Centralise the custom template directory (#10596)

Several configuration sections are using separate settings for custom template directories, which can be confusing. This PR adds a new top-level configuration for a custom template directory which is then used for every module. The only exception is the consent templates, since the consent template directory require a specific hierarchy, so it's probably better that it stays separate from everything else.

12 files changed, 342 insertions, 418 deletions

diff --git a/changelog.d/10596.removal b/changelog.d/10596.removal
new file mode 100644
index 0000000000..e69f632db4
--- /dev/null
+++ b/changelog.d/10596.removal
@@ -0,0 +1 @@
+The `template_dir` configuration settings in the `sso`, `account_validity` and `email` sections of the configuration file are now deprecated in favour of the global `templates.custom_template_directory` setting. See the [upgrade notes](https://matrix-org.github.io/synapse/latest/upgrade.html) for more information.
diff --git a/docs/SUMMARY.md b/docs/SUMMARY.md
index 3d320a1c43..56e0141c2b 100644
--- a/docs/SUMMARY.md
+++ b/docs/SUMMARY.md
@@ -21,6 +21,7 @@
     - [Homeserver Sample Config File](usage/configuration/homeserver_sample_config.md)
     - [Logging Sample Config File](usage/configuration/logging_sample_config.md)
     - [Structured Logging](structured_logging.md)
+    - [Templates](templates.md)
     - [User Authentication](usage/configuration/user_authentication/README.md)
       - [Single-Sign On]()
         - [OpenID Connect](openid.md)
diff --git a/docs/sample_config.yaml b/docs/sample_config.yaml
index aeebcaf45f..3ec76d5abf 100644
--- a/docs/sample_config.yaml
+++ b/docs/sample_config.yaml
@@ -565,6 +565,19 @@ retention:
 #
 #next_link_domain_whitelist: ["matrix.org"]
 
+# Templates to use when generating email or HTML page contents.
+#
+templates:
+  # Directory in which Synapse will try to find template files to use to generate
+  # email or HTML page contents.
+  # If not set, or a file is not found within the template directory, a default
+  # template from within the Synapse package will be used.
+  #
+  # See https://matrix-org.github.io/synapse/latest/templates.html for more
+  # information about using custom templates.
+  #
+  #custom_template_directory: /path/to/custom/templates/
+
 
 ## TLS ##
 
@@ -1895,6 +1908,9 @@ cas_config:
 # Additional settings to use with single-sign on systems such as OpenID Connect,
 # SAML2 and CAS.
 #
+# Server admins can configure custom templates for pages related to SSO. See
+# https://matrix-org.github.io/synapse/latest/templates.html for more information.
+#
 sso:
     # A list of client URLs which are whitelisted so that the user does not
     # have to confirm giving access to their account to the URL. Any client
@@ -1927,169 +1943,6 @@ sso:
     #
     #update_profile_information: true
 
