Clarifications to SSO login/UIA (#2608)

including a bunch of text about security

CONTRIBUTING.rst

@@ -13,7 +13,7 @@ https://github.com/matrix-org/matrix-doc/blob/master/meta/documentation_style.rs
 
 Python code within the ``matrix-doc`` project should follow the same style as
 synapse, which is documented at
-https://github.com/matrix-org/synapse/tree/master/docs/code_style.rst.
+https://github.com/matrix-org/synapse/tree/master/docs/code_style.md.
 
 Matrix-doc workflows
 --------------------

changelogs/client_server/newsfragments/2608.clarification

@@ -0,0 +1 @@
+Clarify the behaviour of SSO login and UI-Auth.

specification/client_server_api.rst

@@ -1,4 +1,4 @@
-.. Copyright 2016 OpenMarket Ltd
+.. Copyright 2016-2020 The Matrix.org Foundation C.I.C.
 ..
 .. Licensed under the Apache License, Version 2.0 (the "License");
 .. you may not use this file except in compliance with the License.
@@ -425,6 +425,8 @@ on the server-side and the user simply needs to provide their credentials again.
 
 In either case, the client's previously known access token will no longer function.
 
+.. _`user-interactive authentication`:
+
 User-Interactive Authentication API
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -813,28 +815,8 @@ Single Sign-On
   provider.
 
 A client wanting to complete authentication using SSO should use the
-`Fallback`_ authentication flow by opening a browser window for
+`Fallback`_ mechanism. See `SSO during User-Interactive Authentication`_ for
-``/_matrix/client/r0/auth/m.login.sso/fallback/web?session=<...>`` with the
+more information.
-session parameter set to the session ID provided by the server.
-
-The homeserver should return a page which asks for the user's confirmation
-before proceeding. For example, the page could say words to the effect of:
-
-  A client is trying to remove a device/add an email address/take over your
-  account. To confirm this action, re-authenticate with single sign-on. If you
-  did not expect this, your account may be compromised!
-
-Once the user has confirmed they should be redirected to the single sign-on
-provider's login page. Once the provider has validated the user, the browser is
-redirected back to the homeserver.
-
-The homeserver then validates the response from the single sign-on provider and
-updates the user-interactive authentication session to mark the single sign-on
-stage has been completed. The browser is shown the fallback authentication
-completion page.
-
-Once the flow has completed, the client retries the request with the session
-only, as above.
 
 Email-based (identity / homeserver)
 <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
@@ -940,6 +922,8 @@ should open is::
 Where ``auth type`` is the type name of the stage it is attempting and
 ``session ID`` is the ID of the session given by the homeserver.
 
+.. _`user-interactive authentication fallback completion`:
+
 This MUST return an HTML page which can perform this authentication stage. This
 page must use the following JavaScript when the authentication has been
 completed:
@@ -1157,7 +1141,7 @@ with ``403 Forbidden`` and an error code of ``M_FORBIDDEN``.
 
 If the homeserver advertises ``m.login.sso`` as a viable flow, and the client
 supports it, the client should redirect the user to the ``/redirect`` endpoint
-for `Single Sign-On <#sso-client-login>`_. After authentication is complete, the
+for `client login via SSO`_. After authentication is complete, the
 client will need to submit a ``/login`` request matching ``m.login.token``.
 
 {{login_cs_http_api}}

specification/modules/sso_login.rst

@@ -1,4 +1,4 @@
-.. Copyright 2019 New Vector Ltd
+.. Copyright 2019-2020 The Matrix.org Foundation C.I.C.
 ..
 .. Licensed under the Apache License, Version 2.0 (the "License");
 .. you may not use this file except in compliance with the License.
@@ -12,34 +12,98 @@
 .. See the License for the specific language governing permissions and
 .. limitations under the License.
 
