diff --git a/CHANGES.md b/CHANGES.md
index 16c11ff0c..b71d4641b 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,7 +1,29 @@
+Synapse 1.27.0rc2 (2021-02-11)
+==============================
+
+Features
+--------
+
+- Further improvements to the user experience of registration via single sign-on. ([\#9297](https://github.com/matrix-org/synapse/issues/9297))
+
+
+Bugfixes
+--------
+
+- Fix ratelimiting introduced in v1.27.0rc1 for invites to respect the `ratelimit` flag on application services. ([\#9302](https://github.com/matrix-org/synapse/issues/9302))
+- Do not automatically calculate `public_baseurl` since it can be wrong in some situations. Reverts behaviour introduced in v1.26.0. ([\#9313](https://github.com/matrix-org/synapse/issues/9313))
+
+
+Improved Documentation
+----------------------
+
+- Clarify the sample configuration for changes made to the template loading code. ([\#9310](https://github.com/matrix-org/synapse/issues/9310))
+
+
 Synapse 1.27.0rc1 (2021-02-02)
 ==============================
 
-Note that this release includes a change in Synapse to use Redis as a cache ─ as well as a pub/sub mechanism ─ if Redis support is enabled. No action is needed by server administrators, and we do not expect resource usage of the Redis instance to change dramatically.
+Note that this release includes a change in Synapse to use Redis as a cache ─ as well as a pub/sub mechanism ─ if Redis support is enabled for workers. No action is needed by server administrators, and we do not expect resource usage of the Redis instance to change dramatically.
 
 This release also changes the callback URI for OpenID Connect (OIDC) identity providers. If your server is configured to use single sign-on via an OIDC/OAuth2 IdP, you may need to make configuration changes. Please review [UPGRADE.rst](UPGRADE.rst) for more details on these changes.
 
diff --git a/changelog.d/9297.feature b/changelog.d/9297.feature
deleted file mode 100644
index a2d0b27da..000000000
--- a/changelog.d/9297.feature
+++ /dev/null
@@ -1 +0,0 @@
-Further improvements to the user experience of registration via single sign-on.
diff --git a/changelog.d/9302.bugfix b/changelog.d/9302.bugfix
deleted file mode 100644
index c1cdea52a..000000000
--- a/changelog.d/9302.bugfix
+++ /dev/null
@@ -1 +0,0 @@
-Fix new ratelimiting for invites to respect the `ratelimit` flag on application services. Introduced in v1.27.0rc1.
diff --git a/changelog.d/9310.doc b/changelog.d/9310.doc
deleted file mode 100644
index f61705b73..000000000
--- a/changelog.d/9310.doc
+++ /dev/null
@@ -1 +0,0 @@
-Clarify the sample configuration for changes made to the template loading code.
diff --git a/docs/sample_config.yaml b/docs/sample_config.yaml
index fbbf71edd..71c3af287 100644
--- a/docs/sample_config.yaml
+++ b/docs/sample_config.yaml
@@ -74,10 +74,6 @@ pid_file: DATADIR/homeserver.pid
 # Otherwise, it should be the URL to reach Synapse's client HTTP listener (see
 # 'listeners' below).
 #
-# If this is left unset, it defaults to 'https://<server_name>/'. (Note that
-# that will not work unless you configure Synapse or a reverse-proxy to listen
-# on port 443.)
-#
 #public_baseurl: https://example.com/
 
 # Set the soft limit on the number of file descriptors synapse can use
@@ -1177,9 +1173,8 @@ account_validity:
   # send an email to the account's email address with a renewal link. By
   # default, no such emails are sent.
   #
-  # If you enable this setting, you will also need to fill out the 'email'
-  # configuration section. You should also check that 'public_baseurl' is set
-  # correctly.
+  # If you enable this setting, you will also need to fill out the 'email' and
+  # 'public_baseurl' configuration sections.
   #
   #renew_at: 1w
 
