diff --git a/hc/accounts/tests/test_check_token.py b/hc/accounts/tests/test_check_token.py
index 4de17e91..d0122949 100644
--- a/hc/accounts/tests/test_check_token.py
+++ b/hc/accounts/tests/test_check_token.py
@@ -60,5 +60,7 @@ class CheckTokenTestCase(BaseTestCase):
 
         # It should not log the user in yet
         self.assertNotIn("_auth_user_id", self.client.session)
+
         # Instead, it should set 2fa_user_id in the session
-        self.assertEqual(self.client.session["2fa_user_id"], self.alice.id)
+        user_id, email, valid_until = self.client.session["2fa_user"]
+        self.assertEqual(user_id, self.alice.id)
diff --git a/hc/accounts/tests/test_login.py b/hc/accounts/tests/test_login.py
index 99cd86e5..80061b0f 100644
--- a/hc/accounts/tests/test_login.py
+++ b/hc/accounts/tests/test_login.py
@@ -124,5 +124,7 @@ class LoginTestCase(BaseTestCase):
 
         # It should not log the user in yet
         self.assertNotIn("_auth_user_id", self.client.session)
+
         # Instead, it should set 2fa_user_id in the session
-        self.assertEqual(self.client.session["2fa_user_id"], self.alice.id)
+        user_id, email, valid_until = self.client.session["2fa_user"]
+        self.assertEqual(user_id, self.alice.id)
diff --git a/hc/accounts/tests/test_login_webauthn.py b/hc/accounts/tests/test_login_webauthn.py
index 092c3296..ac725a95 100644
--- a/hc/accounts/tests/test_login_webauthn.py
+++ b/hc/accounts/tests/test_login_webauthn.py
@@ -1,3 +1,4 @@
+import time
 from unittest.mock import patch
 
 from django.test.utils import override_settings
@@ -11,7 +12,7 @@ class LoginWebAuthnTestCase(BaseTestCase):
 
         # This is the user we're trying to authenticate
         session = self.client.session
-        session["2fa_user_id"] = self.alice.id
+        session["2fa_user"] = [self.alice.id, self.alice.email, (time.time()) + 300]
         session.save()
 
         self.url = "/accounts/login/two_factor/"
@@ -24,6 +25,28 @@ class LoginWebAuthnTestCase(BaseTestCase):
         # It should put a "state" key in the session:
         self.assertIn("state", self.client.session)
 
+    def test_it_requires_unauthenticated_user(self):
+        self.client.login(username="alice@example.org", password="password")
+
+        r = self.client.get(self.url)
+        self.assertEqual(r.status_code, 400)
+
+    def test_it_rejects_changed_email(self):
+        session = self.client.session
+        session["2fa_user"] = [self.alice.id, "eve@example.org", int(time.time())]
+        session.save()
+
+        r = self.client.get(self.url)
+        self.assertEqual(r.status_code, 400)
+
+    def test_it_rejects_old_timestamp(self):
+        session = self.client.session
+        session["2fa_user"] = [self.alice.id, self.alice.email, int(time.time()) - 310]
+        session.save()
+
+        r = self.client.get(self.url)
+        self.assertRedirects(r, "/accounts/login/")
+
     @override_settings(RP_ID=None)
     def test_it_requires_rp_id(self):
         r = self.client.get(self.url)
diff --git a/hc/accounts/views.py b/hc/accounts/views.py
index 724275c9..354b72c8 100644
--- a/hc/accounts/views.py
+++ b/hc/accounts/views.py
@@ -2,6 +2,7 @@ import base64
 from datetime import timedelta as td
 from secrets import token_bytes
 from urllib.parse import urlparse
+import time
 import uuid
 
 from django.db import transaction
@@ -104,7 +105,12 @@ def _redirect_after_login(request):
 
 def _check_2fa(request, user):
     if user.credentials.exists():
-        request.session["2fa_user_id"] = user.id
+        # We have verified user's password or token, and now must
+        # verify their security key. We store the following in user's session:
+        # - user.id, to look up the user in the login_webauthn view
+        # - user.email, to make sure email was not changed between the auth steps
+        # - timestamp, to limit the max time between the auth steps
+        request.session["2fa_user"] = [user.id, user.email, int(time.time())]
 
         path = reverse("hc-login-webauthn")
         redirect_url = request.GET.get("next")
@@ -684,14 +690,26 @@ def _check_credential(request, form, credentials):
 
 
 def login_webauthn(request):
-    if "2fa_user_id" not in request.session:
-        return HttpResponseBadRequest()
-
     # We require RP_ID. Fail predicably if it is not set:
     if not settings.RP_ID:
         return HttpResponse(status=500)
 
-    user = User.objects.get(id=request.session["2fa_user_id"])
+    # Expect an unauthenticated user
+    if request.user.is_authenticated:
+        return HttpResponseBadRequest()
+
+    if "2fa_user" not in request.session:
+        return HttpResponseBadRequest()
+
+    user_id, email, timestamp = request.session["2fa_user"]
+    if timestamp + 300 < time.time():
+        return redirect("hc-login")
+
+    try:
+        user = User.objects.get(id=user_id, email=email)
+    except User.DoesNotExist:
+        return HttpResponseBadRequest()
+
     credentials = [c.unpack() for c in user.credentials.all()]
 
     if request.method == "POST":
@@ -703,7 +721,7 @@ def login_webauthn(request):
             return HttpResponseBadRequest()
 
         request.session.pop("state")
-        request.session.pop("2fa_user_id")
+        request.session.pop("2fa_user")
         auth_login(request, user, "hc.accounts.backends.EmailBackend")
         return _redirect_after_login(request)