diff --git a/browser/GMPolyfill.js b/browser/GMPolyfill.js
new file mode 100644
index 0000000..3e0606d
--- /dev/null
+++ b/browser/GMPolyfill.js
@@ -0,0 +1,107 @@
+/*
+ * Vencord, a modification for Discord's desktop app
+ * Copyright (c) 2022 Vendicated and contributors
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+*/
+
+function fetchOptions(url) {
+    return new Promise((resolve, reject) => {
+        const opt = {
+            method: "OPTIONS",
+            url: url,
+        };
+        opt.onload = resp => resolve(resp.responseHeaders);
+        opt.ontimeout = () => reject("fetch timeout");
+        opt.onerror = () => reject("fetch error");
+        opt.onabort = () => reject("fetch abort");
+        GM_xmlhttpRequest(opt);
+    });
+}
+
+function parseHeaders(headers) {
+    if (!headers)
+        return {};
+    const result = {};
+    const headersArr = headers.trim().split("\n");
+    for (var i = 0; i < headersArr.length; i++) {
+        var row = headersArr[i];
+        var index = row.indexOf(":")
+            , key = row.slice(0, index).trim().toLowerCase()
+            , value = row.slice(index + 1).trim();
+
+        if (result[key] === undefined) {
+            result[key] = value;
+        } else if (Array.isArray(result[key])) {
+            result[key].push(value);
+        } else {
+            result[key] = [result[key], value];
+        }
+    }
+    return result;
+}
+
+// returns true if CORS permits request
+async function checkCors(url, method) {
+    const headers = parseHeaders(await fetchOptions(url));
+
+    const origin = headers["access-control-allow-origin"];
+    if (origin !== "*" && origin !== window.location.origin) return false;
+
+    const methods = headers["access-control-allow-methods"]?.split(/,\s/g);
+    if (methods && !methods.includes(method)) return false;
+
+    return true;
+}
+
+function blobTo(to, blob) {
+    if (to === "arrayBuffer" && blob.arrayBuffer) return blob.arrayBuffer();
+    return new Promise((resolve, reject) => {
+        var fileReader = new FileReader();
+        fileReader.onload = event => resolve(event.target.result);
+        if (to === "arrayBuffer") fileReader.readAsArrayBuffer(blob);
+        else if (to === "text") fileReader.readAsText(blob, "utf-8");
+        else reject("unknown to");
+    });
+}
+
+function GM_fetch(url, opt) {
+    return new Promise((resolve, reject) => {
+        checkCors(url, opt?.method || "GET")
+            .then(can => {
+                if (can) {
+                    // https://www.tampermonkey.net/documentation.php?ext=dhdg#GM_xmlhttpRequest
+                    const options = opt || {};
+                    options.url = url;
+                    options.data = options.body;
+                    options.responseType = "blob";
+                    options.onload = resp => {
+                        var blob = resp.response;
+                        resp.blob = () => Promise.resolve(blob);
+                        resp.arrayBuffer = () => blobTo("arrayBuffer", blob);
+                        resp.text = () => blobTo("text", blob);
+                        resp.json = async () => JSON.parse(await blobTo("text", blob));
+                        resolve(resp);
+                    };
+                    options.ontimeout = () => reject("fetch timeout");
+                    options.onerror = () => reject("fetch error");
+                    options.onabort = () => reject("fetch abort");
+                    GM_xmlhttpRequest(options);
+                } else {
+                    reject("CORS issue");
+                }
+            });
+    });
+}
+export const fetch = GM_fetch;
diff --git a/browser/userscript.meta.js b/browser/userscript.meta.js
index 81cf3e7..5b2a39b 100644
--- a/browser/userscript.meta.js
+++ b/browser/userscript.meta.js
@@ -7,7 +7,7 @@
 // @supportURL      https://github.com/Vendicated/Vencord
 // @license         GPL-3.0
 // @match           *://*.discord.com/*
-// @grant           none
+// @grant           GM_xmlhttpRequest
 // @run-at          document-start
 // @compatible      chrome Chrome + Tampermonkey or Violentmonkey
 // @compatible      firefox Firefox Tampermonkey
diff --git a/scripts/build/buildWeb.mjs b/scripts/build/buildWeb.mjs
index 7508937..c85d8aa 100755
--- a/scripts/build/buildWeb.mjs
+++ b/scripts/build/buildWeb.mjs
@@ -60,13 +60,18 @@ await Promise.all(
         }),
         esbuild.build({
             ...commonOptions,
+            inject: ["browser/GMPolyfill.js", ...(commonOptions?.inject || [])],
+            define: {
+                "window": "unsafeWindow",
+                ...(commonOptions?.define)
+            },
             outfile: "dist/Vencord.user.js",
             banner: {
                 js: readFileSync("browser/userscript.meta.js", "utf-8").replace("%version%", `${PackageJSON.version}.${new Date().getTime()}`)
             },
             footer: {
                 // UserScripts get wrapped in an iife, so define Vencord prop on window that returns our local
-                js: "Object.defineProperty(window,'Vencord',{get:()=>Vencord});"
+                js: "Object.defineProperty(unsafeWindow,'Vencord',{get:()=>Vencord});"
             },
         })
     ]