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 containing a summary of room 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.
Software afectado
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| Plataforma | Paquete | Versiones vulnerables | Corregida en |
|---|---|---|---|
| Python (PyPI) | matrix-synapse | < 1.74.0 | 1.74.0 |
Referencias
- https://github.com/matrix-org/synapse/security/advisories/GHSA-f3wc-3vxv-xmvr
- https://github.com/matrix-org/synapse/pull/14642
- https://github.com/matrix-org/synapse/issues/14492
- https://nvd.nist.gov/vuln/detail/CVE-2023-32323
- https://github.com/matrix-org/synapse
- https://github.com/pypa/advisory-database/tree/main/vulns/matrix-synapse/PYSEC-2023-67.yaml
- https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/UJIJRP5ZH6B3KGFLHCAKR2IX2Y4Z25QD
- Vector CVSS
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:N/I:N/A:L- Debilidad (CWE)
CWE-20- Plataforma
- Python (PyPI)
- Paquete
matrix-synapse- Publicada
- 24.05.2023
- Actualizado
- 11.10.2026 12:19
Fuentes y licencias
OSV / GitHub Advisory Database - PYSEC-2023-67 - PyPA Advisory Database; CC BY 4.0
OSV / GitHub Advisory Database - GHSA-f3wc-3vxv-xmvr - CC BY 4.0
Historial de vulnerabilidades: matrix-synapse
Todas las vulnerabilidades publicadas de este paquete (Python (PyPI)), de la más reciente a la más antigua. Muestra si es un fallo aislado o un paquete que hay que vigilar.
- Entradas en total
- 45
- Primera → última
- 03.2019 05.2026
- Sin corrección
- 0
- Explotada en ataques
- 0
Distribución por gravedad
- Alta 21
- Media 21
- Baja 3
Divulgaciones por año
- 2019: 3
- 2020: 3
- 2021: 12
- 2022: 7
- 2023: 9
- 2024: 7
- 2025: 2
- 2026: 2
-
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