Synapse Matrix has a partial room state leak via Sliding Sync

Impact

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.

Patches

Synapse version 1.120.1 fixes the problem.

Workarounds

Disable Sliding Sync.

References

https://github.com/matrix-org/matrix-spec-proposals/pull/4186
https://github.com/element-hq/synapse/blob/d80cd57c54427687afcb48740d99219c88a0fff1/synapse/config/experimental.py#L341-L344

For more information

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