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+# User-Interactive Authentication for SSO-backed homeserver
+
+Certain endpoints, such as `DELETE /_matrix/client/r0/devices/{deviceId}` and
+`POST /_matrix/client/r0/account/3pid/add`, require the user to reconfirm their
+identity, as a guard against a leaked access token being used to take over an
+entire account.
+
+On a normal homeserver, this is done by prompting the user to enter their
+password. However, on a homeserver where users authenticate via a single-sign-on
+system, the user doesn't have a password registered with the homeserver. Instead
+we need to delegate that check to the SSO system.
+
+At the protocol level, this means adding support for SSO to the
+[user-interactive authentication API](https://matrix.org/docs/spec/client_server/r0.6.0#user-interactive-authentication-api).
+
+In theory, once SSO is added as a possible flow for authentication, any clients
+that already implement the [fallback process for unknown authentication types](https://matrix.org/docs/spec/client_server/r0.6.0#fallback)
+will work fine without modification. It is unknown whether this is widely
+supported among clients.
+
+## Proposal
+
+An [additional authentication type](https://matrix.org/docs/spec/client_server/r0.6.0#authentication-types)
+of `m.login.sso` is added to the user-interactive authentication specification.
+
+There are no additional parameters as part of this authentication type. As per
+the user-interactive authentication specification, the only parameter included in
+the `auth` dictionary should be the session ID from the homeserver, e.g.:
+
+```json
+{
+  "auth": {
+    "session": "<session ID>"
+  }
+}
+```
+
+### Detailed fallback authentication flow:
+
+The following is a re-iteration of the [fallback authentication flow](https://matrix.org/docs/spec/client_server/r0.6.0#fallback), 
+but with details filled in for the proposed new authentication type.
+
+When choosing this authentication flow, the following should occur:
+
+1.  If the client wants to complete authentication using SSO, it opens a browser
+    window for `/_matrix/client/r0/auth/m.login.sso/fallback/web?session=<...>`
+    with session set to the UI-Auth session id (from the "auth" dict).
+
+    The homeserver returns a page which asks for the user's confirmation before
+    proceeding. See the security considerations section below for why this is
+    necessary. 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!
+2.  The link, once the user clicks on it, goes to the SSO provider's page.
+3.  The SSO provider validates the user, and redirects the browser back to the
+    homeserver.
+4.  The homeserver validates the response from the SSO provider, updates the
+    user-interactive auth session to show that the SSO has completed, and
+    [serves the fallback auth completion page as specced](https://matrix.org/docs/spec/client_server/r0.6.0#fallback).
+5.  The client resubmits its original request, with its original session id,
+    which now should complete.
+
+Note that the post-SSO URL on the homeserver is left up to the homeserver
+implementation rather than forming part of the specification, choices might be
+limited by the chosen SSO implementation, for example:
+
+*   SAML2 servers typically only support one URL per service provider, so in
+    practice it will need to be the same as that already used for the login flow
+    (for synapse, it's `/_matrix/saml2/authn_response`) - and the server needs to
+    be able to figure out if it's doing SSO for a login attempt or an SSO
+    attempt.
+*   CAS doesn't have the same restriction.
+
+### Example flow:
+
+A more complete example is provided below in which a user attempts to delete
+a device and is pushed into the user interactive authentication process with
+SSO being the only possible flow.
+
+0.  Client submits the request, which the server says requires SSO:
+
+    ```
+    POST /_matrix/client/r0/delete_devices HTTP/1.1
+    Content-Type: application/json
+    Authorization: Bearer xyzzy
+
+    {
+        "devices": ["FSVVTZRRAA"]
+    }
+
+    HTTP/1.1 401 Unauthorized
+    Content-Type: application/json
+
+    {
+        "flows": [
+            {
+                "stages": [
+                    "m.login.sso"
+                ]
+            }
+        ],
+        "params": {},
+        "session": "dTKfsLHSAJeAhqfxUsvrIVJd"
+    }
+    ```
+
+1.  Client opens a browser window for the fallback endpoint:
+
+    ```
+    GET /_matrix/client/r0/auth/m.login.sso/fallback/web
+        ?session=dTKfsLHSAJeAhqfxUsvrIVJd HTTP/1.1
+
+    HTTP/1.1 200 OK
+
+    <body>
+    A client is trying to remove a device from your account. To confirm this
+    action, <a href="https://sso_provider/validate?SAMLRequest=...">re-authenticate with single sign-on</a>.
