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

@@ -1392,7 +1392,11 @@ sso:
  # phishing attacks from evil.site. To avoid this, include a slash after the
  # hostname: "https://my.client/".
  #
- # By default, this list is empty.
+ # If public_baseurl is set, then the login fallback page (used by clients
+ # that don't have full support for SSO) is always included in this list.
+ #
+ # By default, this list is empty, except if public_baseurl is set (in which
+ # case the login fallback page is the only element in the list).
  #
  #client_whitelist:
  # - https://riot.im/develop

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.
@@ -54,7 +65,11 @@ def generate_config_section(self, **kwargs):
  # phishing attacks from evil.site. To avoid this, include a slash after the
  # hostname: "https://my.client/".
  #
- # By default, this list is empty.
+ # If public_baseurl is set, then the login fallback page (used by clients
+ # that don't have full support for SSO) is always included in this list.
+ #
+ # By default, this list is empty, except if public_baseurl is set (in which
+ # case the login fallback page is the only element in the list).
  #
  #client_whitelist:
  # - https://riot.im/develop

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))