-SSO client login
+SSO client login/authentication
-================
+===============================
 
 .. _module:sso_login:
 
 Single Sign-On (SSO) is a generic term which refers to protocols which allow
 users to log into applications via a single web-based authentication portal.
-Examples include "Central Authentication Service" (CAS) and SAML.
+Examples include OpenID Connect, "Central Authentication Service" (CAS) and SAML.
 
-An overview of the process, as used in Matrix, is as follows:
+This module allows a Matrix homeserver to delegate user authentication to an
+external authentication server supporting one of these protocols. In this
+process, there are three systems involved:
 
-1. The Matrix client instructs the user's browser to navigate to the
+ * A Matrix client, using the APIs defined this specification, which is seeking
-   |/login/sso/redirect|_ endpoint on the user's homeserver.
+   to authenticate a user to a Matrix homeserver.
+
+ * A Matrix homeserver, implementing the APIs defined in this specification, but
+   which is delegating user authentication to the authentication server.
+
+ * An "authentication server", which is responsible for authenticating the
+   user.
+
+This specification is concerned only with communication between the Matrix
+client and the homeserver, and is independent of the SSO protocol used to
+communicate with the authentication server. Different Matrix homeserver
+implementations might support different SSO protocols.
+
+Clients and homeservers implementing the SSO flow will need to consider both login_
+and `user-interactive authentication`_. The flow is
+similar in both cases, but there are slight differences.
+
+Typically, SSO systems require a single "callback" URI to be configured at the
+authentication server. Once the user is authenticated, their browser is
+redirected to that URI. It is up to the Matrix homeserver implementation to
+implement a suitable endpoint. For example, for CAS authentication the
+homeserver should provide a means for the administrator to configure where the
+CAS server is and the REST endpoints which consume the ticket.
+
+Client login via SSO
+---------------------
+
+An overview of the process is as follows:
+
+0. The Matrix client calls |GET /login|_ to find the supported login
+   types, and the homeserver includes a flow with ``"type": "m.login.sso"`` in the
+   response.
+
+1. To initiate the ``m.login.sso`` login type, the Matrix client instructs the
+   user's browser to navigate to the |/login/sso/redirect|_ endpoint on the
+   user's homeserver.
 
 2. The homeserver responds with an HTTP redirect to the SSO user interface,
    which the browser follows.
 
-3. The SSO system authenticates the user.
+3. The authentication server and the homeserver interact to verify the user's
+   identity and other authentication information, potentially using a number of
+   redirects.
 
-4. The SSO server and the homeserver interact to verify the user's identity
+4. The browser is directed to the ``redirectUrl`` provided by the client with
-   and other authentication information, potentially using a number of redirects.
-
-5. The browser is directed to the ``redirectUrl`` provided by the client with
    a ``loginToken`` query parameter for the client to log in with.
 
+5. The client exchanges the login token for an access token by calling the
+   |/login|_ endpoint with a ``type`` of ``m.login.token``.
+
+For native applications, typically steps 1 to 4 are carried out by opening an
+embedded web view.
+
+These steps are illustrated as follows::
+
+  Matrix Client                        Matrix Homeserver      Auth Server
+      |                                       |                   |
+      |-------------(0) GET /login----------->|                   |
+      |<-------------login types--------------|                   |
+      |                                       |                   |
+      |   Webview                             |                   |
+      |      |                                |                   |
+      |----->|                                |                   |
+      |      |--(1) GET /login/sso/redirect-->|                   |
+      |      |<---------(2) 302---------------|                   |
+      |      |                                |                   |
+      |      |<========(3) Authentication process================>|
+      |      |                                |                   |
+      |      |<--(4) redirect to redirectUrl--|                   |
+      |<-----|                                |                   |
+      |                                       |                   |
+      |---(5) POST /login with login token--->|                   |
+      |<-------------access token-------------|                   |
+
+
 .. Note::
    In the older `r0.4.0 version <https://matrix.org/docs/spec/client_server/r0.4.0.html#cas-based-client-login>`_
-   of this specification it was possible to authenticate via CAS when the server
+   of this specification it was possible to authenticate via CAS when the homeserver
    provides a ``m.login.cas`` login flow. This specification deprecates the use
    of ``m.login.cas`` to instead prefer ``m.login.sso``, which is the same process
    with the only change being which redirect endpoint to use: for ``m.login.cas``, use
@@ -47,40 +111,65 @@ An overview of the process, as used in Matrix, is as follows:
    The endpoints are otherwise the same.
 
