Zebra 6.2.1

Changed

• On Testnet, the getblocktemplate RPC no longer switches to a
  minimum-difficulty block template early. Zebra previously treated a template
  as minimum-difficulty as soon as its cur_time came within a fixed 150
  seconds (2 * PoWTargetSpacing after Blossom) of the consensus
  minimum-difficulty threshold, clamping cur_time up to just past the
  threshold. On Testnet this future-dated the block's timestamp and produced
  spurious minimum-difficulty blocks that depress difficulty far below its
  equilibrium. Templates now switch to minimum difficulty only once cur_time
  reaches the consensus threshold itself. This is a Testnet-only,
  template-construction (non-consensus) change: it does not alter block
  validity, and it does not change the difficulty-averaging rule that amplifies
  each minimum-difficulty block into a large difficulty drop (tracked in
  zcash/zips#1321)
  (#10873)

Security

• Allow chain synchronization to immediately retry an honest block body after rejecting a body
  with the same header hash, without waiting for a child block to trigger cleanup.
• Mitigate a peer-driven CPU-exhaustion vector on nodes with NU6.3 (Ironwood) active: reject
  underpaying and structurally invalid shielded mempool transactions before their expensive proof
  verification, and disconnect peers that send transactions with invalid shielded proofs.