-    # Directory in which Synapse will try to find the template files below.
-    # If not set, or the files named below are not found within the template
-    # directory, default templates from within the Synapse package will be used.
-    #
-    # Synapse will look for the following templates in this directory:
-    #
-    # * HTML page to prompt the user to choose an Identity Provider during
-    #   login: 'sso_login_idp_picker.html'.
-    #
-    #   This is only used if multiple SSO Identity Providers are configured.
-    #
-    #   When rendering, this template is given the following variables:
-    #     * redirect_url: the URL that the user will be redirected to after
-    #       login.
-    #
-    #     * server_name: the homeserver's name.
-    #
-    #     * providers: a list of available Identity Providers. Each element is
-    #       an object with the following attributes:
-    #
-    #         * idp_id: unique identifier for the IdP
-    #         * idp_name: user-facing name for the IdP
-    #         * idp_icon: if specified in the IdP config, an MXC URI for an icon
-    #              for the IdP
-    #         * idp_brand: if specified in the IdP config, a textual identifier
-    #              for the brand of the IdP
-    #
-    #   The rendered HTML page should contain a form which submits its results
-    #   back as a GET request, with the following query parameters:
-    #
-    #     * redirectUrl: the client redirect URI (ie, the `redirect_url` passed
-    #       to the template)
-    #
-    #     * idp: the 'idp_id' of the chosen IDP.
-    #
-    # * HTML page to prompt new users to enter a userid and confirm other
-    #   details: 'sso_auth_account_details.html'. This is only shown if the
-    #   SSO implementation (with any user_mapping_provider) does not return
-    #   a localpart.
-    #
-    #   When rendering, this template is given the following variables:
-    #
-    #     * server_name: the homeserver's name.
-    #
-    #     * idp: details of the SSO Identity Provider that the user logged in
-    #       with: an object with the following attributes:
-    #
-    #         * idp_id: unique identifier for the IdP
-    #         * idp_name: user-facing name for the IdP
-    #         * idp_icon: if specified in the IdP config, an MXC URI for an icon
-    #              for the IdP
-    #         * idp_brand: if specified in the IdP config, a textual identifier
-    #              for the brand of the IdP
-    #
-    #     * user_attributes: an object containing details about the user that
-    #       we received from the IdP. May have the following attributes:
-    #
-    #         * display_name: the user's display_name
-    #         * emails: a list of email addresses
-    #
-    #   The template should render a form which submits the following fields:
-    #
-    #     * username: the localpart of the user's chosen user id
-    #
-    # * HTML page allowing the user to consent to the server's terms and
-    #   conditions. This is only shown for new users, and only if
-    #   `user_consent.require_at_registration` is set.
-    #
-    #   When rendering, this template is given the following variables:
-    #
-    #     * server_name: the homeserver's name.
-    #
-    #     * user_id: the user's matrix proposed ID.
-    #
-    #     * user_profile.display_name: the user's proposed display name, if any.
-    #
-    #     * consent_version: the version of the terms that the user will be
-    #       shown
-    #
-    #     * terms_url: a link to the page showing the terms.
-    #
-    #   The template should render a form which submits the following fields:
-    #
-    #     * accepted_version: the version of the terms accepted by the user
-    #       (ie, 'consent_version' from the input variables).
-    #
-    # * HTML page for a confirmation step before redirecting back to the client
-    #   with the login token: 'sso_redirect_confirm.html'.
-    #
-    #   When rendering, this template is given the following variables:
-    #
-    #     * redirect_url: the URL the user is about to be redirected to.
-    #
-    #     * display_url: the same as `redirect_url`, but with the query
-    #                    parameters stripped. The intention is to have a
-    #                    human-readable URL to show to users, not to use it as
-    #                    the final address to redirect to.
-    #
-    #     * server_name: the homeserver's name.
-    #
-    #     * new_user: a boolean indicating whether this is the user's first time
-    #          logging in.
-    #
-    #     * user_id: the user's matrix ID.
-    #
-    #     * user_profile.avatar_url: an MXC URI for the user's avatar, if any.
-    #           None if the user has not set an avatar.
-    #
-    #     * user_profile.display_name: the user's display name. None if the user
-    #           has not set a display name.
-    #
-    # * HTML page which notifies the user that they are authenticating to confirm
-    #   an operation on their account during the user interactive authentication
-    #   process: 'sso_auth_confirm.html'.
-    #
-    #   When rendering, this template is given the following variables:
-    #     * redirect_url: the URL the user is about to be redirected to.
-    #
-    #     * description: the operation which the user is being asked to confirm
-    #
-    #     * idp: details of the Identity Provider that we will use to confirm
-    #       the user's identity: an object with the following attributes:
-    #
-    #         * idp_id: unique identifier for the IdP
-    #         * idp_name: user-facing name for the IdP
-    #         * idp_icon: if specified in the IdP config, an MXC URI for an icon
-    #              for the IdP
-    #         * idp_brand: if specified in the IdP config, a textual identifier
-    #              for the brand of the IdP
-    #
-    # * HTML page shown after a successful user interactive authentication session:
-    #   'sso_auth_success.html'.
-    #
-    #   Note that this page must include the JavaScript which notifies of a successful authentication
-    #   (see https://matrix.org/docs/spec/client_server/r0.6.0#fallback).
-    #
-    #   This template has no additional variables.
-    #
-    # * HTML page shown after a user-interactive authentication session which
-    #   does not map correctly onto the expected user: 'sso_auth_bad_user.html'.
-    #
-    #   When rendering, this template is given the following variables:
-    #     * server_name: the homeserver's name.
-    #     * user_id_to_verify: the MXID of the user that we are trying to
-    #       validate.
-    #
-    # * HTML page shown during single sign-on if a deactivated user (according to Synapse's database)
-    #   attempts to login: 'sso_account_deactivated.html'.
-    #
-    #   This template has no additional variables.
-    #
-    # * HTML page to display to users if something goes wrong during the
-    #   OpenID Connect authentication process: 'sso_error.html'.
-    #
-    #   When rendering, this template is given two variables:
-    #     * error: the technical name of the error
-    #     * error_description: a human-readable message for the error
-    #
-    # You can see the default templates at:
-    # https://github.com/matrix-org/synapse/tree/master/synapse/res/templates
-    #
-    #template_dir: "res/templates"
-
 
 # JSON web token integration. The following settings can be used to make
 # Synapse JSON web tokens for authentication, instead of its internal
@@ -2220,6 +2073,9 @@ ui_auth:
 
 # Configuration for sending emails from Synapse.
 #
+# Server admins can configure custom templates for email content. See
+# https://matrix-org.github.io/synapse/latest/templates.html for more information.
+#
 email:
   # The hostname of the outgoing SMTP server to use. Defaults to 'localhost'.
   #
@@ -2296,49 +2152,6 @@ email:
   #
   #invite_client_location: https://app.element.io
 
