# CVE-2026-109363: Zcash Foundation Zebra denial of service via malformed v6 transaction

Source: https://junglewise.ai/threats/cve-2026-109363-zcashfoundation-zebra-6-4-0-before-6-4-2-parses-version-6  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-10-12. Updated: 2026-10-16  
License: CC BY 4.0 with a link back

CVE-2026-109363 · Severity: high · CVSS 7.5 · Published 2026-10-12

Zebra is a full-node implementation of the Zcash blockchain. An unauthenticated attacker can crash a Zebra node by sending a specially crafted version 6 transaction with an outdated consensus branch identifier, causing the node to abort and become unavailable. The attacker can repeat this attack after each restart, leading to persistent service disruption.

Technologies: [Zcash Foundation Zebra](https://junglewise.ai/threats/technologies/zebra). Vendors: [Zcash Foundation](https://junglewise.ai/threats/vendors/zcash-foundation).

## Quick answers

**Is CVE-2026-109363 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-109363 being exploited in the wild.

**How severe is CVE-2026-109363?** High severity, with a CVSS score of 7.5, published 12 October 2026.

**Which products and versions are affected?** Zcash Foundation Zebra 6.4.0 before 6.4.2.

**Is there a fix for CVE-2026-109363?** Yes: fixed in 6.4.2. Until then: Disable inbound peer connections if possible, though this limits node functionality. Upgrade to v6.4.2 or later as soon as feasible. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-109363 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-109363?** Check whether you run Zcash Foundation Zebra at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- Fixed in 6.4.2

Disable inbound peer connections if possible, though this limits node functionality. Upgrade to v6.4.2 or later as soon as feasible.

## Technical details

The vulnerability is a read/write asymmetry in transaction parsing and serialization: Zebra's parser accepts version 6 transactions with pre-NU6.3 consensus branch IDs that upstream zcash_primitives will refuse to re-serialize. During UnminedTx construction, size computation re-serializes the transaction and treats the result as infallible via .expect(), causing the process to abort when the write fails. An unauthenticated peer can trigger this with a single 1016-byte tx message before handshake completion.

## Affected products

- Zcash Foundation Zebra 6.4.0 before 6.4.2

## Timeline

- 2026-09-25: disclosed: GHSA-h5rr-8pqv-grp9 published
- 2026-10-12: advisory: CVE-2026-109363 published
- 2026-10-12: patched: Fix commit 008abac deployed in v6.4.2

## References

- [https://github.com/ZcashFoundation/zebra/commit/008abac5c99e872f283450d596d95e37851df49a](https://github.com/ZcashFoundation/zebra/commit/008abac5c99e872f283450d596d95e37851df49a)
- [https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-h5rr-8pqv-grp9](https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-h5rr-8pqv-grp9)

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