@@ -1270,7 +1265,8 @@ account_validity:
 # The identity server which we suggest that clients should use when users log
 # in on this server.
 #
-# (By default, no suggestion is made, so it is left up to the client.)
+# (By default, no suggestion is made, so it is left up to the client.
+# This setting is ignored unless public_baseurl is also set.)
 #
 #default_identity_server: https://matrix.org
 
@@ -1295,6 +1291,8 @@ account_validity:
 # by the Matrix Identity Service API specification:
 # https://matrix.org/docs/spec/identity_service/latest
 #
+# If a delegate is specified, the config option public_baseurl must also be filled out.
+#
 account_threepid_delegates:
     #email: https://example.com     # Delegate email sending to example.com
     #msisdn: http://localhost:8090  # Delegate SMS sending to this local process
@@ -1948,9 +1946,9 @@ sso:
     # phishing attacks from evil.site. To avoid this, include a slash after the
     # hostname: "https://my.client/".
     #
-    # The login fallback page (used by clients that don't natively support the
-    # required login flows) is automatically whitelisted in addition to any URLs
-    # in this list.
+    # 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.
     #
diff --git a/synapse/__init__.py b/synapse/__init__.py
index 06b3820be..283d6ffef 100644
--- a/synapse/__init__.py
+++ b/synapse/__init__.py
@@ -48,7 +48,7 @@ try:
 except ImportError:
     pass
 
-__version__ = "1.27.0rc1"
+__version__ = "1.27.0rc2"
 
 if bool(os.environ.get("SYNAPSE_TEST_PATCH_LOG_CONTEXTS", False)):
     # We import here so that we don't have to install a bunch of deps when
diff --git a/synapse/api/urls.py b/synapse/api/urls.py
index e36aeef31..6379c86dd 100644
--- a/synapse/api/urls.py
+++ b/synapse/api/urls.py
@@ -42,6 +42,8 @@ class ConsentURIBuilder:
         """
         if hs_config.form_secret is None:
             raise ConfigError("form_secret not set in config")
+        if hs_config.public_baseurl is None:
+            raise ConfigError("public_baseurl not set in config")
 
         self._hmac_secret = hs_config.form_secret.encode("utf-8")
         self._public_baseurl = hs_config.public_baseurl
diff --git a/synapse/config/cas.py b/synapse/config/cas.py
index daea848d2..dbf508596 100644
--- a/synapse/config/cas.py
+++ b/synapse/config/cas.py
@@ -17,7 +17,7 @@ from typing import Any, List
 
 from synapse.config.sso import SsoAttributeRequirement
 
-from ._base import Config
+from ._base import Config, ConfigError
 from ._util import validate_config
 
 
@@ -35,13 +35,15 @@ class CasConfig(Config):
 
         if self.cas_enabled:
             self.cas_server_url = cas_config["server_url"]
-            public_base_url = cas_config.get("service_url") or self.public_baseurl
-            if public_base_url[-1] != "/":
-                public_base_url += "/"
+
+            # The public baseurl is required because it is used by the redirect
+            # template.
+            public_baseurl = self.public_baseurl
+            if not public_baseurl:
+                raise ConfigError("cas_config requires a public_baseurl to be set")
+
             # TODO Update this to a _synapse URL.
-            self.cas_service_url = (
-                public_base_url + "_matrix/client/r0/login/cas/ticket"
-            )
+            self.cas_service_url = public_baseurl + "_matrix/client/r0/login/cas/ticket"
             self.cas_displayname_attribute = cas_config.get("displayname_attribute")
             required_attributes = cas_config.get("required_attributes") or {}
             self.cas_required_attributes = _parsed_required_attributes_def(
diff --git a/synapse/config/emailconfig.py b/synapse/config/emailconfig.py
index 6a487afd3..d4328c46b 100644
--- a/synapse/config/emailconfig.py
+++ b/synapse/config/emailconfig.py
@@ -166,6 +166,11 @@ class EmailConfig(Config):
             if not self.email_notif_from:
                 missing.append("email.notif_from")
 