-  # Directory in which Synapse will try to find the template files below.
-  # If not set, or the files named below are not found within the template
-  # directory, default templates from within the Synapse package will be used.
-  #
-  # Synapse will look for the following templates in this directory:
-  #
-  # * The contents of email notifications of missed events: 'notif_mail.html' and
-  #   'notif_mail.txt'.
-  #
-  # * The contents of account expiry notice emails: 'notice_expiry.html' and
-  #   'notice_expiry.txt'.
-  #
-  # * The contents of password reset emails sent by the homeserver:
-  #   'password_reset.html' and 'password_reset.txt'
-  #
-  # * An HTML page that a user will see when they follow the link in the password
-  #   reset email. The user will be asked to confirm the action before their
-  #   password is reset: 'password_reset_confirmation.html'
-  #
-  # * HTML pages for success and failure that a user will see when they confirm
-  #   the password reset flow using the page above: 'password_reset_success.html'
-  #   and 'password_reset_failure.html'
-  #
-  # * The contents of address verification emails sent during registration:
-  #   'registration.html' and 'registration.txt'
-  #
-  # * HTML pages for success and failure that a user will see when they follow
-  #   the link in an address verification email sent during registration:
-  #   'registration_success.html' and 'registration_failure.html'
-  #
-  # * The contents of address verification emails sent when an address is added
-  #   to a Matrix account: 'add_threepid.html' and 'add_threepid.txt'
-  #
-  # * HTML pages for success and failure that a user will see when they follow
-  #   the link in an address verification email sent when an address is added
-  #   to a Matrix account: 'add_threepid_success.html' and
-  #   'add_threepid_failure.html'
-  #
-  # You can see the default templates at:
-  # https://github.com/matrix-org/synapse/tree/master/synapse/res/templates
-  #
-  #template_dir: "res/templates"
-
   # Subjects to use when sending emails from Synapse.
   #
   # The placeholder '%(app)s' will be replaced with the value of the 'app_name'
diff --git a/docs/templates.md b/docs/templates.md
new file mode 100644
index 0000000000..a240f58b54
--- /dev/null
+++ b/docs/templates.md
@@ -0,0 +1,239 @@
+# Templates
+
+Synapse uses parametrised templates to generate the content of emails it sends and
+webpages it shows to users.
+
+By default, Synapse will use the templates listed [here](https://github.com/matrix-org/synapse/tree/master/synapse/res/templates).
+Server admins can configure an additional directory for Synapse to look for templates
+in, allowing them to specify custom templates:
+
+```yaml
+templates:
+  custom_templates_directory: /path/to/custom/templates/
+```
+
+If this setting is not set, or the files named below are not found within the directory,
+default templates from within the Synapse package will be used.
+
+Templates that are given variables when being rendered are rendered using [Jinja 2](https://jinja.palletsprojects.com/en/2.11.x/).
+Templates rendered by Jinja 2 can also access two functions on top of the functions
+already available as part of Jinja 2:
+
+```python
+format_ts(value: int, format: str) -> str
+```
+
+Formats a timestamp in milliseconds.
+
+Example: `reason.last_sent_ts|format_ts("%c")`
+
+```python
+mxc_to_http(value: str, width: int, height: int, resize_method: str = "crop") -> str
+```
+
+Turns a `mxc://` URL for media content into an HTTP(S) one using the homeserver's
+`public_baseurl` configuration setting as the URL's base.
+
+Example: `message.sender_avatar_url|mxc_to_http(32,32)`
+
+
+## Email templates
+
+Below are the templates Synapse will look for when generating the content of an email:
+
+* `notif_mail.html` and `notif_mail.txt`: The contents of email notifications of missed
+  events.
+  When rendering, this template is given the following variables:
+    * `user_display_name`: the display name for the user receiving the notification
+    * `unsubscribe_link`: the link users can click to unsubscribe from email notifications
+    * `summary_text`: a summary of the notification(s). The text used can be customised
+      by configuring the various settings in the `email.subjects` section of the
+      configuration file.
+    * `rooms`: a list of rooms containing events to include in the email. Each element is
+      an object with the following attributes:
+        * `title`: a human-readable name for the room
+        * `hash`: a hash of the ID of the room
+        * `invite`: a boolean, which is `True` if the room is an invite the user hasn't
+          accepted yet, `False` otherwise
+        * `notifs`: a list of events, or an empty list if `invite` is `True`. Each element
+          is an object with the following attributes:
+            * `link`: a `matrix.to` link to the event
+            * `ts`: the time in milliseconds at which the event was received
+            * `messages`: a list of messages containing one message before the event, the
+              message in the event, and one message after the event. Each element is an
+              object with the following attributes:
+                * `event_type`: the type of the event
+                * `is_historical`: a boolean, which is `False` if the message is the one
+                  that triggered the notification, `True` otherwise
+                * `id`: the ID of the event
+                * `ts`: the time in milliseconds at which the event was sent
+                * `sender_name`: the display name for the event's sender
+                * `sender_avatar_url`: the avatar URL (as a `mxc://` URL) for the event's
+                  sender
+                * `sender_hash`: a hash of the user ID of the sender
+        * `link`: a `matrix.to` link to the room
+    * `reason`: information on the event that triggered the email to be sent. It's an
+      object with the following attributes:
+        * `room_id`: the ID of the room the event was sent in
+        * `room_name`: a human-readable name for the room the event was sent in
+        * `now`: the current time in milliseconds
+        * `received_at`: the time in milliseconds at which the event was received
+        * `delay_before_mail_ms`: the amount of time in milliseconds Synapse always waits
+          before ever emailing about a notification (to give the user a chance to respond
+          to other push or notice the window)
+        * `last_sent_ts`: the time in milliseconds at which a notification was last sent
+          for an event in this room
+        * `throttle_ms`: the minimum amount of time in milliseconds between two
+          notifications can be sent for this room
+* `password_reset.html` and `password_reset.txt`: The contents of password reset emails
+  sent by the homeserver.
+  When rendering, these templates are given a `link` variable which contains the link the
+  user must click in order to reset their password.
+* `registration.html` and `registration.txt`: The contents of address verification emails
+  sent during registration.
+  When rendering, these templates are given a `link` variable which contains the link the
+  user must click in order to validate their email address.
+* `add_threepid.html` and `add_threepid.txt`: The contents of address verification emails
+  sent when an address is added to a Matrix account.
+  When rendering, these templates are given a `link` variable which contains the link the
+  user must click in order to validate their email address.
+
+
+## HTML page templates for registration and password reset
+
+Below are the templates Synapse will look for when generating pages related to
+registration and password reset:
+
+* `password_reset_confirmation.html`: An HTML page that a user will see when they follow
+  the link in the password reset email. The user will be asked to confirm the action
