diff --git a/src/firebase.ts b/src/firebase.ts
index f349fa4..406390f 100644
--- a/src/firebase.ts
+++ b/src/firebase.ts
@@ -2,32 +2,48 @@ import { AsyncMiddleware, Fallback } from './types'
 import { FIREBASE_COOKIE_KEY } from './constants'
 import { NextRequest } from 'next/server'
 import { handleFallback } from './handle-fallback'
-import { decodeProtectedHeader, jwtVerify, JWK, importJWK } from 'jose'
+import { decodeProtectedHeader, jwtVerify, importJWK } from 'jose'
+import { toJwk } from 'js-x509-utils'
 
-export const makeFirebaseInspector = (fallback: Fallback): AsyncMiddleware => {
+export const makeFirebaseInspector = (
+  fallback: Fallback,
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  customHandler?: (payload: any) => boolean
+): AsyncMiddleware => {
   return async (request, event) => {
-    const ok = await verifyFirebaseIdToken(request)
+    const ok = await verifyFirebaseIdToken(request, customHandler)
     if (ok) return
     return handleFallback(fallback, request, event)
   }
 }
 
-const JWK_ENDPOINT =
-  'https://www.googleapis.com/service_accounts/v1/jwk/securetoken@system.gserviceaccount.com'
-
-const verifyFirebaseIdToken = async (req: NextRequest): Promise<boolean> => {
-  const token = req.cookies[FIREBASE_COOKIE_KEY]
+const verifyFirebaseIdToken = async (
+  req: NextRequest,
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  customHandler?: (payload: any) => boolean
+): Promise<boolean> => {
+  const cookieKey =
+    process.env.FORTRESS_FIREBASE_COOKIE_KEY ?? FIREBASE_COOKIE_KEY
+  const token = req.cookies[cookieKey]
   if (!token) return false
 
-  const { keys }: { keys: JWK[] } = await fetch(JWK_ENDPOINT).then((res) =>
-    res.json()
-  )
+  const endpoint =
+    process.env.FORTRESS_FIREBASE_MODE === 'session'
+      ? 'https://www.googleapis.com/identitytoolkit/v3/relyingparty/publicKeys'
+      : 'https://www.googleapis.com/robot/v1/metadata/x509/securetoken@system.gserviceaccount.com'
+
+  try {
+    const keys: Record<string, string> = await fetch(endpoint).then((res) =>
+      res.json()
+    )
 
-  const { kid } = decodeProtectedHeader(token)
-  const jwk = keys.find((key) => key.kid === kid)
-  if (!jwk) return false
+    const { kid = '', alg } = decodeProtectedHeader(token)
+    const jwk = await toJwk(keys[kid], 'pem')
 
-  return jwtVerify(token, await importJWK(jwk))
-    .then(() => true)
-    .catch(() => false)
+    return jwtVerify(token, await importJWK({ ...jwk, alg }))
+      .then((res) => customHandler?.(res.payload) ?? true)
+      .catch(() => false)
+  } catch (_) {
+    return false
+  }
 }