Synapse Outgoing federation to specific hosts can be disabled by sending malicious invites in pypi/matrix-synapse


CVE-2023-32323, GHSA-f3wc-3vxv-xmvr

Package Slug



Synapse Outgoing federation to specific hosts can be disabled by sending malicious invites



A malicious user on a Synapse homeserver X with permission to create certain state events can disable outbound federation from X to an arbitrary homeserver Y.

Synapse instances with federation disabled are not affected.


The Matrix protocol allows homeservers to provide an invite_room_state field on a room invite containing a summary of room state. In versions of Synapse up to and including v1.73.0, Synapse does not limit the size of invite_room_state, meaning that it was possible to create an arbitrarily large invite event.

An attacker with an account on a vulnerable Synapse homeserver X could exploit this by having X create an over-sized invite event in a room with a user from another homeserver Y. Once acknowledged by the invitee's homeserver, the invite event would be sent in a batch of events to Y. If the malicious invite is so large that the entire batch is rejected as too large, X's outgoing traffic to Y would become "stuck", meaning that messages and state events created by X would remain unseen by Y.


Synapse 1.74 refuses to create oversized invite_room_state fields. Server operators should upgrade to Synapse 1.74 or newer urgently.


There are no robust workarounds.

This attack needs an account on Synapse homeserver X to deny federation from X to another homeserver Y. As a partial mitigation, Synapse operators can disable open registration to limit the ability of attackers to create new accounts on homeserver X.

If homeserver X has been attacked in this way, restarting it will resume outgoing federation by entering "catchup mode". For catchup mode to ignore the oversized invites, every attacked room must have a correctly-sized event sent by X which is newer than any oversized invite. This is difficult to arrange, and does not prevent the attacker from repeating their attack.


  • was caused by this issue.
  • includes the patch described above.

For more information

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Affected Versions

All versions before 1.74.0


Upgrade to version 1.74.0 or above.

Last Modified