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Support OIDC backchannel logouts #11414

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Oct 31, 2022
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Support OIDC backchannel logouts #11414

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49fe26e
Support back-channel logouts from OIDC providers
sandhose Sep 16, 2022
a0bb53d
Apply suggestions from code review
sandhose Dec 7, 2021
a5d74fa
Validate `exp` and `nbf` claims in logout tokens if present
sandhose Dec 7, 2021
51f3cd8
Docstrings on issuer/client_id properties
sandhose Dec 7, 2021
1fd3658
Misc changes
sandhose Sep 16, 2022
17580ae
Check the `sub` claim when receiving a logout token
sandhose Sep 21, 2022
42df126
Revoke in-flight logins
sandhose Oct 26, 2022
55076e1
Fix unit tests on old version of Twisted
sandhose Oct 26, 2022
eb7537a
Docs
sandhose Oct 26, 2022
d19bc83
Apply suggestions from code review
sandhose Oct 28, 2022
0f22917
Docs
sandhose Oct 28, 2022
99f58d0
Apply suggestions from code review
sandhose Oct 28, 2022
48777f3
Refactor OIDC backchannel logout test config
sandhose Oct 31, 2022
e3c2db6
Move the `is_access_token_valid` method to the `RestHelper`
sandhose Oct 31, 2022
9cc5d40
Clarification around logout token verification
sandhose Oct 31, 2022
c0b4986
Clarify the string split while manually parsing the JWT
sandhose Oct 31, 2022
6deef8d
Move the IDP session revocation logic to the SSO handler
sandhose Oct 31, 2022
e0748f6
Use the issuer from the metadata, not the config when verifying logou…
sandhose Oct 31, 2022
626d778
Merge branch 'develop' into sandhose/oidc-logout
sandhose Oct 31, 2022
8d32796
Fix remaining issues raised in code review
sandhose Oct 31, 2022
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1 change: 1 addition & 0 deletions changelog.d/11414.feature
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Support back-channel logouts from OpenID Connect providers.
14 changes: 14 additions & 0 deletions docs/openid.md
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Expand Up @@ -49,6 +49,13 @@ setting in your configuration file.
See the [configuration manual](usage/configuration/config_documentation.md#oidc_providers) for some sample settings, as well as
the text below for example configurations for specific providers.

## OIDC Back-Channel Logout

Synapse supports receiving [OpenID Connect Back-Channel Logout](https://openid.net/specs/openid-connect-backchannel-1_0.html) notifications.

This lets the OpenID Connect Provider notify Synapse when a user logs out, so that Synapse can end that user session.
This feature can be enabled by setting the `backchannel_logout_enabled` property to `true` in the provider configuration, and setting the following URL as destination for Back-Channel Logout notifications in your OpenID Connect Provider: `[synapse public baseurl]/_synapse/client/oidc/backchannel_logout`

## Sample configs

Here are a few configs for providers that should work with Synapse.
Expand Down Expand Up @@ -123,6 +130,9 @@ oidc_providers:

[Keycloak][keycloak-idp] is an opensource IdP maintained by Red Hat.

Keycloak supports OIDC Back-Channel Logout, which sends logout notification to Synapse, so that Synapse users get logged out when they log out from Keycloak.
This can be optionally enabled by setting `backchannel_logout_enabled` to `true` in the Synapse configuration, and by setting the "Backchannel Logout URL" in Keycloak.

Follow the [Getting Started Guide](https://www.keycloak.org/getting-started) to install Keycloak and set up a realm.

1. Click `Clients` in the sidebar and click `Create`
Expand All @@ -144,6 +154,8 @@ Follow the [Getting Started Guide](https://www.keycloak.org/getting-started) to
| Client Protocol | `openid-connect` |
| Access Type | `confidential` |
| Valid Redirect URIs | `[synapse public baseurl]/_synapse/client/oidc/callback` |
| Backchannel Logout URL (optional) | `[synapse public baseurl]/_synapse/client/oidc/backchannel_logout` |
| Backchannel Logout Session Required (optional) | `On` |