+  before their password is reset.
+  When rendering, this template is given the following variables:
+    * `sid`: the session ID for the password reset
+    * `token`: the token for the password reset
+    * `client_secret`: the client secret for the password reset
+* `password_reset_success.html` and `password_reset_failure.html`: HTML pages for success
+  and failure that a user will see when they confirm the password reset flow using the
+  page above.
+  When rendering, `password_reset_success.html` is given no variable, and
+  `password_reset_failure.html` is given a `failure_reason`, which contains the reason
+  for the password reset failure. 
+* `registration_success.html` and `registration_failure.html`: HTML pages for success and
+  failure that a user will see when they follow the link in an address verification email
+  sent during registration.
+  When rendering, `registration_success.html` is given no variable, and
+  `registration_failure.html` is given a `failure_reason`, which contains the reason
+  for the registration failure.
+* `add_threepid_success.html` and `add_threepid_failure.html`: HTML pages for success and
+  failure that a user will see when they follow the link in an address verification email
+  sent when an address is added to a Matrix account.
+  When rendering, `add_threepid_success.html` is given no variable, and
+  `add_threepid_failure.html` is given a `failure_reason`, which contains the reason
+  for the registration failure.
+
+
+## HTML page templates for Single Sign-On (SSO)
+
+Below are the templates Synapse will look for when generating pages related to SSO:
+
+* `sso_login_idp_picker.html`: HTML page to prompt the user to choose an
+  Identity Provider during login.
+  This is only used if multiple SSO Identity Providers are configured.
+  When rendering, this template is given the following variables:
+    * `redirect_url`: the URL that the user will be redirected to after
+      login.
+    * `server_name`: the homeserver's name.
+    * `providers`: a list of available Identity Providers. Each element is
+      an object with the following attributes:
+        * `idp_id`: unique identifier for the IdP
+        * `idp_name`: user-facing name for the IdP
+        * `idp_icon`: if specified in the IdP config, an MXC URI for an icon
+             for the IdP
+        * `idp_brand`: if specified in the IdP config, a textual identifier
+             for the brand of the IdP
+  The rendered HTML page should contain a form which submits its results
+  back as a GET request, with the following query parameters:
+    * `redirectUrl`: the client redirect URI (ie, the `redirect_url` passed
+      to the template)
+    * `idp`: the 'idp_id' of the chosen IDP.
+* `sso_auth_account_details.html`: HTML page to prompt new users to enter a
+  userid and confirm other details. This is only shown if the
+  SSO implementation (with any `user_mapping_provider`) does not return
+  a localpart.
+  When rendering, this template is given the following variables:
+    * `server_name`: the homeserver's name.
+    * `idp`: details of the SSO Identity Provider that the user logged in
+      with: an object with the following attributes:
+        * `idp_id`: unique identifier for the IdP
+        * `idp_name`: user-facing name for the IdP
+        * `idp_icon`: if specified in the IdP config, an MXC URI for an icon
+             for the IdP
+        * `idp_brand`: if specified in the IdP config, a textual identifier
+             for the brand of the IdP
+    * `user_attributes`: an object containing details about the user that
+      we received from the IdP. May have the following attributes:
+        * display_name: the user's display_name
+        * emails: a list of email addresses
+  The template should render a form which submits the following fields:
+    * `username`: the localpart of the user's chosen user id
+* `sso_new_user_consent.html`: HTML page allowing the user to consent to the
+  server's terms and conditions. This is only shown for new users, and only if
+  `user_consent.require_at_registration` is set.
+  When rendering, this template is given the following variables:
+    * `server_name`: the homeserver's name.
+    * `user_id`: the user's matrix proposed ID.
+    * `user_profile.display_name`: the user's proposed display name, if any.
+    * consent_version: the version of the terms that the user will be
+      shown
+    * `terms_url`: a link to the page showing the terms.
+  The template should render a form which submits the following fields:
+    * `accepted_version`: the version of the terms accepted by the user
+      (ie, 'consent_version' from the input variables).
+* `sso_redirect_confirm.html`: HTML page for a confirmation step before redirecting back
+  to the client with the login token.
+  When rendering, this template is given the following variables:
+    * `redirect_url`: the URL the user is about to be redirected to.
+    * `display_url`: the same as `redirect_url`, but with the query
+                   parameters stripped. The intention is to have a
+                   human-readable URL to show to users, not to use it as
+                   the final address to redirect to.
+    * `server_name`: the homeserver's name.
+    * `new_user`: a boolean indicating whether this is the user's first time
+         logging in.
+    * `user_id`: the user's matrix ID.
+    * `user_profile.avatar_url`: an MXC URI for the user's avatar, if any.
+          `None` if the user has not set an avatar.
+    * `user_profile.display_name`: the user's display name. `None` if the user
+          has not set a display name.
+* `sso_auth_confirm.html`: HTML page which notifies the user that they are authenticating
+  to confirm an operation on their account during the user interactive authentication
+  process.
+  When rendering, this template is given the following variables:
+    * `redirect_url`: the URL the user is about to be redirected to.
+    * `description`: the operation which the user is being asked to confirm
+    * `idp`: details of the Identity Provider that we will use to confirm
+      the user's identity: an object with the following attributes:
+        * `idp_id`: unique identifier for the IdP
+        * `idp_name`: user-facing name for the IdP
+        * `idp_icon`: if specified in the IdP config, an MXC URI for an icon
+             for the IdP
+        * `idp_brand`: if specified in the IdP config, a textual identifier
+             for the brand of the IdP
+* `sso_auth_success.html`: HTML page shown after a successful user interactive
+  authentication session.
+  Note that this page must include the JavaScript which notifies of a successful
+  authentication (see https://matrix.org/docs/spec/client_server/r0.6.0#fallback).
+  This template has no additional variables.
+* `sso_auth_bad_user.html`: HTML page shown after a user-interactive authentication
+  session which does not map correctly onto the expected user.
+  When rendering, this template is given the following variables:
+    * `server_name`: the homeserver's name.
+    * `user_id_to_verify`: the MXID of the user that we are trying to
+      validate.
+* `sso_account_deactivated.html`: HTML page shown during single sign-on if a deactivated
+  user (according to Synapse's database) attempts to login.
+  This template has no additional variables.
+* `sso_error.html`: HTML page to display to users if something goes wrong during the
+  OpenID Connect authentication process.
+  When rendering, this template is given two variables:
+    * `error`: the technical name of the error
+    * `error_description`: a human-readable message for the error
diff --git a/docs/upgrade.md b/docs/upgrade.md
index 8831c9d6cf..1c459d8e2b 100644
--- a/docs/upgrade.md
+++ b/docs/upgrade.md
@@ -112,6 +112,17 @@ environment variable.
 See [using a forward proxy with Synapse documentation](setup/forward_proxy.md) for
 details.
 
