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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.

In theory, once SSO is added as a possible flow for authentication, any clients that already implement the fallback process for unknown authentication types will work fine without modification. It is unknown whether this is widely supported among clients.

Proposal

An additional authentication type 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.:

{
  "auth": {
    "session": "<session ID>"
  }
}

Detailed fallback authentication flow:

The following is a re-iteration of the fallback authentication flow, but with details filled in for the proposed new authentication type.

When choosing this authentication flow, the following should occur:

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!
The link, once the user clicks on it, goes to the SSO provider's page.
The SSO provider validates the user, and redirects the browser back to the homeserver.
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.
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.

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"
}

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>

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>

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>
```

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>

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.

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