Zebra: Cached Mempool Verification Bypasses Consensus Rules for Ahead-of-Tip Blocks

CVE-2026-40880: Cached Mempool Verification Bypasses Consensus Rules for Ahead-of-Tip Blocks

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By carefully submitting a transaction that is valid for height H+1 but invalid for H+2 and then mining that transaction in a block at height H+2, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network.

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 prior to version 4.3.1. (Some older versions are not affected but are no longer supported by the network)

Description

The vulnerability exists due to a performance optimization whose goal is to improve performance of transaction validation if the transaction was previously accepted into the mempool (and thus was verified as valid). That however did not take into account that a transaction can be valid for a specific height, but invalid at higher heights; for example, it can contain an expiry height, a lock time, and it is always bound to a network upgrade, all of which are height-dependant.

An attacker (specifically a malicious miner) could exploit this by (e.g. in the expiry height case):

• Submitting a transaction with expiry height H+1 (where H is the height of the current tip)
• Mining a block H+1, and a block H+2 that contains that same transaction, and submitting block H+2 before H+1
• Zebra nodes would accept H+2 as valid (pending contextual verification) and wait for block H+1; when it arrives, Zebra would commit both blocks even if H+2 contains an expired transaction
• Other nodes (like zcashd or zebrad 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 4.3.1.

We removed the performance optimization altogether, since we deemed it too risky.

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.

Credits

Thanks to @sangsoo-osec for a thorough advisory submission that noticed the lock time issue, and to @shieldedonly with an also thorough advisory (that was submitted while we were working on the first one) who noticed that the issue applied to other aspects of the transaction validation.