+## Deprecation of `template_dir`
+
+The `template_dir` settings in the `sso`, `account_validity` and `email` sections of the
+configuration file are now deprecated. Server admins should use the new
+`templates.custom_template_directory` setting in the configuration file and use one single
+custom template directory for all aforementioned features. Template file names remain
+unchanged. See [the related documentation](https://matrix-org.github.io/synapse/latest/templates.html)
+for more information and examples.
+
+We plan to remove support for these settings in October 2021.
+
 
 # Upgrading to v1.39.0
 
diff --git a/synapse/config/account_validity.py b/synapse/config/account_validity.py
index 9acce5996e..52e63ab1f6 100644
--- a/synapse/config/account_validity.py
+++ b/synapse/config/account_validity.py
@@ -78,6 +78,11 @@ class AccountValidityConfig(Config):
         )
 
         # Read and store template content
+        custom_template_directories = (
+            self.root.server.custom_template_directory,
+            account_validity_template_dir,
+        )
+
         (
             self.account_validity_account_renewed_template,
             self.account_validity_account_previously_renewed_template,
@@ -88,5 +93,5 @@ class AccountValidityConfig(Config):
                 "account_previously_renewed.html",
                 invalid_token_template_filename,
             ],
-            (td for td in (account_validity_template_dir,) if td),
+            (td for td in custom_template_directories if td),
         )
diff --git a/synapse/config/emailconfig.py b/synapse/config/emailconfig.py
index fc74b4a8b9..4477419196 100644
--- a/synapse/config/emailconfig.py
+++ b/synapse/config/emailconfig.py
@@ -258,7 +258,12 @@ class EmailConfig(Config):
                     add_threepid_template_success_html,
                 ],
                 (
-                    td for td in (template_dir,) if td
+                    td
+                    for td in (
+                        self.root.server.custom_template_directory,
+                        template_dir,
+                    )
+                    if td
                 ),  # Filter out template_dir if not provided
             )
 