 Client behaviour
-----------------
+~~~~~~~~~~~~~~~~
 
 The client starts the process by instructing the browser to navigate to
 |/login/sso/redirect|_ with an appropriate ``redirectUrl``. Once authentication
 is successful, the browser will be redirected to that ``redirectUrl``.
 
-.. TODO-spec
+{{sso_login_redirect_cs_http_api}}
 
-   Should we recommend some sort of CSRF protection here (specifically, we
+Security considerations
-   should guard against people accidentally logging in by sending them a link
++++++++++++++++++++++++
-   to ``/login/sso/redirect``.
 
-   Maybe we should recommend that the ``redirectUrl`` should contain a CSRF
+1. CSRF attacks via manipulation of parameters on the ``redirectUrl``
-   token which the client should then check before sending the login token to
-   ``/login``?
 
-{{sso_login_redirect_cs_http_api}}
+   Clients should validate any requests to the ``redirectUrl``. In particular, it
+   may be possible for attackers to falsify any query parameters, leading to
+   cross-site request forgery (CSRF) attacks.
+
+   For example, consider a web-based client at ``https://client.example.com``,
+   which wants to initiate SSO login on the homeserver at ``server.example.org``.
+   It does this by storing the homeserver name in a query parameter for the
+   ``redirectUrl``: it redirects to
+   ``https://server.example.org/login/sso/redirect?redirectUrl=https://client.example.com?hs=server.example.org``.
+
+   An attacker could trick a victim into following a link to
+   ``https://server.example.org/login/sso/redirect?redirectUrl=https://client.example.com?hs=evil.com``,
+   which would result in the client sending a login token for the victim's
+   account to the attacker-controlled site ``evil.com``.
+
+   To guard against this, clients MUST NOT store state (such as the address of
+   the homeserver being logged into) anywhere it can be modified by external
+   processes.
+
+   Instead, the state could be stored in `localStorage
+   <https://developer.mozilla.org/en-US/docs/Web/API/Window/localStorage>`_ or
+   in a cookie.
+
+2. For added security, clients SHOULD include a unique identifier in the
+   ``redirectUrl`` and reject any callbacks that do not contain a recognised
+   identifier, to guard against unsolicited login attempts and replay attacks.
 
 Server behaviour
-----------------
+~~~~~~~~~~~~~~~~
+
+Redirecting to the Authentication server
+++++++++++++++++++++++++++++++++++++++++
 
-The URI for the SSO system to be used should be configured on the server by the
+The server should handle
-server administrator. The server is expected to set up any endpoints required to
+``/_matrix/client/%CLIENT_MAJOR_VERSION%/login/sso/redirect`` as follows:
-interact with that SSO system. For example, for CAS authentication the homeserver
-should provide a means for the administrator to configure where the CAS server is
-and the REST endpoints which consume the ticket. A good reference for how CAS could
-be implemented is available in the older `r0.4.0 version <https://matrix.org/docs/spec/client_server/r0.4.0.html#cas-based-client-login>`_
-of this specification.
 
-Handling the redirect endpoint
+#. It should build a suitable request for the SSO system.
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-When responding to the ``/login/sso/redirect`` endpoint, the server must
+#. It should store enough state that the flow can be securely resumed after the
-generate a URI for the SSO login page with any appropriate parameters.
+   SSO process completes. One way to do this is by storing a cookie which is
+   stored in the user's browser, by adding a ``Set-Cookie`` header to the response.
+
+#. It should redirect the user's browser to the SSO login page with the
+   appropriate parameters.
+
+See also the "Security considerations" below.
 
