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Spam checker callbacks

Spam checker callbacks allow module developers to implement spam mitigation actions for Synapse instances. Spam checker callbacks can be registered using the module API's register_spam_checker_callbacks method.

Callbacks

The available spam checker callbacks are:

check_event_for_spam

First introduced in Synapse v1.37.0 Signature extended to support Allow and Code in Synapse v1.60.0 Boolean and string return value types deprecated in Synapse v1.60.0

async def check_event_for_spam(event: "synapse.module_api.EventBase") -> Union["synapse.module_api.ALLOW", "synapse.module_api.error.Codes", str, bool]

Called when receiving an event from a client or via federation. The callback must return either:

  • synapse.module_api.ALLOW, to allow the operation. Other callbacks may still decide to reject it.
  • synapse.api.Codes to reject the operation with an error code. In case of doubt, synapse.api.error.Codes.FORBIDDEN is a good error code.
  • (deprecated) a str to reject the operation and specify an error message. Note that clients typically will not localize the error message to the user's preferred locale.
  • (deprecated) on False, behave as ALLOW. Deprecated as confusing, as some callbacks in expect True to allow and others True to reject.
  • (deprecated) on True, behave as synapse.api.error.Codes.FORBIDDEN. Deprecated as confusing, as some callbacks in expect True to allow and others True to reject.

If multiple modules implement this callback, they will be considered in order. If a callback returns synapse.module_api.ALLOW, Synapse falls through to the next one. The value of the first callback that does not return synapse.module_api.ALLOW will be used. If this happens, Synapse will not call any of the subsequent implementations of this callback.

user_may_join_room

First introduced in Synapse v1.37.0

async def user_may_join_room(user: str, room: str, is_invited: bool) -> bool

Called when a user is trying to join a room. The module must return a bool to indicate whether the user can join the room. Return False to prevent the user from joining the room; otherwise return True to permit the joining.

The user is represented by their Matrix user ID (e.g. @alice:example.com) and the room is represented by its Matrix ID (e.g. !room:example.com). The module is also given a boolean to indicate whether the user currently has a pending invite in the room.

This callback isn't called if the join is performed by a server administrator, or in the context of a room creation.

If multiple modules implement this callback, they will be considered in order. If a callback returns True, Synapse falls through to the next one. The value of the first callback that does not return True will be used. If this happens, Synapse will not call any of the subsequent implementations of this callback.

user_may_invite

First introduced in Synapse v1.37.0

async def user_may_invite(inviter: str, invitee: str, room_id: str) -> bool

Called when processing an invitation. The module must return a bool indicating whether the inviter can invite the invitee to the given room. Both inviter and invitee are represented by their Matrix user ID (e.g. @alice:example.com). Return False to prevent the invitation; otherwise return True to permit it.

If multiple modules implement this callback, they will be considered in order. If a callback returns True, Synapse falls through to the next one. The value of the first callback that does not return True will be used. If this happens, Synapse will not call any of the subsequent implementations of this callback.

user_may_send_3pid_invite

First introduced in Synapse v1.45.0

async def user_may_send_3pid_invite(
    inviter: str,
    medium: str,
    address: str,
    room_id: str,
) -> bool

Called when processing an invitation using a third-party identifier (also called a 3PID, e.g. an email address or a phone number). The module must return a bool indicating whether the inviter can invite the invitee to the given room. Return False to prevent the invitation; otherwise return True to permit it.

The inviter is represented by their Matrix user ID (e.g. @alice:example.com), and the invitee is represented by its medium (e.g. "email") and its address (e.g. [email protected]). See the Matrix specification for more information regarding third-party identifiers.

For example, a call to this callback to send an invitation to the email address [email protected] would look like this:

await user_may_send_3pid_invite(
    "@bob:example.com",  # The inviter's user ID
    "email",  # The medium of the 3PID to invite
    "[email protected]",  # The address of the 3PID to invite
    "!some_room:example.com",  # The ID of the room to send the invite into
)

Note: If the third-party identifier is already associated with a matrix user ID, user_may_invite will be used instead.

If multiple modules implement this callback, they will be considered in order. If a callback returns True, Synapse falls through to the next one. The value of the first callback that does not return True will be used. If this happens, Synapse will not call any of the subsequent implementations of this callback.

user_may_create_room

First introduced in Synapse v1.37.0

async def user_may_create_room(user: str) -> bool

Called when processing a room creation request. The module must return a bool indicating whether the given user (represented by their Matrix user ID) is allowed to create a room. Return False to prevent room creation; otherwise return True to permit it.

If multiple modules implement this callback, they will be considered in order. If a callback returns True, Synapse falls through to the next one. The value of the first callback that does not return True will be used. If this happens, Synapse will not call any of the subsequent implementations of this callback.

user_may_create_room_alias

First introduced in Synapse v1.37.0

async def user_may_create_room_alias(user: str, room_alias: "synapse.types.RoomAlias") -> bool

Called when trying to associate an alias with an existing room. The module must return a bool indicating whether the given user (represented by their Matrix user ID) is allowed to set the given alias. Return False to prevent the alias creation; otherwise return True to permit it.

