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.
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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 findverifiedunminedtx 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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