Astro is a web framework. Prior to version 5.15.9, when using Astro's Cloudflare adapter (@astrojs/cloudflare) with output: 'server', the image optimization endpoint (/_image) contains a critical vulnerability in the isRemoteAllowed() function that unconditionally allows data: protocol URLs. This enables Cross-Site Scripting (XSS) attacks through malicious SVG payloads, bypassing domain restrictions and Content Security Policy protections. This issue has been patched in version 5.15.9.

Affected: npm: astro

**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`

```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:

```ts const href = ctx.url.searchPar…