Package Instance

Synapse's invalid device keys degrade federation functionality
Lack of validation for device keys in Synapse before 1.138.3 and in Synapse 1.139.0 allow an attacker registered on the victim homeserver to degrade federation functionality, unpredictably breaking outbound federation to other homeservers.

Synapse allows unsupported content types to lead to memory exhaustion
In Synapse before 1.120.1, `multipart/form-data` requests can in certain configurations transiently increase memory consumption beyond expected levels while processing the request, which can be used to amplify denial of service attacks.

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](mailto:security@element.io).

Synapse can be forced to thumbnail unexpected file formats, invoking external, potentially untrustworthy decoders
In Synapse versions before 1.120.1, enabling the `dynamic_thumbnails` option or processing a specially crafted request could trigger the decoding and thumbnail generation of uncommon image formats, potentially invoking external tools like Ghostscript for processing.

This significantly expands the attack surface in a historically vulnerable area, presenting a risk that far outweighs the benefit, particularly since these formats are rarely used on the open web or within the Matrix ecosystem.

For a list of image formats, as well as decoding libraries and helper programs used, see [the Pillow documentation](https://pillow.readthedocs.io/en/stable/handbook/image-file-formats.html).

Synapse Matrix has a partial room state leak via Sliding Sync
The Sliding Sync feature on Synapse versions between 1.113.0rc1 and 1.120.0 can leak partial room state changes to users no longer in a room. Non-state events, like messages, are unaffected.

Synapse vulnerable to federation denial of service via malformed events
A malicious server can craft events with a `depth` outside the integer range allowed by Canonical JSON. When such an event is received by Synapse version up to 1.127.0, it prevents it from federating with other servers. The vulnerability has been exploited in the wild.

Synapse allows a a malformed invite to break the invitee's `/sync`
Synapse versions before 1.120.1 fail to properly validate invites received over federation. This vulnerability allows a malicious server to send a specially crafted invite that disrupts the invited user's `/sync` functionality.