+            # public_baseurl is required to build password reset and validation links that
+            # will be emailed to users
+            if config.get("public_baseurl") is None:
+                missing.append("public_baseurl")
+
             if missing:
                 raise ConfigError(
                     MISSING_PASSWORD_RESET_CONFIG_ERROR % (", ".join(missing),)
@@ -264,6 +269,9 @@ class EmailConfig(Config):
             if not self.email_notif_from:
                 missing.append("email.notif_from")
 
+            if config.get("public_baseurl") is None:
+                missing.append("public_baseurl")
+
             if missing:
                 raise ConfigError(
                     "email.enable_notifs is True but required keys are missing: %s"
diff --git a/synapse/config/oidc_config.py b/synapse/config/oidc_config.py
index 9d8196d8c..d081f36fa 100644
--- a/synapse/config/oidc_config.py
+++ b/synapse/config/oidc_config.py
@@ -53,7 +53,10 @@ class OIDCConfig(Config):
                     "Multiple OIDC providers have the idp_id %r." % idp_id
                 )
 
-        self.oidc_callback_url = self.public_baseurl + "_synapse/client/oidc/callback"
+        public_baseurl = self.public_baseurl
+        if public_baseurl is None:
+            raise ConfigError("oidc_config requires a public_baseurl to be set")
+        self.oidc_callback_url = public_baseurl + "_synapse/client/oidc/callback"
 
     @property
     def oidc_enabled(self) -> bool:
diff --git a/synapse/config/registration.py b/synapse/config/registration.py
index afb3e0b2a..ead007ba5 100644
--- a/synapse/config/registration.py
+++ b/synapse/config/registration.py
@@ -49,6 +49,10 @@ class AccountValidityConfig(Config):
 
             self.startup_job_max_delta = self.period * 10.0 / 100.0
 
+        if self.renew_by_email_enabled:
+            if "public_baseurl" not in synapse_config:
+                raise ConfigError("Can't send renewal emails without 'public_baseurl'")
+
         template_dir = config.get("template_dir")
 
         if not template_dir:
@@ -105,6 +109,13 @@ class RegistrationConfig(Config):
         account_threepid_delegates = config.get("account_threepid_delegates") or {}
         self.account_threepid_delegate_email = account_threepid_delegates.get("email")
         self.account_threepid_delegate_msisdn = account_threepid_delegates.get("msisdn")
+        if self.account_threepid_delegate_msisdn and not self.public_baseurl:
+            raise ConfigError(
+                "The configuration option `public_baseurl` is required if "
+                "`account_threepid_delegate.msisdn` is set, such that "
+                "clients know where to submit validation tokens to. Please "
+                "configure `public_baseurl`."
+            )
 
         self.default_identity_server = config.get("default_identity_server")
         self.allow_guest_access = config.get("allow_guest_access", False)
@@ -227,9 +238,8 @@ class RegistrationConfig(Config):
           # send an email to the account's email address with a renewal link. By
           # default, no such emails are sent.
           #
-          # If you enable this setting, you will also need to fill out the 'email'
-          # configuration section. You should also check that 'public_baseurl' is set
-          # correctly.
+          # If you enable this setting, you will also need to fill out the 'email' and
+          # 'public_baseurl' configuration sections.
           #
           #renew_at: 1w
 
@@ -320,7 +330,8 @@ class RegistrationConfig(Config):
         # The identity server which we suggest that clients should use when users log
         # in on this server.
         #
-        # (By default, no suggestion is made, so it is left up to the client.)
+        # (By default, no suggestion is made, so it is left up to the client.
+        # This setting is ignored unless public_baseurl is also set.)
         #
         #default_identity_server: https://matrix.org
 