@@ -299,7 +304,14 @@ class EmailConfig(Config):
                 self.email_notif_template_text,
             ) = self.read_templates(
                 [notif_template_html, notif_template_text],
-                (td for td in (template_dir,) if td),
+                (
+                    td
+                    for td in (
+                        self.root.server.custom_template_directory,
+                        template_dir,
+                    )
+                    if td
+                ),  # Filter out template_dir if not provided
             )
 
             self.email_notif_for_new_users = email_config.get(
@@ -322,7 +334,14 @@ class EmailConfig(Config):
                 self.account_validity_template_text,
             ) = self.read_templates(
                 [expiry_template_html, expiry_template_text],
-                (td for td in (template_dir,) if td),
+                (
+                    td
+                    for td in (
+                        self.root.server.custom_template_directory,
+                        template_dir,
+                    )
+                    if td
+                ),  # Filter out template_dir if not provided
             )
 
         subjects_config = email_config.get("subjects", {})
@@ -354,6 +373,9 @@ class EmailConfig(Config):
             """\
         # Configuration for sending emails from Synapse.
         #
+        # Server admins can configure custom templates for email content. See
+        # https://matrix-org.github.io/synapse/latest/templates.html for more information.
+        #
         email:
           # The hostname of the outgoing SMTP server to use. Defaults to 'localhost'.
           #
@@ -430,49 +452,6 @@ class EmailConfig(Config):
           #
           #invite_client_location: https://app.element.io
 
-          # Directory in which Synapse will try to find the template files below.
-          # If not set, or the files named below are not found within the template
-          # directory, default templates from within the Synapse package will be used.
-          #
-          # Synapse will look for the following templates in this directory:
-          #
-          # * The contents of email notifications of missed events: 'notif_mail.html' and
-          #   'notif_mail.txt'.
-          #
-          # * The contents of account expiry notice emails: 'notice_expiry.html' and
-          #   'notice_expiry.txt'.
-          #
-          # * The contents of password reset emails sent by the homeserver:
-          #   'password_reset.html' and 'password_reset.txt'
-          #
-          # * An HTML page that a user will see when they follow the link in the password
-          #   reset email. The user will be asked to confirm the action before their
-          #   password is reset: 'password_reset_confirmation.html'
-          #
-          # * HTML pages for success and failure that a user will see when they confirm
-          #   the password reset flow using the page above: 'password_reset_success.html'
-          #   and 'password_reset_failure.html'
-          #
-          # * The contents of address verification emails sent during registration:
-          #   'registration.html' and 'registration.txt'
-          #
-          # * HTML pages for success and failure that a user will see when they follow
-          #   the link in an address verification email sent during registration:
-          #   'registration_success.html' and 'registration_failure.html'
-          #
-          # * The contents of address verification emails sent when an address is added
-          #   to a Matrix account: 'add_threepid.html' and 'add_threepid.txt'
-          #
-          # * HTML pages for success and failure that a user will see when they follow
-          #   the link in an address verification email sent when an address is added
-          #   to a Matrix account: 'add_threepid_success.html' and
-          #   'add_threepid_failure.html'
-          #
-          # You can see the default templates at:
-          # https://github.com/matrix-org/synapse/tree/master/synapse/res/templates
-          #
-          #template_dir: "res/templates"
-
           # Subjects to use when sending emails from Synapse.
           #
           # The placeholder '%%(app)s' will be replaced with the value of the 'app_name'
diff --git a/synapse/config/server.py b/synapse/config/server.py
index 187b4301a0..8494795919 100644
--- a/synapse/config/server.py
+++ b/synapse/config/server.py
@@ -710,6 +710,18 @@ class ServerConfig(Config):
             # Turn the list into a set to improve lookup speed.
             self.next_link_domain_whitelist = set(next_link_domain_whitelist)
 
+        templates_config = config.get("templates") or {}
+        if not isinstance(templates_config, dict):
+            raise ConfigError("The 'templates' section must be a dictionary")
+
+        self.custom_template_directory = templates_config.get(
+            "custom_template_directory"
+        )
+        if self.custom_template_directory is not None and not isinstance(
+            self.custom_template_directory, str
+        ):
+            raise ConfigError("'custom_template_directory' must be a string")
+
     def has_tls_listener(self) -> bool:
         return any(listener.tls for listener in self.listeners)
 
@@ -1284,6 +1296,19 @@ class ServerConfig(Config):
         # all domains.
         #
         #next_link_domain_whitelist: ["matrix.org"]
+
+        # Templates to use when generating email or HTML page contents.
+        #
+        templates:
+          # Directory in which Synapse will try to find template files to use to generate
+          # email or HTML page contents.
+          # If not set, or a file is not found within the template directory, a default
+          # template from within the Synapse package will be used.
+          #
+          # See https://matrix-org.github.io/synapse/latest/templates.html for more
+          # information about using custom templates.
+          #
+          #custom_template_directory: /path/to/custom/templates/
         """
             % locals()
         )
diff --git a/synapse/config/sso.py b/synapse/config/sso.py
index 4b590e0535..fe1177ab81 100644
--- a/synapse/config/sso.py
+++ b/synapse/config/sso.py
@@ -45,6 +45,11 @@ class SSOConfig(Config):
         self.sso_template_dir = sso_config.get("template_dir")
 
