowed
if (url.protocol === 'data:') {
return true;
}

// Non-http(s) protocols are never allowed
if (!['http:', 'https:'].includes(url.protocol)) {
return false;
}
// ... further http/https allow-list checks
}

In the Cloudflare adapter, the /_image endpoint contains logic similar to:

const href = ctx.url.searchParams.get('href');
if (!href) {
// return error
}

if (isRemotePath(href)) {
if (isRemoteAllowed(href, imageConfig) === false) {
// return error
} else {
//redirect to return the image
return Response.redirect(href, 302);
}
}

Because data: URLs are considered “allowed”, a request such as: https://example.com/_image?href=data:image/svg+xml;base64,PHN2Zy... (base64-encoded malicious SVG)

triggers a 302 redirect directly to the data: URL, causing the browser to render and execute the malicious JavaScript inside the SVG.

Proof of Concept (PoC)

1. Create a minimal Astro project with Cloudflare adapter (output: 'server').
2. Deploy to Cloudflare Pages or Workers.
3. Request the image endpoint with the following payload: