Support OIDC backchannel logouts (#11414)

If configured an OIDC IdP can log a user's session out of
Synapse when they log out of the identity provider.

The IdP sends a request directly to Synapse (and must be
configured with an endpoint) when a user logs out.

changelog.d/11414.feature

@@ -0,0 +1 @@
+Support back-channel logouts from OpenID Connect providers.

docs/openid.md

@@ -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.
@@ -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`
@@ -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:
@@ -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.

docs/usage/configuration/config_documentation.md

@@ -3021,6 +3021,15 @@ 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`.
+  Defaults to `false`.
+
+* `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 `true`. Defaults to `false`.
+
+  You might want to disable this if the `subject_claim` returned by the mapping provider is not `sub`.
 
 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

synapse/config/oidc.py

@@ -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"],
@@ -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),
@@ -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
+
+    # Whether Synapse should ignore the `sub` claim in backchannel logouts or not.
+    backchannel_logout_ignore_sub: bool
+
     # Whether to skip metadata verification
     skip_verification: bool