         # Read templates from disk
+        custom_template_directories = (
+            self.root.server.custom_template_directory,
+            self.sso_template_dir,
+        )
+
         (
             self.sso_login_idp_picker_template,
             self.sso_redirect_confirm_template,
@@ -63,7 +68,7 @@ class SSOConfig(Config):
                 "sso_auth_success.html",
                 "sso_auth_bad_user.html",
             ],
-            (td for td in (self.sso_template_dir,) if td),
+            (td for td in custom_template_directories if td),
         )
 
         # These templates have no placeholders, so render them here
@@ -94,6 +99,9 @@ class SSOConfig(Config):
         # Additional settings to use with single-sign on systems such as OpenID Connect,
         # SAML2 and CAS.
         #
+        # Server admins can configure custom templates for pages related to SSO. See
+        # https://matrix-org.github.io/synapse/latest/templates.html for more information.
+        #
         sso:
             # A list of client URLs which are whitelisted so that the user does not
             # have to confirm giving access to their account to the URL. Any client
@@ -125,167 +133,4 @@ class SSOConfig(Config):
             # information when first signing in. Defaults to false.
             #
             #update_profile_information: true
-
-            # Directory in which Synapse will try to find the template files below.
-            # If not set, or the files named below are not found within the template
-            # directory, default templates from within the Synapse package will be used.
-            #
-            # Synapse will look for the following templates in this directory:
-            #
-            # * HTML page to prompt the user to choose an Identity Provider during
-            #   login: 'sso_login_idp_picker.html'.
-            #
-            #   This is only used if multiple SSO Identity Providers are configured.
-            #
-            #   When rendering, this template is given the following variables:
-            #     * redirect_url: the URL that the user will be redirected to after
-            #       login.
-            #
-            #     * server_name: the homeserver's name.
-            #
-            #     * providers: a list of available Identity Providers. Each element is
-            #       an object with the following attributes:
-            #
-            #         * idp_id: unique identifier for the IdP
-            #         * idp_name: user-facing name for the IdP
-            #         * idp_icon: if specified in the IdP config, an MXC URI for an icon
-            #              for the IdP
-            #         * idp_brand: if specified in the IdP config, a textual identifier
-            #              for the brand of the IdP
-            #
-            #   The rendered HTML page should contain a form which submits its results
-            #   back as a GET request, with the following query parameters:
-            #
-            #     * redirectUrl: the client redirect URI (ie, the `redirect_url` passed
-            #       to the template)
-            #
-            #     * idp: the 'idp_id' of the chosen IDP.
-            #
-            # * HTML page to prompt new users to enter a userid and confirm other
-            #   details: 'sso_auth_account_details.html'. This is only shown if the
-            #   SSO implementation (with any user_mapping_provider) does not return
-            #   a localpart.
-            #
-            #   When rendering, this template is given the following variables:
-            #
-            #     * server_name: the homeserver's name.
-            #
-            #     * idp: details of the SSO Identity Provider that the user logged in
-            #       with: an object with the following attributes:
-            #
-            #         * idp_id: unique identifier for the IdP
-            #         * idp_name: user-facing name for the IdP
-            #         * idp_icon: if specified in the IdP config, an MXC URI for an icon
-            #              for the IdP
-            #         * idp_brand: if specified in the IdP config, a textual identifier
-            #              for the brand of the IdP
-            #
-            #     * user_attributes: an object containing details about the user that
-            #       we received from the IdP. May have the following attributes:
-            #
-            #         * display_name: the user's display_name
-            #         * emails: a list of email addresses
-            #
-            #   The template should render a form which submits the following fields:
-            #
-            #     * username: the localpart of the user's chosen user id
-            #
-            # * HTML page allowing the user to consent to the server's terms and
-            #   conditions. This is only shown for new users, and only if
-            #   `user_consent.require_at_registration` is set.
-            #
-            #   When rendering, this template is given the following variables:
-            #
-            #     * server_name: the homeserver's name.
-            #
-            #     * user_id: the user's matrix proposed ID.
-            #
-            #     * user_profile.display_name: the user's proposed display name, if any.
-            #
-            #     * consent_version: the version of the terms that the user will be
-            #       shown
-            #
-            #     * terms_url: a link to the page showing the terms.
-            #
-            #   The template should render a form which submits the following fields:
-            #
-            #     * accepted_version: the version of the terms accepted by the user
-            #       (ie, 'consent_version' from the input variables).
-            #
-            # * HTML page for a confirmation step before redirecting back to the client
-            #   with the login token: 'sso_redirect_confirm.html'.
-            #
-            #   When rendering, this template is given the following variables:
-            #
-            #     * redirect_url: the URL the user is about to be redirected to.
-            #
-            #     * display_url: the same as `redirect_url`, but with the query
-            #                    parameters stripped. The intention is to have a
-            #                    human-readable URL to show to users, not to use it as
-            #                    the final address to redirect to.
-            #
-            #     * server_name: the homeserver's name.
-            #
-            #     * new_user: a boolean indicating whether this is the user's first time
-            #          logging in.
-            #
-            #     * user_id: the user's matrix ID.
-            #
-            #     * user_profile.avatar_url: an MXC URI for the user's avatar, if any.
-            #           None if the user has not set an avatar.
-            #
-            #     * user_profile.display_name: the user's display name. None if the user
-            #           has not set a display name.
-            #
-            # * HTML page which notifies the user that they are authenticating to confirm
-            #   an operation on their account during the user interactive authentication
-            #   process: 'sso_auth_confirm.html'.
-            #
-            #   When rendering, this template is given the following variables:
-            #     * redirect_url: the URL the user is about to be redirected to.
-            #
-            #     * description: the operation which the user is being asked to confirm
-            #
-            #     * idp: details of the Identity Provider that we will use to confirm
-            #       the user's identity: an object with the following attributes:
-            #
-            #         * idp_id: unique identifier for the IdP
-            #         * idp_name: user-facing name for the IdP
-            #         * idp_icon: if specified in the IdP config, an MXC URI for an icon
-            #              for the IdP
-            #         * idp_brand: if specified in the IdP config, a textual identifier
-            #              for the brand of the IdP
-            #
-            # * HTML page shown after a successful user interactive authentication session:
-            #   'sso_auth_success.html'.
-            #
-            #   Note that this page must include the JavaScript which notifies of a successful authentication
-            #   (see https://matrix.org/docs/spec/client_server/r0.6.0#fallback).
-            #
-            #   This template has no additional variables.
-            #
-            # * HTML page shown after a user-interactive authentication session which
-            #   does not map correctly onto the expected user: 'sso_auth_bad_user.html'.
-            #
-            #   When rendering, this template is given the following variables:
-            #     * server_name: the homeserver's name.
-            #     * user_id_to_verify: the MXID of the user that we are trying to
-            #       validate.
-            #
-            # * HTML page shown during single sign-on if a deactivated user (according to Synapse's database)
-            #   attempts to login: 'sso_account_deactivated.html'.
-            #
-            #   This template has no additional variables.
-            #
-            # * HTML page to display to users if something goes wrong during the
-            #   OpenID Connect authentication process: 'sso_error.html'.
-            #
-            #   When rendering, this template is given two variables:
-            #     * error: the technical name of the error
-            #     * error_description: a human-readable message for the error
-            #
-            # You can see the default templates at:
-            # https://github.com/matrix-org/synapse/tree/master/synapse/res/templates
-            #
-            #template_dir: "res/templates"
         """
diff --git a/synapse/module_api/__init__.py b/synapse/module_api/__init__.py
index 82725853bc..2f99d31c42 100644
--- a/synapse/module_api/__init__.py
+++ b/synapse/module_api/__init__.py
@@ -91,6 +91,7 @@ class ModuleApi:
         self._state = hs.get_state_handler()
         self._clock: Clock = hs.get_clock()
         self._send_email_handler = hs.get_send_email_handler()
+        self.custom_template_dir = hs.config.server.custom_template_directory
 