5. Click `Save`
6. On the Credentials tab, update the fields:
Expand All @@ -167,7 +179,9 @@ oidc_providers:
config:
localpart_template: "{{ user.preferred_username }}"
display_name_template: "{{ user.name }}"
backchannel_logout_enabled: true # Optional
```

### Auth0

[Auth0][auth0] is a hosted SaaS IdP solution.
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6 changes: 6 additions & 0 deletions docs/usage/configuration/config_documentation.md
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Expand Up @@ -3014,6 +3014,12 @@ Options for each entry include:
which is set to the claims returned by the UserInfo Endpoint and/or
in the ID Token.

* `backchannel_logout_enabled`: set to true to process OIDC Back-Channel Logout notifications.
Those notifications are expected to be received on `/_synapse/client/oidc/backchannel_logout`.
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* `backchannel_logout_ignore_sub`: by default, the OIDC Back-Channel Logout feature checks that the
`sub` claim matches the subject claim received during login. This check can be disabled by setting
this to `false`.
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It is possible to configure Synapse to only allow logins if certain attributes
match particular values in the OIDC userinfo. The requirements can be listed under
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12 changes: 12 additions & 0 deletions synapse/config/oidc.py
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Expand Up @@ -123,6 +123,8 @@ def oidc_enabled(self) -> bool:
"userinfo_endpoint": {"type": "string"},
"jwks_uri": {"type": "string"},
"skip_verification": {"type": "boolean"},
"backchannel_logout_enabled": {"type": "boolean"},
"backchannel_logout_ignore_sub": {"type": "boolean"},
"user_profile_method": {
"type": "string",
"enum": ["auto", "userinfo_endpoint"],
Expand Down Expand Up @@ -292,6 +294,10 @@ def _parse_oidc_config_dict(
token_endpoint=oidc_config.get("token_endpoint"),
userinfo_endpoint=oidc_config.get("userinfo_endpoint"),
jwks_uri=oidc_config.get("jwks_uri"),
backchannel_logout_enabled=oidc_config.get("backchannel_logout_enabled", False),
backchannel_logout_ignore_sub=oidc_config.get(
"backchannel_logout_ignore_sub", False
),
skip_verification=oidc_config.get("skip_verification", False),
user_profile_method=oidc_config.get("user_profile_method", "auto"),
allow_existing_users=oidc_config.get("allow_existing_users", False),
Expand Down Expand Up @@ -368,6 +374,12 @@ class OidcProviderConfig:
# "openid" scope is used.
jwks_uri: Optional[str]

# Whether Synapse should react to backchannel logouts
backchannel_logout_enabled: bool
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It makes me slightly sad to add another configuration option here, but I don't think there's much we can do here to avoid it?

The metadata seems to include whether this feature is supported or not. Can we auto-enable it in those cases?

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I mean, we could probably completely skip the configuration parameter. The only real benefit right now are warnings when the config enables backchannel logouts, but the OP does not advertise proper support.

Right now the behaviour is to delete the device, which does a "hard" logout. From a client perspective, it means it won't keep E2EE data, so you can't get back the deleted device. Maybe that's not the right behaviour, maybe that should be configurable.

Would it make sense to have instead a config parameter backchannel_logout_behaviour with the following values: logout (the default), soft_logout, nothing, and maybe implement soft_logout/other behaviours later?

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Right now the behaviour is to delete the device, which does a "hard" logout. From a client perspective, it means it won't keep E2EE data, so you can't get back the deleted device. Maybe that's not the right behaviour, maybe that should be configurable.

Would it make sense to have instead a config parameter backchannel_logout_behaviour with the following values: logout (the default), soft_logout, nothing, and maybe implement soft_logout/other behaviours later?

I don't think doing a hard logout makes sense, actually. It likely makes more sense to do a soft-logout? I'm not really sure what a backchannel-logout is supposed to represent, but logging out of the IdP causing Synapse to destroy a session, causing a user to lose decryption keys sounds not great.

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I haven't really found a good way to soft-logout a user :(
It looks like soft-logouts are only sent when the token expired.

but logging out of the IdP causing Synapse to destroy a session, causing a user to lose decryption keys sounds not great.

I agree, but on the other hand leaving 'dead' devices on the account doesn't seem great either. Would you be fine with leaving this behaviour for now, see how it feels with whatever customer is asking for this change, and iterate on it later if it makes more sense to do a soft-logout?


# Whetever Synapse should ignore the `sub` claim in backchannel logouts or not
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backchannel_logout_ignore_sub: bool

# Whether to skip metadata verification
skip_verification: bool

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