Zebra Consensus Failure due to Improper SIGHASH_SINGLE Validation

Zebra version 4.4.0 improperly validates V5 transparent transactions using the SIGHASH_SINGLE signature flag. Specifically, it fails to enforce a ZIP-244 consensus rule that requires validation to fail when an input is signed with SIGHASH_SINGLE and there is no transparent output at the same index. Instead, Zebra asks the underlying sighash library to compute a digest, resulting in a digest over an empty output set. This divergence from zcashd, which correctly rejects such transactions, could allow an attacker to create a consensus split between Zebra and zcashd nodes. The vulnerability exists due to a missed check in Zebra’s V5 sighash callback, which calls librustzcash’s ZIP-244 implementation. The issue was addressed in Zebra 4.4.1.

Attack Chain

1. Attacker crafts a V5 transaction with two or more transparent inputs.
2. The crafted transaction includes fewer transparent outputs than inputs.
3. The attacker signs an input whose index has no matching output (vout entry) with SIGHASH_SINGLE (0x03) or SIGHASH_SINGLE|ANYONECANPAY (0x83).
4. Zebra’s sighash callback incorrectly computes a digest for the invalid input using librustzcash, rather than failing the validation.
5. The attacker broadcasts the malicious transaction to the Zcash network.
6. Zebra nodes verify the transaction’s transparent script using the incorrectly computed digest and accept the transaction (and any block containing it).
7. zcashd nodes reject the transaction due to the invalid SIGHASH_SINGLE signature.
8. This divergence creates a consensus split, potentially isolating Zebra nodes from the rest of the network.

Impact

This consensus failure could lead to network partitioning, service disruption, and potential double-spend attacks against affected Zebra nodes. While the impact is currently mitigated by the dominance of zcashd among miners, a successful attack could still disrupt services relying on Zebra nodes, cause financial losses for affected users, and damage the reputation of the Zebra project.

Recommendation

• Upgrade to Zebra version 4.4.1 or later immediately to remediate the vulnerability.
• Monitor network traffic for unusual transaction patterns, especially V5 transactions with SIGHASH_SINGLE signatures.
• Deploy the Sigma rules in this brief to your SIEM to detect potential exploitation attempts based on transaction characteristics.
• Review the fix in Zebra 4.4.1 (GHSA-pvmv-cwg8-v6c8) to understand the corrected validation logic.