Repository: matrix-org/synapse

Issue:
Use OpenID Connect users with existing accounts
I have a homeserver that is using LDAP and I want to migrate to OpenID Connect. This requires that existing users can login with OpenID Connect. If an existing user tries to login, I get the following error and login doesn't work: `synapse.handlers.oidc_handler.MappingException: mxid '@snowball:test.example.com' is already taken`

I think this behavior is reasonable as a default because merging with existing users may be a security problem if the admin isn't careful.

[This patch](https://github.com/BBBSnowball/synapse/commit/a1821f320f7a42682a71b79eede1c03879c26c44) adds a config option to allow using existing users. This seems to work for me but needs more testing.

What do you think? Is this a good solution? Are there any better ways to migrate from LDAP (or other password login) to OIDC?


Discussion:

Resolving patch:
diff --git a/synapse/config/oidc_config.py b/synapse/config/oidc_config.py
--- a/synapse/config/oidc_config.py
+++ b/synapse/config/oidc_config.py
@@ -56,6 +56,7 @@ def read_config(self, config, **kwargs):
         self.oidc_userinfo_endpoint = oidc_config.get("userinfo_endpoint")
         self.oidc_jwks_uri = oidc_config.get("jwks_uri")
         self.oidc_skip_verification = oidc_config.get("skip_verification", False)
+        self.oidc_allow_existing_users = oidc_config.get("allow_existing_users", False)
 
         ump_config = oidc_config.get("user_mapping_provider", {})
         ump_config.setdefault("module", DEFAULT_USER_MAPPING_PROVIDER)
@@ -158,6 +159,11 @@ def generate_config_section(self, config_dir_path, server_name, **kwargs):
           #
           #skip_verification: true
 
+          # Uncomment to allow a user logging in via OIDC to match a pre-existing account instead
+          # of failing. This could be used if switching from password logins to OIDC. Defaults to false.
+          #
+          #allow_existing_users: true
+
           # An external module can be provided here as a custom solution to mapping
           # attributes returned from a OIDC provider onto a matrix user.
           #
diff --git a/synapse/handlers/oidc_handler.py b/synapse/handlers/oidc_handler.py
--- a/synapse/handlers/oidc_handler.py
+++ b/synapse/handlers/oidc_handler.py
@@ -114,6 +114,7 @@ def __init__(self, hs: "HomeServer"):
             hs.config.oidc_user_mapping_provider_config
         )  # type: OidcMappingProvider
         self._skip_verification = hs.config.oidc_skip_verification  # type: bool
+        self._allow_existing_users = hs.config.oidc_allow_existing_users  # type: bool
 
         self._http_client = hs.get_proxied_http_client()
         self._auth_handler = hs.get_auth_handler()
@@ -849,7 +850,8 @@ async def _map_userinfo_to_user(
         If we don't find the user that way, we should register the user,
         mapping the localpart and the display name from the UserInfo.
 
-        If a user already exists with the mxid we've mapped, raise an exception.
+        If a user already exists with the mxid we've mapped and allow_existing_users
+        is disabled, raise an exception.
 
         Args:
             userinfo: an object representing the user
@@ -905,21 +907,31 @@ async def _map_userinfo_to_user(
 
         localpart = map_username_to_mxid_localpart(attributes["localpart"])
 
-        user_id = UserID(localpart, self._hostname)
-        if await self._datastore.get_users_by_id_case_insensitive(user_id.to_string()):
-            # This mxid is taken
-            raise MappingException(
-                "mxid '{}' is already taken".format(user_id.to_string())
+        user_id = UserID(localpart, self._hostname).to_string()
+        users = await self._datastore.get_users_by_id_case_insensitive(user_id)
+        if users:
+            if self._allow_existing_users:
+                if len(users) == 1:
+                    registered_user_id = next(iter(users))
+                elif user_id in users:
+                    registered_user_id = user_id
+                else:
+                    raise MappingException(
+                        "Attempted to login as '{}' but it matches more than one user inexactly: {}".format(
+                            user_id, list(users.keys())
+                        )
+                    )
+            else:
+                # This mxid is taken
+                raise MappingException("mxid '{}' is already taken".format(user_id))
+        else:
+            # It's the first time this user is logging in and the mapped mxid was
+            # not taken, register the user
+            registered_user_id = await self._registration_handler.register_user(
+                localpart=localpart,
+                default_display_name=attributes["display_name"],
+                user_agent_ips=(user_agent, ip_address),
             )
-
-        # It's the first time this user is logging in and the mapped mxid was
-        # not taken, register the user
-        registered_user_id = await self._registration_handler.register_user(
-            localpart=localpart,
-            default_display_name=attributes["display_name"],
-            user_agent_ips=(user_agent, ip_address),
-        )
-
         await self._datastore.record_user_external_id(
             self._auth_provider_id, remote_user_id, registered_user_id,
         )
diff --git a/synapse/storage/databases/main/registration.py b/synapse/storage/databases/main/registration.py
--- a/synapse/storage/databases/main/registration.py
+++ b/synapse/storage/databases/main/registration.py
@@ -379,7 +379,7 @@ def f(txn):
 
     async def get_user_by_external_id(
         self, auth_provider: str, external_id: str
-    ) -> str:
+    ) -> Optional[str]:
         """Look up a user by their external auth id
 
         Args:
@@ -387,7 +387,7 @@ async def get_user_by_external_id(
             external_id: id on that system
 
         Returns:
-            str|None: the mxid of the user, or None if they are not known
+            the mxid of the user, or None if they are not known
         """
         return await self.db_pool.simple_select_one_onecol(
             table="user_external_ids",


Test patch:
diff --git a/tests/handlers/test_oidc.py b/tests/handlers/test_oidc.py
--- a/tests/handlers/test_oidc.py
+++ b/tests/handlers/test_oidc.py
@@ -617,3 +617,38 @@ def test_map_userinfo_to_user(self):
             )
         )
         self.assertEqual(mxid, "@test_user_2:test")
+
+        # Test if the mxid is already taken
+        store = self.hs.get_datastore()
+        user3 = UserID.from_string("@test_user_3:test")
+        self.get_success(
+            store.register_user(user_id=user3.to_string(), password_hash=None)
+        )
+        userinfo = {"sub": "test3", "username": "test_user_3"}
+        e = self.get_failure(
+            self.handler._map_userinfo_to_user(
+                userinfo, token, "user-agent", "10.10.10.10"
+            ),
+            MappingException,
+        )
+        self.assertEqual(str(e.value), "mxid '@test_user_3:test' is already taken")
+
+    @override_config({"oidc_config": {"allow_existing_users": True}})
+    def test_map_userinfo_to_existing_user(self):
+        """Existing users can log in with OpenID Connect when allow_existing_users is True."""
+        store = self.hs.get_datastore()
+        user4 = UserID.from_string("@test_user_4:test")
+        self.get_success(
+            store.register_user(user_id=user4.to_string(), password_hash=None)
+        )
+        userinfo = {
+            "sub": "test4",
+            "username": "test_user_4",
+        }
+        token = {}
+        mxid = self.get_success(
+            self.handler._map_userinfo_to_user(
+                userinfo, token, "user-agent", "10.10.10.10"
+            )
+        )
+        self.assertEqual(mxid, "@test_user_4:test")