PYSEC-2026-1611 Synapse allows a a malformed invite to break the invitee's `/sync`

Impact 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. Patches Synapse 1.120.1 rejects such invalid invites received ov...