Zebra's Block Validator Undercounts Coinbase and P2SH Sigops

Zebra's block validator undercounts transparent signature operations against the 20000-sigop block limit (MAX_BLOCK_SIGOPS), allowing it to accept blocks that zcashd rejects with bad-blk-sigops. A miner who produces such a block can split the network: Zebra nodes follow the offending chain while zcashd nodes do not.

Two distinct undercounts:

A: Coinbase Hidden Legacy Sigops

zcashd's GetLegacySigOpCount() includes the coinbase input's scriptSig. Zebra's Sigops impl skipped the coinbase input entirely, so up to ~98 sigops (the 100-byte coinbase script length cap, less the height prefix) could be hidden inside the coinbase scriptSig without being charged against the block limit.

B: Aggregate P2SH Sigops.

zcashd's GetP2SHSigOpCount() parses each P2SH input's redeem script with accurate=true and sums those sigops into the block-wide total via ConnectBlock. The check is per-block, not per-transaction, and the limit applies regardless of who mines the offending block — a miner just needs to include enough P2SH-spending transactions whose redeem scripts together exceed 20000 sigops. Zebra computed P2SH sigops only on the mempool-acceptance path (used for ZIP-317 weighting) and never accumulated them during block validation. A block whose aggregate redeem-script sigops exceed 20000 (e.g. 1334 P2SH spends × 15 sigops = 20010) would be accepted by Zebra and rejected by zcashd.

Patches

Fixed in this release: https://github.com/ZcashFoundation/zebra/releases/tag/v4.4.0.

Workarounds

None. Operators relying on Zebra for consensus should upgrade.

Resources

• MAX_BLOCK_SIGOPS constant inherited from Bitcoin via the Zcash protocol spec's §7.6 catch-all "Other rules inherited from Bitcoin", tracked for explicit documentation in zcash/zips#568.
• zcashd GetLegacySigOpCount: https://github.com/zcash/zcash/blob/v6.11.0/src/main.cpp#L826-L836
• zcashd GetP2SHSigOpCount: https://github.com/zcash/zcash/blob/v6.11.0/src/main.cpp#L840-L852
• zcashd ConnectBlock aggregates per-tx sigops and compares against MAX_BLOCK_SIGOPS.