Synapse pagination Denial of Service

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.