This release includes four security fixes:

• Prevent an attack where a federated server could send redactions for arbitrary events in v1 and v2 rooms.
• Prevent a denial-of-service attack where cycles of redaction events would make Synapse spin infinitely.
• Prevent an attack where users could be joined or parted from public rooms without their consent.
• Fix a vulnerability where a federated server could spoof read-receipts from users on other servers.

See https://github.com/matrix-org/synapse/releases/tag/v1.2.1 for complete details.