Jwt Attack
CVE-2026-34377
HIGH
GHSA-3vmh-33xr-9cqh
Attack ChainAIDerived
Hypothetical attack flow derived from CVE metadata – not a confirmed exploit chain
-
AccessMalicious miner crafts transaction with valid txid
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ExploitProvides invalid authorization data in V5 block
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ExecutionZebra verification cache accepts invalid block
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ImpactConsensus split occurs between vulnerable nodes and network
Vulnerability Assessment
| Weakness |
CWE
OWASP Top 10:2025
A04:2025 - Cryptographic Failures
ASVS 5.0
V11 - Cryptography Signature verification
|
| Exploitation Conditions | Attacker must be a malicious miner with high-privilege network access to the Zcash blockchain. Requires crafting a block with a valid transaction txid but invalid authorization data. Only affects vulnerable Zebra nodes; does not impact Zcashd or patched Zebra versions. |
| Impact | This vulnerability presents significant real-world risk despite the absence of public exploit code or CISA KEV listing. The CVSS assessment is not available, but the attack profile indicates Network attack vector with low complexity for actors possessing mining capabilities (high privilege requirement in cryptocurrency context). The EPSS data is absent, suggesting limited historical exploitation signals. However, the technical nature enables deterministic exploitation: any miner observing mempool transactions on Zebra nodes can craft consensus-splitting blocks with minimal effort. The SSVC framework suggests moderate priority: exploitation requires miner-level resources (not automatable by commodity attackers), but technical impact is high as it directly compromises consensus integrity. The practical risk is elevated by the cryptocurrency context where consensus failures enable double-spend attacks and network fragmentation undermines trust. While authentication is not relevant (mining operations are permissionless), the requirement for mining capability constrains the attacker pool to entities with significant hash power or mining pool access. Tags indicating Information Disclosure and JWT Attack appear misclassified for this consensus vulnerability. The 4.3.0 patch availability and lack of workarounds make this a mandatory upgrade for production Zebra nodes, though the absence of observed exploitation suggests the vulnerability window may not have been widely abused. |
| Exploit ScenarioDerived | A malicious miner monitors the mempool of target Zebra nodes and identifies a valid V5 transaction with distinctive characteristics. The miner constructs a block containing a modified version of this transaction preserving the original txid (transaction structure) but replacing the Authorization Data Root with invalid signatures or zero-knowledge proofs. When the miner successfully mines this block and broadcasts it to the network, vulnerable Zebra nodes check their mempool cache, find the matching txid from the legitimate transaction, and incorrectly skip authorization validation. These nodes accept and commit the invalid block while Zcashd nodes and non-vulnerable Zebra nodes reject it due to failing cryptographic checks. The network partitions with vulnerable nodes following a minority chain, enabling the attacker to potentially execute double-spend attacks against services relying on isolated nodes or causing service disruption through blockchain reorganization confusion. |
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Alert meAnalysisAIv1 – Mar 30, 2026
Zebra cryptocurrency nodes prior to version 4.3.0 can be forced into consensus split by malicious miners who craft blocks containing V5 transactions with matching txids but invalid authorization data. The vulnerability stems from a cache lookup that used ZIP-244 txid (which excludes authorization data) to bypass full verification, allowing nodes to accept blocks with invalid signatures. While this does not enable invalid transaction acceptance, it isolates vulnerable nodes from the Zcash network, creating fork conditions exploitable for service disruption and potential double-spend scenarios against partitioned nodes. No public exploit code or CISA KEV listing exists, but the technical complexity is low for actors with mining capabilities. Affected products are zebrad and zebra-consensus Rust packages supporting Network Upgrade 5 (V5 transactions). Vendor-released patch: Zebra 4.3.0.
