@astrojs/node's trailing slash handling causes open redirect issue

Summary

Following github.com/withastro/astro/security/advisories/…, there's still an Open Redirect vulnerability in a subset of Astro deployment scenarios.

Details

Astro 5.12.8 fixed a case where https://example.com//astro.build/press would redirect to the external origin //astro.build/press. However, with the Node deployment adapter in standalone mode and trailingSlash set to "always" in the Astro configuration, https://example.com//astro.build/press still redirects to //astro.build/press.

Proof of Concept

1. Create a new minimal Astro project (astro@5.12.8)

2. Configure it to use the Node adapter (@astrojs/node@9.4.0) and force trailing slashes:

   // astro.config.mjs
   import { defineConfig } from 'astro/config';
   import node from '@astrojs/node';
   
   export default defineConfig({
     trailingSlash: 'always',
     adapter: node({ mode: 'standalone' }),
   });

3. Build the site by running astro build.

4. Run the server, e.g. with astro preview.

5. Append //astro.build/press to the preview URL, e.g. localhost//astro.build/press

6. The site will redirect to the external Astro Build origin.

Example reproduction

1. Open this StackBlitz reproduction.

2. Open the preview in a separate window so the StackBlitz embed doesn't cause security errors.

3. Append //astro.build/press to the preview URL, e.g. https://x.local-corp.webcontainer.io//astro.build/press.

4. See it redirect to the external Astro Build origin.

Impact

This is classified as an Open Redirection vulnerability (CWE-601). It affects any user who clicks on a specially crafted link pointing to the affected domain. Since the domain appears legitimate, victims may be tricked into trusting the redirected page, leading to possible credential theft, malware distribution, or other phishing-related attacks.

No authentication is required to exploit this vulnerability. Any unauthenticated user can trigger the redirect by clicking a malicious link.