Clarify security note regarding the domain Synapse is hosted on.

README.rst

@@ -149,21 +149,45 @@ For details on having Synapse manage your federation TLS certificates
 automatically, please see `<docs/ACME.md>`_.
 
 
-Security Note
+Security note
 =============
 
-Matrix serves raw user generated data in some APIs - specifically the `content
-repository endpoints <https://matrix.org/docs/spec/client_server/latest.html#get-matrix-media-r0-download-servername-mediaid>`_.
+Matrix serves raw, user-supplied data in some APIs -- specifically the `content
+repository endpoints`_.
 
-Whilst we have tried to mitigate against possible XSS attacks (e.g.
-https://github.com/matrix-org/synapse/pull/1021) we recommend running
-matrix homeservers on a dedicated domain name, to limit any malicious user generated
-content served to web browsers a matrix API from being able to attack webapps hosted
-on the same domain.  This is particularly true of sharing a matrix webclient and
-server on the same domain.
+.. _content repository endpoints: https://matrix.org/docs/spec/client_server/latest.html#get-matrix-media-r0-download-servername-mediaid
 
-See https://github.com/vector-im/riot-web/issues/1977 and
-https://developer.github.com/changes/2014-04-25-user-content-security for more details.
+Whilst we make a reasonable effort to mitigate against XSS attacks (for
+instance, by using `CSP`_), a Matrix homeserver should not be hosted on a
+domain hosting other web applications. This especially applies to sharing
+the domain with Matrix web clients and other sensitive applications like
+webmail. See
+https://developer.github.com/changes/2014-04-25-user-content-security for more
+information.
+
+.. _CSP: https://github.com/matrix-org/synapse/pull/1021
+
+Ideally, the homeserver should not simply be on a different subdomain, but on
+a completely different `registered domain`_ (also known as top-level site or
+eTLD+1). This is because `some attacks`_ are still possible as long as the two
+applications share the same registered domain.
+
+.. _registered domain: https://tools.ietf.org/html/draft-ietf-httpbis-rfc6265bis-03#section-2.3
+
+.. _some attacks: https://en.wikipedia.org/wiki/Session_fixation#Attacks_using_cross-subdomain_cookie
+
+To illustrate this with an example, if your Element Web or other sensitive web
+application is hosted on ``A.example1.com``, you should ideally host Synapse on
+``example2.com``. Some amount of protection is offered by hosting on
+``B.example1.com`` instead, so this is also acceptable in some scenarios.
+However, you should *not* host your Synapse on ``A.example1.com``.
+
+Note that all of the above refers exclusively to the domain used in Synapse's
+``public_baseurl`` setting. In particular, it has no bearing on the domain
+mentioned in MXIDs hosted on that server.
+
+Following this advice ensures that even if an XSS is found in Synapse, the
+impact to other applications will be minimal.
 
 
 Upgrading an existing Synapse

changelog.d/9221.doc

@@ -0,0 +1 @@
+Clarify security note regarding hosting Synapse on the same domain as other web applications.