If multiple modules implement this callback, they will be considered in order. If a callback returns True, Synapse falls through to the next one. The value of the first callback that does not return True will be used. If this happens, Synapse will not call any of the subsequent implementations of this callback.

user_may_publish_room

First introduced in Synapse v1.37.0

async def user_may_publish_room(user: str, room_id: str) -> bool

Called when trying to publish a room to the homeserver's public rooms directory. The module must return a bool indicating whether the given user (represented by their Matrix user ID) is allowed to publish the given room. Return False to prevent the room from being published; otherwise return True to permit its publication.

If multiple modules implement this callback, they will be considered in order. If a callback returns True, Synapse falls through to the next one. The value of the first callback that does not return True will be used. If this happens, Synapse will not call any of the subsequent implementations of this callback.

check_username_for_spam

First introduced in Synapse v1.37.0

async def check_username_for_spam(user_profile: synapse.module_api.UserProfile) -> bool

Called when computing search results in the user directory. The module must return a bool indicating whether the given user should be excluded from user directory searches. Return True to indicate that the user is spammy and exclude them from search results; otherwise return False.

The profile is represented as a dictionary with the following keys:

  • user_id: str. The Matrix ID for this user.
  • display_name: Optional[str]. The user's display name, or None if this user has not set a display name.
  • avatar_url: Optional[str]. The mxc:// URL to the user's avatar, or None if this user has not set an avatar.

The module is given a copy of the original dictionary, so modifying it from within the module cannot modify a user's profile when included in user directory search results.

If multiple modules implement this callback, they will be considered in order. If a callback returns False, Synapse falls through to the next one. The value of the first callback that does not return False will be used. If this happens, Synapse will not call any of the subsequent implementations of this callback.

check_registration_for_spam

First introduced in Synapse v1.37.0

async def check_registration_for_spam(
    email_threepid: Optional[dict],
    username: Optional[str],
    request_info: Collection[Tuple[str, str]],
    auth_provider_id: Optional[str] = None,
) -> "synapse.spam_checker_api.RegistrationBehaviour"

Called when registering a new user. The module must return a RegistrationBehaviour indicating whether the registration can go through or must be denied, or whether the user may be allowed to register but will be shadow banned.

The arguments passed to this callback are:

  • email_threepid: The email address used for registering, if any.
  • username: The username the user would like to register. Can be None, meaning that Synapse will generate one later.
  • request_info: A collection of tuples, which first item is a user agent, and which second item is an IP address. These user agents and IP addresses are the ones that were used during the registration process.
  • auth_provider_id: The identifier of the SSO authentication provider, if any.

If multiple modules implement this callback, they will be considered in order. If a callback returns RegistrationBehaviour.ALLOW, Synapse falls through to the next one. The value of the first callback that does not return RegistrationBehaviour.ALLOW will be used. If this happens, Synapse will not call any of the subsequent implementations of this callback.

check_media_file_for_spam

First introduced in Synapse v1.37.0

async def check_media_file_for_spam(
    file_wrapper: "synapse.rest.media.v1.media_storage.ReadableFileWrapper",
    file_info: "synapse.rest.media.v1._base.FileInfo",
) -> bool

Called when storing a local or remote file. The module must return a bool indicating whether the given file should be excluded from the homeserver's media store. Return True to prevent this file from being stored; otherwise return False.

If multiple modules implement this callback, they will be considered in order. If a callback returns False, Synapse falls through to the next one. The value of the first callback that does not return False will be used. If this happens, Synapse will not call any of the subsequent implementations of this callback.

should_drop_federated_event

First introduced in Synapse v1.60.0

async def should_drop_federated_event(event: "synapse.events.EventBase") -> bool

Called when checking whether a remote server can federate an event with us. Returning True from this function will silently drop a federated event and split-brain our view of a room's DAG, and thus you shouldn't use this callback unless you know what you are doing.

If multiple modules implement this callback, they will be considered in order. If a callback returns False, Synapse falls through to the next one. The value of the first callback that does not return False will be used. If this happens, Synapse will not call any of the subsequent implementations of this callback.

Example

The example below is a module that implements the spam checker callback check_event_for_spam to deny any message sent by users whose Matrix user IDs are mentioned in a configured list, and registers a web resource to the path /_synapse/client/list_spam_checker/is_evil that returns a JSON object indicating whether the provided user appears in that list.

import json
from typing import Union

from twisted.web.resource import Resource
from twisted.web.server import Request

from synapse.module_api import ModuleApi


class IsUserEvilResource(Resource):
    def __init__(self, config):
        super(IsUserEvilResource, self).__init__()
        self.evil_users = config.get("evil_users") or []

    def render_GET(self, request: Request):
        user = request.args.get(b"user")[0].decode()
        request.setHeader(b"Content-Type", b"application/json")
        return json.dumps({"evil": user in self.evil_users}).encode()


class ListSpamChecker:
    def __init__(self, config: dict, api: ModuleApi):
        self.api = api
        self.evil_users = config.get("evil_users") or []

        self.api.register_spam_checker_callbacks(
            check_event_for_spam=self.check_event_for_spam,
        )

        self.api.register_web_resource(
            path="/_synapse/client/list_spam_checker/is_evil",
            resource=IsUserEvilResource(config),
        )

    async def check_event_for_spam(self, event: "synapse.events.EventBase") -> Union[bool, str]:
        return event.sender not in self.evil_users