consolidate cross-origin proxy and add ssrf guard

packages/shim/src/util/proxy.js

@@ -0,0 +1,54 @@
+// Single round-trip through the server's /api/proxy endpoint for cross-origin requests.
+// Encodes a binary request body to base64, returns the upstream response with its body as an ArrayBuffer.
+// Throws an Error carrying the server's message on failure.
+
+import { arrayBufferToBase64, base64ToArrayBuffer } from "./base64.js";
+
+export async function proxyFetch({ url, method, headers, body, contentType }) {
+  let encodedBody = null;
+  let binary = false;
+
+  if (body instanceof ArrayBuffer) {
+    encodedBody = arrayBufferToBase64(body);
+    binary = true;
+  } else if (body instanceof Uint8Array) {
+    encodedBody = arrayBufferToBase64(body.buffer);
+    binary = true;
+  } else if (body != null) {
+    encodedBody = body;
+  }
+
+  const payload = {
+    url,
+    method: method || "GET",
+    headers: headers || {},
+    body: encodedBody,
+    binary,
+  };
+
+  if (contentType !== undefined) {
+    payload.contentType = contentType;
+  }
+
+  // Use native fetch to avoid an unnecessary call through the shim. proxy is already same origin.
+  const nativeFetch = window.__originalFetch || fetch;
+
+  const res = await nativeFetch("/api/proxy", {
+    method: "POST",
+    headers: { "Content-Type": "application/json" },
+    body: JSON.stringify(payload),
+  });
+
+  if (!res.ok) {
+    const err = await res.json().catch(() => ({}));
+    throw new Error(err.error || "Proxy request failed");
+  }
+
+  const result = await res.json();
+
+  return {
+    status: result.status,
+    headers: result.headers,
+    body: base64ToArrayBuffer(result.body),
+  };
+}