Always whitelist the login fallback for SSO

changelog.d/7153.feature

@@ -0,0 +1 @@
+Always whitelist the login fallback in the SSO configuration if `public_baseurl` is set.

docs/sample_config.yaml

@@ -1392,6 +1392,10 @@ sso:
     # phishing attacks from evil.site. To avoid this, include a slash after the
     # hostname: "https://my.client/".
     #
+    # If public_baseurl is set, then the login fallback page (used by clients
+    # that don't natively support the required login flows) is whitelisted in
+    # addition to any URLs in this list.
+    #
     # By default, this list is empty.
     #
     #client_whitelist:

synapse/config/sso.py

@@ -39,6 +39,17 @@ def read_config(self, config, **kwargs):
 
         self.sso_client_whitelist = sso_config.get("client_whitelist") or []
 
+        # Attempt to also whitelist the server's login fallback, since that fallback sets
+        # the redirect URL to itself (so it can process the login token then return
+        # gracefully to the client). This would make it pointless to ask the user for
+        # confirmation, since the URL the confirmation page would be showing wouldn't be
+        # the client's.
+        # public_baseurl is an optional setting, so we only add the fallback's URL to the
+        # list if it's provided (because we can't figure out what that URL is otherwise).
+        if self.public_baseurl:
+            login_fallback_url = self.public_baseurl + "_matrix/static/client/login"
+            self.sso_client_whitelist.append(login_fallback_url)
+
     def generate_config_section(self, **kwargs):
         return """\
         # Additional settings to use with single-sign on systems such as SAML2 and CAS.
@@ -54,6 +65,10 @@ def generate_config_section(self, **kwargs):
             # phishing attacks from evil.site. To avoid this, include a slash after the
             # hostname: "https://my.client/".
             #
+            # If public_baseurl is set, then the login fallback page (used by clients
+            # that don't natively support the required login flows) is whitelisted in
+            # addition to any URLs in this list.
+            #
             # By default, this list is empty.
             #
             #client_whitelist:

tests/rest/client/v1/test_login.py

@@ -350,7 +350,14 @@ def test_cas_redirect_confirm(self):
     def test_cas_redirect_whitelisted(self):
         """Tests that the SSO login flow serves a redirect to a whitelisted url
         """
-        redirect_url = "https://legit-site.com/"
+        self._test_redirect("https://legit-site.com/")
+
+    @override_config({"public_baseurl": "https://example.com"})
+    def test_cas_redirect_login_fallback(self):
+        self._test_redirect("https://example.com/_matrix/static/client/login")
+
+    def _test_redirect(self, redirect_url):
+        """Tests that the SSO login flow serves a redirect for the given redirect URL."""
         cas_ticket_url = (
             "/_matrix/client/r0/login/cas/ticket?redirectUrl=%s&ticket=ticket"
             % (urllib.parse.quote(redirect_url))