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GHSA-jv4h-j224-23cc
No affected components available
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_SIGOPSconstant 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.zcashdGetLegacySigOpCount: https://github.com/zcash/zcash/blob/v6.11.0/src/main.cpp#L826-L836zcashdGetP2SHSigOpCount: https://github.com/zcash/zcash/blob/v6.11.0/src/main.cpp#L840-L852zcashdConnectBlockaggregates per-tx sigops and compares againstMAX_BLOCK_SIGOPS.
The vulnerability can be exploited over the network without needing physical access. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights. No user interaction is needed for the attacker to exploit this vulnerability. The impact is confined to the system where the vulnerability exists. There is a high impact on the integrity of the data.
Exploitation attempts have been detected. Elevated vigilance and prompt remediation are advised.
The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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