+    If you did not expect this, your account may be compromised!
+    </body>
+    ```
+
+2.  The user clicks the confirmation link which goes to the SSO provider's site:
+
+    ```
+    GET https://sso_provider/validate?SAMLRequest=<etc> HTTP/1.1
+
+    <SAML/CAS or other SSO data>
+    ```
+
+3.  The SSO provider validates the user and ends up redirecting the browser back
+    to the homeserver. The example below shows a 302 for simplicity, this might
+    vary based on SSO implementation.
+
+    ```
+    HTTP/1.1 302 Moved
+    Location: https://homeserver/_matrix/saml2/authn_response?
+        SAMLResponse=<etc>
+    ```
+
+4.  The browser sends the SSO response to the homeserver, which validates it and
+    shows the fallback auth completion page:
+
+    ```
+    GET /_matrix/saml2/authn_response?SAMLResponse=<etc>
+
+
+    HTTP/1.1 200 OK
+
+    <script>
+    if (window.onAuthDone) {
+        window.onAuthDone();
+    } else if (window.opener && window.opener.postMessage) {
+         window.opener.postMessage("authDone", "*");
+    }
+    </script>
+
+    <p>Thank you.</p>
+    <p>You may now close this window and return to the application.</p>
+    ```
+
+5.  The client closes the browser popup if it is still open, and resubmits its
+    original request, which now succeeds:
+
+    ```
+    POST /_matrix/client/r0/delete_devices HTTP/1.1
+    Content-Type: application/json
+    Authorization: Bearer xyzzy
+
+    {
+        "auth": {
+          "session": "dTKfsLHSAJeAhqfxUsvrIVJd"
+        }
+    }
+
+    HTTP/1.1 200 OK
+    Content-Type: application/json
+
+    {}
+    ```
+
+## Alternatives
+
+An alternative client flow where the fallback auth ends up redirecting to a
+given URI, instead of doing JavaScript `postMessage` foo could be considered.
+This is probably an orthogonal change to the fallback auth though.
+
+## Security considerations
+
+### Why we need user to confirm before the SSO flow
+
+Recall that the thing we are trying to guard against here is a single leaked
+access-token being used to take over an entire account. So let's assume the
+attacker has got hold of an access token for your account. What happens if the
+confirmation step is skipped?
+
+The attacker, who has your access token, starts a UI Authentication session to
+add their email address to your account.
+
+They then sends you a link "hey, check out this cool video!"; the link leads (via
+an innocent-looking URL shortener or some other phishing technique) to
+`/_matrix/client/r0/auth/m.login.sso/fallback/web?session=<...>`, with the ID of
+the session that he just created.
+
+Since there is no confirmation step, the server redirects directly to the SSO
+provider.
+
+It's common for SSO providers to redirect straight back to the app if you've
+recently authenticated with them; even in the best case, the SSO provider shows
+an innocent message along the lines of "Confirm that you want to sign in to
+<your Matrix homeserver>".
+
+After redirecting back to the homeserver, the SSO is completed and the
+attacker's session is validated. They are now able to make their malicious
+change to your account.
+
+This problem can be mitigated by clearly telling the user what is about to happen.
+
+### Reusing User Interactive Authentication sessions
+
+The security of this relies on User Interactive Authentication sessions only
+being used for the same request as they were initiated for. This security is not
+only a concern for the proposed SSO authentication type. It is not believed
+that this is currently enforced in implementations.
+
+## Unstable prefix
+
+A vendor prefix of `org.matrix.login.sso` is proposed (instead of `m.login.sso`)
+until this is part of the specification.