         try:
             app_name = self._hs.config.email_app_name
@@ -679,7 +680,7 @@ class ModuleApi:
         """
         return self._hs.config.read_templates(
             filenames,
-            (td for td in (custom_template_directory,) if td),
+            (td for td in (self.custom_template_dir, custom_template_directory) if td),
         )
 
 
diff --git a/synapse/rest/synapse/client/new_user_consent.py b/synapse/rest/synapse/client/new_user_consent.py
index 488b97b32e..fc62a09b7f 100644
--- a/synapse/rest/synapse/client/new_user_consent.py
+++ b/synapse/rest/synapse/client/new_user_consent.py
@@ -46,6 +46,8 @@ class NewUserConsentResource(DirectServeHtmlResource):
         self._consent_version = hs.config.consent.user_consent_version
 
         def template_search_dirs():
+            if hs.config.server.custom_template_directory:
+                yield hs.config.server.custom_template_directory
             if hs.config.sso.sso_template_dir:
                 yield hs.config.sso.sso_template_dir
             yield hs.config.sso.default_template_dir
diff --git a/synapse/rest/synapse/client/pick_username.py b/synapse/rest/synapse/client/pick_username.py
index ab24ec0a8e..c15b83c387 100644
--- a/synapse/rest/synapse/client/pick_username.py
+++ b/synapse/rest/synapse/client/pick_username.py
@@ -74,6 +74,8 @@ class AccountDetailsResource(DirectServeHtmlResource):
         self._sso_handler = hs.get_sso_handler()
 
         def template_search_dirs():
+            if hs.config.server.custom_template_directory:
+                yield hs.config.server.custom_template_directory
             if hs.config.sso.sso_template_dir:
                 yield hs.config.sso.sso_template_dir
             yield hs.config.sso.default_template_dir