CVE-2026-40880 — zebra-consensus
CVE-2026-40880 is a CWE-1025 vulnerability in zebra-consensus. A fix is available for zebra-consensus — see the affected versions and patch details below.
Zebra: Cached Mempool Verification Bypasses Consensus Rules for Ahead-of-Tip Blocks
Exploitation Status
No confirmed exploitation observed yet
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-40880.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
Real-World Exposure
zebra-consensus🦀zebradReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects crates.io packages — download data is not available via public APIs for these ecosystems.
Description
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(whereHis the height of the current tip) - Mining a block
H+1, and a blockH+2that contains that same transaction, and submitting blockH+2beforeH+1 - Zebra nodes would accept
H+2as valid (pending contextual verification) and wait for blockH+1; when it arrives, Zebra would commit both blocks even ifH+2contains an expired transaction - Other nodes (like
zcashdorzebradnodes 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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🦀crates.io | zebra-consensus | all versions | 5.0.2cargo update -p zebra-consensus --precise 5.0.2 |
| 🦀crates.io | zebrad | all versions | 4.3.1cargo update -p zebrad --precise 4.3.1 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for zebra-consensus, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update zebra-consensus to 5.0.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-40880 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-40880 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-40880. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-40880 in your dependencies?
O3 Security finds CVE-2026-40880 across crates.io dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.