 .. TODO-spec:
 
@@ -89,24 +178,169 @@ generate a URI for the SSO login page with any appropriate parameters.
    endpoint, and to give more meaningful errors in the case of
    faulty/poorly-configured clients.
 
-Handling the authentication endpoint
+Handling the callback from the Authentication server
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+++++++++++++++++++++++++++++++++++++++++++++++++++++
+
+Note that there will normally be a single callback URI which is used for both login
+and user-interactive authentication: it is up to the homeserver implementation
+to distinguish which is taking place.
+
+The homeserver should validate the response from the SSO system: this may
+require additional calls to the authentication server, and/or may require
+checking a signature on the response.
+
+The homeserver then proceeds as follows:
+
+#. The homeserver MUST map the user details received from the authentication
+   server to a valid `Matrix user identifier <../appendices.html#user-identifiers>`_.
+   The guidance in `Mapping from other character sets
+   <../appendices.html#mapping-from-other-character-sets>`_ may be useful.
+
+#. If the generated user identifier represents a new user, it should be
+   registered as a new user.
+
+#. The homeserver should generate a short-term login token. This is an opaque
+   token, suitable for use with the ``m.login.token`` type of the |/login|_
+   API. The lifetime of this token SHOULD be limited to around five
+   seconds.
+
+#. The homeserver adds a query parameter of ``loginToken``, with the value of
+   the generated login token, to the ``redirectUrl`` given in the
+   ``/_matrix/client/%CLIENT_MAJOR_VERSION%/login/sso/redirect``
+   request. (Note: ``redirectURL`` may or may not include existing query
+   parameters. If it already includes one or more ``loginToken`` parameters,
+   they should be removed before adding the new one.)
+
+#. The homeserver redirects the user's browser to the URI thus built.
+
+Security considerations
+~~~~~~~~~~~~~~~~~~~~~~~
+
+1. Homeservers should ensure that login tokens are not sent to malicious
+   clients.
+
+   For example, consider a homeserver at ``server.example.org``. An attacker tricks
+   a victim into following a link to
+   ``https://server.example.org/login/sso/redirect?redirectUrl=https://evil.com``,
+   resulting in a login token being sent to the attacker-controlled site
+   ``evil.com``. This is a form of cross-site request forgery (CSRF).
+
+   To mitigate this, Homeservers SHOULD confirm with the user that they are
+   happy to grant access to their matrix account to the site named in the
+   ``redirectUrl``. This can be done either *before* redirecting to the SSO
+   login page when handling the
+   ``/_matrix/client/%CLIENT_MAJOR_VERSION%/login/sso/redirect`` endpoint, or
+   *after* login when handling the callback from the authentication server. (If
+   the check is performed before redirecting, it is particularly important that
+   the homeserver guards against unsolicited authentication attempts as below).
+
+   It may be appropriate to whitelist a set of known-trusted client URLs in
+   this process. In particular, the homeserver's own `login fallback`_
+   implementation could be excluded.
+
+2. For added security, homeservers SHOULD guard against unsolicited
+   authentication attempts by tracking pending requests. One way to do this is
+   to set a cookie when handling
+   ``/_matrix/client/%CLIENT_MAJOR_VERSION%/login/sso/redirect``, which is
+   checked and cleared when handling the callback from the authentication
+   server.
+
+SSO during User-Interactive Authentication
+------------------------------------------
+
+`User-interactive authentication`_ is used by client-server
+endpoints which require additional confirmation of the user's identity (beyond
+holding an access token). Typically this means that the user must re-enter
+their password, but for homeservers which delegate to an SSO server, this means
+redirecting to the authentication server during user-interactive auth.
+
+The implemementation of this is based on the `Fallback`_ mechanism for
+user-interactive auth.
+
+Client behaviour
+----------------
+
+Clients do not need to take any particular additional steps beyond ensuring
+that the fallback mechanism has been implemented, and treating the
+``m.login.sso`` authentication type the same as any other unknown type
+(i.e. they should open a browser window for
+``/_matrix/client/%CLIENT_MAJOR_VERSION%/auth/m.login.sso/fallback/web?session=<session_id>``.
+Once the flow has completed, the client retries the request with the session
+only.)
+
+Server behaviour
+----------------
+
+Redirecting to the Authentication server
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The server should handle
+``/_matrix/client/%CLIENT_MAJOR_VERSION%/auth/m.login.sso/fallback/web`` in
+much the same way as
+``/_matrix/client/%CLIENT_MAJOR_VERSION%/login/sso/redirect``, which is to say:
+
+#. It should build a suitable request for the SSO system.
+
+#. It should store enough state that the flow can be securely resumed after the
+   SSO process completes. One way to do this is by storing a cookie which is
+   stored in the user's browser, by adding a ``Set-Cookie`` header to the response.
+
+#. It should redirect the user's browser to the SSO login page with the
+   appropriate parameters.
+
+See also the "Security considerations" below.
+
+Handling the callback from the Authentication server
+++++++++++++++++++++++++++++++++++++++++++++++++++++
+
+Note that there will normally be a single callback URI which is used for both login
+and user-interactive authentication: it is up to the homeserver implementation
+to distinguish which is taking place.
+
+The homeserver should validate the response from the SSO system: this may
+require additional calls to the authentication server, and/or may require
+checking a signature on the response.
+
+The homeserver then returns the `user-interactive authentication fallback
+completion`_ page to the user's browser.
+
+Security considerations
++++++++++++++++++++++++
+
+1. Confirming the operation
+
+   The homeserver SHOULD confirm that the user is happy for the operation to go
+   ahead. The goal of the user-interactive authentication operation is to guard
+   against a compromised ``access_token`` being used to take over the user's
+   account. Simply redirecting the user to the SSO system is insufficient,
+   since they may not realise what is being asked of them, or the SSO system
+   may even confirm the authentication automatically.
+
+   For example, the homeserver might serve a page with words to the effect of:
+
+     A client is trying to remove a device from your account. To confirm this
+     action, re-authenticate with single sign-on. If you did not expect this, your
+     account may be compromised!
 
