CVE-2026-34377

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CVE-2026-34377: Consensus Failure via Crafted V5 Authorization Data

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By matching a valid transaction's txid while providing invalid authorization data, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network.  To be clear, this would not allow invalid transactions to be accepted but could result in a consensus split between vulnerable Zebra nodes and invulnerable Zebra and Zcashd nodes.

Severity

High - This is a Consensus Vulnerability that could allow a malicious miner to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Affected Versions

All Zebra versions supporting V5 transactions (Network Upgrade 5 and later) prior to version 4.3.0.

Description

The vulnerability exists in the findverifiedunmined_tx function within transaction.rs. This function was designed to optimize block verification by checking if a transaction was already verified in the mempool.

The lookup mechanism used the ZIP-244 txid as the unique key. However, for V5 transactions, the txid specifically excludes the Authorization Data Root (signatures and proofs). Because Zebra returned a "verified" status based solely on the txid, it skipped the essential checkv5auth() call for the transaction version provided in the block.

An attacker (specifically a malicious miner) could exploit this by:

1. Observing a valid V5 transaction broadcast to the network and entering a Zebra node's mempool.
2. Creating a block containing a modified version of that transaction. The modified version has the same txid but contains invalid signatures or proofs.
3. The affected Zebra node identifies the txid in its mempool and incorrectly assumes the block's version of the transaction is already verified.
4. The node commits the block with the invalid transaction data.
5. Other nodes (like zcashd or zebra nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

• Attack Vector: Network (specifically via a malicious miner).
• Effect: Network partition/consensus split.
• Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.0. 

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 or later immediately. 

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Resources

• Zebra 4.3.0 Release Announcement
• ZIP-244: Transaction Identifier

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