Impact

Local authenticated users can cause Synapse to starve other requests of CPU and lead to other requests failing, causing other users to be denied service.

Homeservers that trust all their local users are not at risk.

Patches

Update to Synapse 1.152.1 or later.

Workarounds

If Synapse is deployed behind a reverse proxy, the reverse proxy could be configured to limit the rate of user requests, preventing or increasing the difficulty of the attack.

Identifiers

- ELEMENTSEC-2026-1706

For more information

If you have any questions or comments about this advisory, please email us at security at element.io.

Impact

In federated rooms, malicious homeservers can craft room events in such a way that prevents Synapse from providing full history to paginating clients.

Clients could therefore fail to display room history.

Patches

Update to Synapse 1.152.1 or later.

Workarounds

There are no known workarounds for this issue.

Identifiers

- ELEMENTSEC-2025-1636

For more information

If you have any questions or comments about this advisory, please email us at security at element.io.