RemediationAI
Upgrade immediately to Zebra version 4.3.0 or later, which implements corrected verification logic ensuring authorization data integrity is validated even when transactions match mempool txids. The fix modifies the find_verified_unmined_tx cache lookup to perform full check_v5_auth() validation instead of relying solely on txid matching for verification bypass decisions. Download the patched version from the official Zebra repository and follow standard node upgrade procedures including graceful shutdown, binary replacement, and restart with configuration validation. The Zebra 4.3.0 release announcement at https://zfnd.org/zebra-4-3-0-critical-security-fixes-zip-235-support-and-performance-improvements/ provides detailed upgrade instructions and additional security improvements included in this release. No workarounds exist for this vulnerability as the flaw is fundamental to the transaction verification cache architecture. Node operators running vulnerable versions face consensus isolation risk and should prioritize this upgrade above routine maintenance. After upgrading, verify the node successfully syncs with the canonical Zcash chain by comparing block heights and hashes with reference nodes or block explorers.
Affected Packages
Source: OSV (Open Source Vulnerabilities)
Affects your stack
Affects 2 packages in cargo
Checking this CVE against your own stacks and servers is part of the Basic plan and up.
Technical ContextAI · show details
This vulnerability exploits the ZIP-244 transaction identifier specification used in Zcash V5 transactions. ZIP-244 defines txid calculation to specifically exclude the Authorization Data Root (signatures and zero-knowledge proofs), creating a separation between transaction structure identity and cryptographic validity. Zebra's find_verified_unmined_tx function in transaction.rs implemented a performance optimization by caching verified transactions from the mempool using txid as the lookup key. When processing blocks, if a matching txid was found in cache, the node skipped the check_v5_auth() call that validates signatures and proofs. This represents CWE-347 (Improper Verification of Cryptographic Signature), where the system trusted structural identity (txid match) as proof of cryptographic validity. The affected Rust packages (pkg:rust/zebrad and pkg:rust/zebra-consensus) form the core consensus layer of Zebra nodes. The vulnerability only affects V5 transaction processing introduced with Network Upgrade 5, making pre-NU5 blocks immune. The cache optimization logic correctly assumed that mempool transactions were fully validated, but failed to account for the scenario where a miner could reference the same txid with substituted authorization data in a block proposal.
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Severity by source
CVSS:4.0/AV:N/AC:L/AT:N/PR:H/UI:N/VC:N/VI:H/VA:H/SC:N/SI:H/SA:H
Primary rating from Vendor (https://github.com/ZcashFoundation/zebra) · only source for this CVE.
CVSS VectorVendor: https://github.com/ZcashFoundation/zebra
Lifecycle Timeline
2DescriptionCVE.org
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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 find_verified_unmined_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 check_v5_auth() call for the transaction version provided in the block.
An attacker (specifically a malicious miner) could exploit this by:
- Observing a valid V5 transaction broadcast to the network and entering a Zebra node's mempool.
- Creating a block containing a modified version of that transaction. The modified version has the same
txidbut contains invalid signatures or proofs. - The affected Zebra node identifies the
txidin its mempool and incorrectly assumes the block's version of the transaction is already verified. - The node commits the block with the invalid transaction data.
- Other nodes (like
zcashdorzebranodes 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
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Executive ViewAI
Zebra cryptocurrency nodes prior to version 4.3.0 are vulnerable to consensus-split attacks by malicious miners, which can isolate affected nodes from the Zcash network and create conditions for double-spending against partitioned infrastructure. Organizations operating Zebra nodes or relying on Zcash network participation face operational disruption and potential financial impact if network consensus is compromised by coordinated mining actors.
Apply vendor patch: Zebra 4.3.0. If immediate patching is not feasible, isolate vulnerable nodes from untrusted mining sources and implement monitoring for block rejection anomalies and network fork conditions until patch deployment is complete.
Threat Intelligence Overview
NVD
CVE.org
EPSS
Vulnrichment
VulDB
How this decision was reached
The engine reads each signal below and applies a fixed list of rules from top to bottom. The first rule that matches sets the recommended action – like a statement, every line is shown.
What every action level means
SSVC AssessmentENISA
Priority Score
Attack Technique
Information disclosure occurs when an application unintentionally exposes sensitive data that aids attackers in reconnaissance or directly compromises security.
Affected Products (CPE)CVE.org
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