fix(api): prevent creation of Response objects outside of request time

src/backend-emulator/main.js

@@ -21,6 +21,24 @@ const workerShims = {
 
 let listeners = [];
 
+let isResponseConstructorAllowed = false;
+
+/**
+ * SafeResponse wraps a fetch Response to yell loudly if constructed at an unsafe time.
+ * Cloudflare will reject all Response objects that aren't created during a request, so no pre-generation is allowed.
+ */
+class SafeResponse extends fetch.Response {
+    constructor(...args) {
+        super(...args);
+
+        if (!isResponseConstructorAllowed) {
+            throw new Error(
+                'Response object created outside of request context. This will be rejected by Cloudflare.'
+            );
+        }
+    }
+}
+
 const context = () =>
     vm.createContext(
         {
@@ -30,7 +48,7 @@ const context = () =>
                     listeners.push(fn);
                 }
             },
-            Response: fetch.Response,
+            Response: SafeResponse,
             URL: URL,
             crypto: crypto,
             setTimeout: setTimeout,
@@ -75,6 +93,7 @@ const server = http.createServer((req, res) => {
             console.log(
                 `${loggedStatus} [${timeEnd - timeStart}ms] - ${req.method} ${req.url}`
             );
+            isResponseConstructorAllowed = false;
         },
         request: new fetch.Request(
             new URL(`http://${req.headers.host || 'localhost'}${req.url}`),
@@ -95,6 +114,7 @@ const server = http.createServer((req, res) => {
         return;
     }
 
+    isResponseConstructorAllowed = true;
     for (let listener of listeners) {
         try {
             listener(event);

src/backend-worker/handlers/get-picker-data.ts

@@ -7,7 +7,8 @@ import {
 import { respond, withSession } from '../utils/api-tools';
 import { getGuild, getGuildData, getGuildMemberRoles } from '../utils/guild';
 
-const fail = respond(
+const fail = () =>
+    respond(
         {
             error: 'guild not found',
         },
@@ -38,14 +39,14 @@ export const GetPickerData = withSession(
         // Save a Discord API request by checking if this user is a member by session first
         const checkGuild = guilds.find((guild) => guild.id === guildID);
         if (!checkGuild) {
-            return fail;
+            return fail();
         }
 
         const guild = await getGuild(guildID, {
             skipCachePull: url.searchParams.has('__no_cache'),
         });
         if (!guild) {
-            return fail;
+            return fail();
         }
 
         const memberRolesP = getGuildMemberRoles({
@@ -57,7 +58,7 @@ export const GetPickerData = withSession(
 
         const [guildData, memberRoles] = await Promise.all([guildDataP, memberRolesP]);
         if (!memberRoles) {
-            return fail;
+            return fail();
         }
 
         const presentableGuild: PresentableGuild = {

src/backend-worker/handlers/login-callback.ts

@@ -22,7 +22,7 @@ const AuthErrorResponse = (extra?: string) =>
     );
 
 export const LoginCallback = resolveFailures(
-    AuthErrorResponse(),
+    AuthErrorResponse,
     async (request: Request): Promise<Response> => {
         const query = new URL(request.url).searchParams;
 

src/backend-worker/router.ts

@@ -34,7 +34,7 @@ export class Router {
         this.routingTree[lowerMethod][rootPath] = handler;
     }
 
-    async handle(request: Request): Promise<Response> | Response {
+    async handle(request: Request): Promise<Response> {
         const url = new URL(request.url);
 
         if (url.pathname === '/' || url.pathname === '') {

src/backend-worker/utils/api-tools.ts

@@ -16,14 +16,16 @@ export const respond = (obj: Record<string, any>, init?: ResponseInit) =>
     new Response(JSON.stringify(obj), init);
 
 export const resolveFailures = (
-    handleWith: Response,
+    handleWith: () => Response,
     handler: (request: Request) => Promise<Response> | Response
 ) => async (request: Request): Promise<Response> => {
     try {
         return handler(request);
     } catch (e) {
         console.error(e);
-        return handleWith || respond({ error: 'internal server error' }, { status: 500 });
+        return (
+            handleWith() || respond({ error: 'internal server error' }, { status: 500 })
+        );
     }
 };
 
@@ -109,7 +111,7 @@ export const cacheLayer = <Identity, Data>(
 const NotAuthenticated = (extra?: string) =>
     respond(
         {
-            err: extra || 'not authenticated',
+            error: extra || 'not authenticated',
         },
         { status: 403 }
     );