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

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ZEBRA is a Zcash node written entirely in Rust. Prior to zebrad version 4.3.1 and zebra-consensus version 5.0.2, 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. This vulnerability is fixed in zebrad version 4.3.1 and zebra-consensus version 5.0.2.

Source published

21 Apr 2026 19:18:22 UTC

Source modified

21 Apr 2026 19:52:58 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

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CVSS 4.0 · 7.2 CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:N/VI:H/VA:H/SC:N/SI:H/SA:H

ZEBRA is a Zcash node written entirely in Rust. Prior to zebrad version 4.3.1 and zebra-consensus version 5.0.2, 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. This vulnerability is fixed in zebrad version 4.3.1 and zebra-consensus version 5.0.2.

Source published

Apr 21, 2026, 7:18:22 PM (timezone not stated)

Source modified

Apr 21, 2026, 7:52:58 PM (timezone not stated)

Observed here

28 Oct 2026 05:02:39 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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EUVD CVSS 4.0 · 7.2 CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:N/VI:H/VA:H/SC:N/SI:H/SA:H

ZEBRA is a Zcash node written entirely in Rust. Prior to zebrad version 4.3.1 and zebra-consensus version 5.0.2, 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. This vulnerability is fixed in zebrad version 4.3.1 and zebra-consensus version 5.0.2.

Source published

21 Apr 2026 20:17:01 UTC

Source modified

26 Oct 2026 21:10:00 UTC

Observed here

27 Oct 2026 10:10:07 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 4.0 · [email protected] · Secondary · 7.2 CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:N/VI:H/VA:H/SC:N/SI:H/SA:H/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

CVSS 3.1 · [email protected] · Primary · 8.1 CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:H

# 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.

Source published

18 Apr 2026 00:41:54 UTC

Source modified

5 May 2026 16:12:16 UTC

Observed here

24 Oct 2026 13:19:54 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V4 · CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:N/VI:H/VA:H/SC:N/SI:H/SA:H

Publisher rating · HIGH