Issue with support for Google Provider Authentication Across Multiple Subdomains in NextAuth v5

[hereisabrams]

I'm currently implementing NextAuth v5 in my application and aiming to enable Google Provider authentication across multiple subdomains (like app.x.com app1.x.com app2.x.com always pointing to the same app

My auth.ts

`
import GoogleProvider from "next-auth/providers/google";
import { cert } from "firebase-admin/app";
import MicrosoftEntraID from "next-auth/providers/microsoft-entra-id";
import NextAuth, { NextAuthConfig } from "next-auth";
import { FirestoreAdapter } from "@auth/firebase-adapter";
import { GOOGLE_AUTH_SCOPES } from "@/utils/global/GlobalVariables";

const microsoftTenantId = "common";
export const { auth, handlers, signIn, signOut } = NextAuth((req) => {
const host =
req?.headers?.get("x-original-host") ||
req?.headers?.get("x-forwarded-host") ||
req?.headers?.get("host");
const proto =
req?.headers?.get("x-forwarded-proto") || (host.includes("localhost") ? "http" : "https");
const baseUrl = ${proto}://${host};
const fullUrl = new URL(req?.url, https://${req?.headers?.get("host")});
console.log("Full URL:", fullUrl?.href);
console.log("baseUrl", baseUrl);

  const commonAuthOptions: NextAuthConfig = {
      providers: [
          GoogleProvider({
              clientId: process.env.GOOGLE_CLIENT_ID!,
              clientSecret: process.env.GOOGLE_CLIENT_SECRET!,
              allowDangerousEmailAccountLinking: true,
              authorization: {
                  params: {
                      redirect_uri: `${baseUrl}/api/auth/callback/google`,
                      scope: GOOGLE_AUTH_SCOPES,
                      expire_in: 43200, // 12 hours
                      access_type: "offline",
                      prompt: "consent",
                  },
              },
          }),
          MicrosoftEntraID({
              clientId: `${process.env.AUTH_MICROSOFT_ENTRA_ID_ID}`,
              clientSecret: `${process.env.AUTH_MICROSOFT_ENTRA_ID_SECRET}`,
              token: {
                  url: `https://login.microsoftonline.com/${microsoftTenantId}/oauth2/v2.0/token`,
              },
              userinfo: { url: "https://graph.microsoft.com/oidc/userinfo" },
              authorization: {
                  url: `https://login.microsoftonline.com/${microsoftTenantId}/oauth2/v2.0/authorize`,
                  params: {
                      scope: "Mail.Send User.Read Mail.Read email Mail.ReadBasic Mail.ReadWrite offline_access openid profile Directory.Read.All Directory.ReadWrite.All Group.Read.All Group.ReadWrite.All Group-Conversation.ReadWrite.All",
                  },
              },
              allowDangerousEmailAccountLinking: true,
              issuer: `https://login.microsoftonline.com/${microsoftTenantId}/v2.0`,
          }),
      ],
      session: {
          strategy: "jwt",
      },
      trustHost: true,
      adapter: FirestoreAdapter({
          credential: cert({
              privateKey: process.env.NEXT_FIREBASE_PRIVATE_KEY!.replace(/\\n/g, "\n"),
              clientEmail: process.env.NEXT_FIREBASE_CLIENT_EMAIL,
              projectId: process.env.NEXT_FIREBASE_PROJECTID,
          }),
      }),
      debug: true,
      pages: {
          error: "/error",
      },
      redirectProxyUrl: baseUrl,
      basePath: `${baseUrl}/api/auth`,
  };
  return commonAuthOptions;

});
`

my route.ts

import { handlers } from "@/utils/lib/auth/Auth"; export const { GET, POST } = handlers;

With the current setup, authentication works correctly on a single subdomain using the AUTH_URL. However, when trying a different subdomain I am faced with this error on the url - /error?error=Configuration