2 files changed, 65 insertions, 0 deletions
diff --git a/synapse/rest/__init__.py b/synapse/rest/__init__.py
index 1be9c47c61..53b8c319a6 100644
--- a/synapse/rest/__init__.py
+++ b/synapse/rest/__init__.py
@@ -22,6 +22,7 @@ from synapse.rest.client import (
     account_validity,
     appservice_ping,
     auth,
+    auth_issuer,
     capabilities,
     devices,
     directory,
@@ -148,3 +149,4 @@ class ClientRestResource(JsonResource):
             mutual_rooms.register_servlets(hs, client_resource)
             login_token_request.register_servlets(hs, client_resource)
             rendezvous.register_servlets(hs, client_resource)
+            auth_issuer.register_servlets(hs, client_resource)
diff --git a/synapse/rest/client/auth_issuer.py b/synapse/rest/client/auth_issuer.py
new file mode 100644
index 0000000000..77b9720956
--- /dev/null
+++ b/synapse/rest/client/auth_issuer.py
@@ -0,0 +1,63 @@
+# Copyright 2023 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.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+import logging
+import typing
+from typing import Tuple
+
+from synapse.api.errors import Codes, SynapseError
+from synapse.http.server import HttpServer
+from synapse.http.servlet import RestServlet
+from synapse.http.site import SynapseRequest
+from synapse.rest.client._base import client_patterns
+from synapse.types import JsonDict
+
+if typing.TYPE_CHECKING:
+    from synapse.server import HomeServer
+
+
+logger = logging.getLogger(__name__)
+
+
+class AuthIssuerServlet(RestServlet):
+    """
+    Advertises what OpenID Connect issuer clients should use to authorise users.
+    """
+
+    PATTERNS = client_patterns(
+        "/org.matrix.msc2965/auth_issuer$",
+        unstable=True,
+        releases=(),
+    )
+
+    def __init__(self, hs: "HomeServer"):
+        super().__init__()
+        self._config = hs.config
+
+    async def on_GET(self, request: SynapseRequest) -> Tuple[int, JsonDict]:
+        if self._config.experimental.msc3861.enabled:
+            return 200, {"issuer": self._config.experimental.msc3861.issuer}
+        else:
+            # Wouldn't expect this to be reached: the servelet shouldn't have been
+            # registered. Still, fail gracefully if we are registered for some reason.
+            raise SynapseError(
+                404,
+                "OIDC discovery has not been configured on this homeserver",
+                Codes.NOT_FOUND,
+            )
+
+
+def register_servlets(hs: "HomeServer", http_server: HttpServer) -> None:
+    # We use the MSC3861 values as they are used by multiple MSCs
+    if hs.config.experimental.msc3861.enabled:
+        AuthIssuerServlet(hs).register(http_server)