@@ -345,6 +356,8 @@ class RegistrationConfig(Config):
         # by the Matrix Identity Service API specification:
         # https://matrix.org/docs/spec/identity_service/latest
         #
+        # If a delegate is specified, the config option public_baseurl must also be filled out.
+        #
         account_threepid_delegates:
             #email: https://example.com     # Delegate email sending to example.com
             #msisdn: http://localhost:8090  # Delegate SMS sending to this local process
diff --git a/synapse/config/saml2_config.py b/synapse/config/saml2_config.py
index 1820614bc..4b494f217 100644
--- a/synapse/config/saml2_config.py
+++ b/synapse/config/saml2_config.py
@@ -188,6 +188,8 @@ class SAML2Config(Config):
         import saml2
 
         public_baseurl = self.public_baseurl
+        if public_baseurl is None:
+            raise ConfigError("saml2_config requires a public_baseurl to be set")
 
         if self.saml2_grandfathered_mxid_source_attribute:
             optional_attributes.add(self.saml2_grandfathered_mxid_source_attribute)
diff --git a/synapse/config/server.py b/synapse/config/server.py
index b5e82ba3d..a635b8a7d 100644
--- a/synapse/config/server.py
+++ b/synapse/config/server.py
@@ -230,11 +230,7 @@ class ServerConfig(Config):
         self.print_pidfile = config.get("print_pidfile")
         self.user_agent_suffix = config.get("user_agent_suffix")
         self.use_frozen_dicts = config.get("use_frozen_dicts", False)
-        self.public_baseurl = config.get("public_baseurl") or "https://%s/" % (
-            self.server_name,
-        )
-        if self.public_baseurl[-1] != "/":
-            self.public_baseurl += "/"
+        self.public_baseurl = config.get("public_baseurl")
 
         # Whether to enable user presence.
         self.use_presence = config.get("use_presence", True)
@@ -386,6 +382,9 @@ class ServerConfig(Config):
             config_path=("federation_ip_range_blacklist",),
         )
 
+        if self.public_baseurl is not None:
+            if self.public_baseurl[-1] != "/":
+                self.public_baseurl += "/"
         self.start_pushers = config.get("start_pushers", True)
 
         # (undocumented) option for torturing the worker-mode replication a bit,
@@ -813,10 +812,6 @@ class ServerConfig(Config):
         # Otherwise, it should be the URL to reach Synapse's client HTTP listener (see
         # 'listeners' below).
         #
-        # If this is left unset, it defaults to 'https://<server_name>/'. (Note that
-        # that will not work unless you configure Synapse or a reverse-proxy to listen
-        # on port 443.)
-        #
         #public_baseurl: https://example.com/
 
         # Set the soft limit on the number of file descriptors synapse can use
diff --git a/synapse/config/sso.py b/synapse/config/sso.py
index b94d3cd5e..07ba217f8 100644
--- a/synapse/config/sso.py
+++ b/synapse/config/sso.py
@@ -81,8 +81,11 @@ class SSOConfig(Config):
         # 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.
-        login_fallback_url = self.public_baseurl + "_matrix/static/client/login"
-        self.sso_client_whitelist.append(login_fallback_url)
+        # 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 """\
@@ -100,9 +103,9 @@ class SSOConfig(Config):
             # phishing attacks from evil.site. To avoid this, include a slash after the
             # hostname: "https://my.client/".
             #
-            # The login fallback page (used by clients that don't natively support the
-            # required login flows) is automatically whitelisted in addition to any URLs
-            # in this list.
+            # 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.
             #
diff --git a/synapse/handlers/identity.py b/synapse/handlers/identity.py
index 4f7137539..8fc1e8b91 100644
--- a/synapse/handlers/identity.py
+++ b/synapse/handlers/identity.py
@@ -504,6 +504,10 @@ class IdentityHandler(BaseHandler):
         except RequestTimedOutError:
             raise SynapseError(500, "Timed out contacting identity server")
 
