# CVE-2026-104426: Zebra inefficient algorithmic complexity in block validation

Source: https://junglewise.ai/threats/cve-2026-104426-zebra-before-6-1-0-contains-an-inefficient-algorithmic  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-10-02. Updated: 2026-10-05  
License: CC BY 4.0 with a link back

CVE-2026-104426 · Severity: medium · CVSS 5.9 · Published 2026-10-02

Zebra is a cryptocurrency node implementation that validates blockchain blocks. An attacker can craft a malicious block containing roughly 26,000 minimal transactions that causes the validation process to stall for over 52 seconds on typical hardware, effectively creating a denial-of-service against network nodes. The stall is temporary and does not corrupt data or crash the software, but significantly impacts node responsiveness.

Vendors: [ZcashFoundation](https://junglewise.ai/threats/vendors/zcashfoundation).

## Quick answers

**Is CVE-2026-104426 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-104426 being exploited in the wild.

**How severe is CVE-2026-104426?** Medium severity, with a CVSS score of 5.9, published 2 October 2026.

**Which products and versions are affected?** ZcashFoundation Zebra before 6.1.0.

**Is there a fix for CVE-2026-104426?** Yes: fixed in 6.1.0. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-104426 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-104426?** Check whether you run ZcashFoundation 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 6.1.0

## Technical details

The vulnerability is an inefficient algorithmic complexity flaw (CWE-407) in the remaining_transaction_value function within utxo.rs. The code clones and converts the entire block-level spent-UTXO map once per transaction during contextual verification, resulting in O(N²) complexity instead of linear O(N). An attacker must produce a valid proof-of-work block or seed the mempool with single-input transactions; the malicious block must pass proof-of-work validation before reaching the vulnerable code path.

## Affected products

- ZcashFoundation Zebra before 6.1.0

## Timeline

- 2026-07-17: disclosed: GitHub security advisory GHSA-4g24-549m-hp75 published
- 2026-10-02: advisory: CVE-2026-104426 published on NVD

## References

- [https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-4g24-549m-hp75](https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-4g24-549m-hp75)
- [https://www.vulncheck.com/advisories/zebra-before-6.1.0-quadratic-complexity-dos-via-block-transparent-value-check](https://www.vulncheck.com/advisories/zebra-before-6.1.0-quadratic-complexity-dos-via-block-transparent-value-check)