Matrix developers report:
The [synapse 1.7.1] release includes several security fixes as well
as a fix to a bug exposed by the security fixes. All previous releases
of Synapse are affected. Administrators are encouraged to upgrade as
soon as possible.
Fix a bug which could cause room events to be incorrectly authorized
using events from a different room.
Fix a bug causing responses to the /context client endpoint to not
use the pruned version of the event.
Fix a cause of state resets in room versions 2 onwards.