CVE-2023-32323 – matrix-synapse

Package

Manager: pip
Name: matrix-synapse
Vulnerable Version: >=0 <1.74.0

Severity

Level: Medium

CVSS v3.1: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:N/I:N/A:L

CVSS v4.0: CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:N/SC:N/SI:N/SA:L

EPSS: 0.00127 pctl0.32783

Details

Synapse Outgoing federation to specific hosts can be disabled by sending malicious invites ### Impact 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. #### Details The Matrix protocol allows homeservers to provide an `invite_room_state` field on a [room invite](https://spec.matrix.org/v1.5/client-server-api/#mroommember) containing a [summary of room state](https://spec.matrix.org/v1.5/client-server-api/#stripped-state). In versions of Synapse up to and including v1.73.0, Synapse did 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. ### Patches Synapse 1.74 refuses to create oversized `invite_room_state` fields. Server operators should upgrade to Synapse 1.74 or newer urgently. ### Workarounds 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. ### References - https://github.com/matrix-org/synapse/issues/14492 was caused by this issue. - https://github.com/matrix-org/synapse/issues/14642 includes the patch described above. ### For more information If you have any questions or comments about this advisory, e-mail us at [security@matrix.org](mailto:security@matrix.org).

Metadata

Created: 2023-05-24T17:28:26Z
Modified: 2025-02-13T18:54:39Z
Source: https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2023/05/GHSA-f3wc-3vxv-xmvr/GHSA-f3wc-3vxv-xmvr.json
CWE IDs: ["CWE-20"]
Alternative ID: GHSA-f3wc-3vxv-xmvr
Finding: F184
Auto approve: 1