Fix redirect-after-login when using TOTP

If user has both WebAuthn and TOTP configured,
when logging in, they will be asked to choose between
"Use security keys" and "Use authenticator app".
The "Use authenticator app" is a link to a different
page (/accounts/login/two_factor/totp/). This commit makes
sure the ?next= query parameter is preserved when navigating
to that page.

For reference, the ?next= query parameter is the URL we should
redirect to after a successful login. Use case:
User is logged out. They click on a bookmarked "Check Details"
link. They get redirected to the login form. After
entering username & password and completing 2FA,
they get redirected to the "Check Details" page they
originally wanted to visit.

hc/accounts/tests/test_login_webauthn.py

@@ -33,6 +33,14 @@ class LoginWebAuthnTestCase(BaseTestCase):
         r = self.client.get(self.url)
         self.assertContains(r, "Use authenticator app")
 
+    def test_it_preserves_next_parameter_in_totp_url(self):
+        self.profile.totp = "0" * 32
+        self.profile.save()
+
+        url = self.url + "?next=" + self.channels_url
+        r = self.client.get(url)
+        self.assertContains(r, "/login/two_factor/totp/?next=" + self.channels_url)
+
     def test_it_requires_unauthenticated_user(self):
         self.client.login(username="[email protected]", password="password")
 

hc/accounts/views.py

@@ -827,9 +827,17 @@ def login_webauthn(request):
     options, state = FIDO2_SERVER.authenticate_begin(credentials)
     request.session["state"] = state
 
+    totp_url = None
+    if user.profile.totp:
+        totp_url = reverse("hc-login-totp")
+
+        redirect_url = request.GET.get("next")
+        if _allow_redirect(redirect_url):
+            totp_url += "?next=%s" % redirect_url
+
     ctx = {
         "options": base64.b64encode(cbor.encode(options)).decode(),
-        "offer_totp": True if user.profile.totp else False,
+        "totp_url": totp_url,
     }
     return render(request, "accounts/login_webauthn.html", ctx)
 

templates/accounts/login_webauthn.html

@@ -13,7 +13,7 @@
     <h1>Two-factor Authentication</h1>
 
     <div id="pick-method">
-        {% if offer_totp %}
+        {% if totp_url %}
         <p>Please select how you want to authenticate.</p>
         {% else %}
         <p>
@@ -26,8 +26,8 @@
             id="use-key-btn"
             type="button"
             class="btn btn-primary">Use security key</button>
-        {% if offer_totp %}
-        <a href="{% url 'hc-login-totp' %}" class="btn btn-default">
+        {% if totp_url %}
+        <a href="{{ totp_url }}" class="btn btn-default">
             Use authenticator app
         </a>
         {% endif %}