Zebra Consensus Divergence in Transparent Sighash Hash-Type Handling (CVE-2026-44497)

Zebra, a Zcash node implementation, versions prior to 4.4.0 are vulnerable to a critical consensus divergence issue. This flaw, identified as CVE-2026-44497, stems from inadequate error handling when processing invalid sighash types during signature hash computation. Specifically, when an undefined hash type is encountered, Zebra’s foreign function interface (FFI) does not properly propagate the error from the Rust-based sighash computation callback to the C++ verification code. Consequently, the C++ checker may use a stale digest from a previous valid signature validation, leading to the acceptance of invalid transactions. This discrepancy can create a consensus split between Zebra and zcashd nodes, potentially disrupting the Zcash network. The vulnerability was introduced as a side effect of a previous fix (GHSA-8m29-fpq5-89jj).

Attack Chain

1. Attacker crafts a malicious transaction.
2. Transaction contains a transparent output.
3. The output is spent by a script that includes OP_CHECKSIGVERIFY and OP_CHECKSIG.
4. OP_CHECKSIGVERIFY is executed with a valid hash type, priming the C++ sighash buffer with a valid digest.
5. OP_CHECKSIG is executed with an undefined hash type.
6. Zebra’s Rust callback returns None due to the undefined hash type, but the C++ checker does not receive this signal.
7. The C++ checker verifies the invalid signature against the stale digest in the buffer.
8. Zebra incorrectly accepts the spend, while zcashd rejects it, leading to a consensus split.

Impact

The vulnerability can lead to a consensus failure within the Zcash network. An attacker can exploit this to cause network partitioning, where different nodes have conflicting views of the blockchain’s state. This can lead to service disruption for users relying on affected Zebra nodes. Furthermore, the vulnerability could potentially be exploited for double-spend attacks if a malicious miner relies on Zebra’s faulty validation results. While the impact is mitigated by the prevalence of zcashd among miners, any miner or template pipeline relying on Zebra’s validation is at risk.

Recommendation

• Upgrade all Zebra nodes to version 4.4.0 or later immediately to address CVE-2026-44497.
• Monitor Zebra node logs for unexpected consensus errors or forks following the upgrade.
• Evaluate the feasibility of implementing custom monitoring to detect divergence between Zebra and zcashd validation results within your environment.