Allow existing users to login via OpenID Connect. (#8345)

Co-authored-by: Benjamin Koch <bbbsnowball@gmail.com>

This adds configuration flags that will match a user to pre-existing users
when logging in via OpenID Connect. This is useful when switching to
an existing SSO system.

3 files changed, 35 insertions, 17 deletions

diff --git a/synapse/config/oidc_config.py b/synapse/config/oidc_config.py
index e0939bce84..70fc8a2f62 100644
--- a/synapse/config/oidc_config.py
+++ b/synapse/config/oidc_config.py
@@ -56,6 +56,7 @@ class OIDCConfig(Config):
         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 @@ class OIDCConfig(Config):
           #
           #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
index 4230dbaf99..0e06e4408d 100644
--- a/synapse/handlers/oidc_handler.py
+++ b/synapse/handlers/oidc_handler.py
@@ -114,6 +114,7 @@ class OidcHandler:
             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 @@ class OidcHandler:
         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 @@ class OidcHandler:
 
         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
index 33825e8949..48ce7ecd16 100644
--- a/synapse/storage/databases/main/registration.py
+++ b/synapse/storage/databases/main/registration.py
@@ -393,7 +393,7 @@ class RegistrationWorkerStore(SQLBaseStore):
 
     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:
@@ -401,7 +401,7 @@ class RegistrationWorkerStore(SQLBaseStore):
             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",