-Once the homeserver has verified the user's identity with the SSO system, it
+   This confirmation could take place before redirecting to the SSO
-MUST map the user ID to a valid `Matrix user identifier <../index.html#user-identifiers>`_.
+   authentication page (when handling the
-The guidance in `Mapping from other character sets
+   ``/_matrix/client/%CLIENT_MAJOR_VERSION%/auth/m.login.sso/fallback/web``
-<../index.html#mapping-from-other-character-sets>`_ may be useful.
+   endpoint), or *after* authentication when handling the callback from the
+   authentication server. (If the check is performed before redirecting, it is
+   particularly important that the homeserver guards against unsolicited
+   authentication attempts as below).
 
-If the generated user identifier represents a new user, it should be registered
+2. For added security, homeservers SHOULD guard against unsolicited
-as a new user.
+   authentication attempts by tracking pending requests. One way to do this is
+   to set a cookie when handling
+   ``/_matrix/client/%CLIENT_MAJOR_VERSION%/auth/m.login.sso/fallback/web``,
+   which is checked and cleared when handling the callback from the
+   authentication server.
 
-Finally, the server should generate a short-term login token. The generated
-token should be a macaroon, suitable for use with the ``m.login.token`` type of
-the |/login|_ API, and `token-based interactive login <#token-based>`_. The
-lifetime of this token SHOULD be limited to around five seconds. This token is
-given to the client via the ``loginToken`` query parameter previously mentioned.
 
 
+.. |GET /login| replace:: ``GET /login``
+.. _GET /login: #get-matrix-client-%CLIENT_MAJOR_VERSION%-login
 .. |/login| replace:: ``/login``
 .. _/login: #post-matrix-client-%CLIENT_MAJOR_VERSION%-login
 .. |/login/sso/redirect| replace:: ``/login/sso/redirect``