+        # It is already checked that public_baseurl is configured since this code
+        # should only be used if account_threepid_delegate_msisdn is true.
+        assert self.hs.config.public_baseurl
+
         # we need to tell the client to send the token back to us, since it doesn't
         # otherwise know where to send it, so add submit_url response parameter
         # (see also MSC2078)
diff --git a/synapse/rest/well_known.py b/synapse/rest/well_known.py
index 241fe746d..f591cc6c5 100644
--- a/synapse/rest/well_known.py
+++ b/synapse/rest/well_known.py
@@ -34,6 +34,10 @@ class WellKnownBuilder:
         self._config = hs.config
 
     def get_well_known(self):
+        # if we don't have a public_baseurl, we can't help much here.
+        if self._config.public_baseurl is None:
+            return None
+
         result = {"m.homeserver": {"base_url": self._config.public_baseurl}}
 
         if self._config.default_identity_server:
diff --git a/synapse/util/templates.py b/synapse/util/templates.py
index 7e5109d20..392dae4a4 100644
--- a/synapse/util/templates.py
+++ b/synapse/util/templates.py
@@ -17,7 +17,7 @@
 
 import time
 import urllib.parse
-from typing import TYPE_CHECKING, Callable, Iterable, Union
+from typing import TYPE_CHECKING, Callable, Iterable, Optional, Union
 
 import jinja2
 
@@ -74,14 +74,23 @@ def build_jinja_env(
     return env
 
 
-def _create_mxc_to_http_filter(public_baseurl: str) -> Callable:
+def _create_mxc_to_http_filter(
+    public_baseurl: Optional[str],
+) -> Callable[[str, int, int, str], str]:
     """Create and return a jinja2 filter that converts MXC urls to HTTP
 
     Args:
         public_baseurl: The public, accessible base URL of the homeserver
     """
 
-    def mxc_to_http_filter(value, width, height, resize_method="crop"):
+    def mxc_to_http_filter(
+        value: str, width: int, height: int, resize_method: str = "crop"
+    ) -> str:
+        if not public_baseurl:
+            raise RuntimeError(
+                "public_baseurl must be set in the homeserver config to convert MXC URLs to HTTP URLs."
+            )
+
         if value[0:6] != "mxc://":
             return ""
 
diff --git a/tests/rest/client/v1/test_login.py b/tests/rest/client/v1/test_login.py
index ceb4ad236..49543d9ac 100644
--- a/tests/rest/client/v1/test_login.py
+++ b/tests/rest/client/v1/test_login.py
@@ -680,10 +680,12 @@ class CASTestCase(unittest.HomeserverTestCase):
         self.redirect_path = "_synapse/client/login/sso/redirect/confirm"
 
         config = self.default_config()
+        config["public_baseurl"] = (
+            config.get("public_baseurl") or "https://matrix.goodserver.com:8448"
+        )
         config["cas_config"] = {
             "enabled": True,
             "server_url": CAS_SERVER,
-            "service_url": "https://matrix.goodserver.com:8448",
         }
 
         cas_user_id = "username"
diff --git a/tests/rest/test_well_known.py b/tests/rest/test_well_known.py
index c5e44af9f..14de0921b 100644
--- a/tests/rest/test_well_known.py
+++ b/tests/rest/test_well_known.py
@@ -40,3 +40,12 @@ class WellKnownTests(unittest.HomeserverTestCase):
                 "m.identity_server": {"base_url": "https://testis"},
             },
         )
+
+    def test_well_known_no_public_baseurl(self):
+        self.hs.config.public_baseurl = None
+
+        channel = self.make_request(
+            "GET", "/.well-known/matrix/client", shorthand=False
+        )
+
+        self.assertEqual(channel.code, 404)
diff --git a/tests/utils.py b/tests/utils.py
index 68033d753..840b657f8 100644
--- a/tests/utils.py
+++ b/tests/utils.py
@@ -159,6 +159,7 @@ def default_config(name, parse=False):
         },
         "rc_3pid_validation": {"per_second": 10000, "burst_count": 10000},
         "saml2_enabled": False,
+        "public_baseurl": None,
         "default_identity_server": None,
         "key_refresh_interval": 24 * 60 * 60 * 1000,
         "old_signing_keys": {},