# CVE-2026-104420: Zebra peer misbehavior scoring bypass in block gossip

Source: https://junglewise.ai/threats/cve-2026-104420-zebra-before-6-3-0-contains-a-protection-mechanism-failure-that  
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-2026-104420 · Severity: medium · CVSS 5.3 · Published 2026-10-02

Zebra is the reference implementation of the Zcash blockchain consensus protocol. An unauthenticated attacker can repeatedly send invalid blocks to a Zebra node without being banned, forcing the node to waste bandwidth and CPU on verification. The vulnerability does not crash the node or affect the blockchain; it only disables the ban mechanism that normally stops abusive peers.

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

## Quick answers

**Is CVE-2026-104420 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-104420 being exploited in the wild.

**How severe is CVE-2026-104420?** Medium severity, with a CVSS score of 5.3, published 2 October 2026.

**Which products and versions are affected?** Zcash Foundation Zebra before 6.3.0, from 2.3.0 to 6.2.3.

**Is there a fix for CVE-2026-104420?** Yes: fixed in 6.3.0. Until then: None that preserve normal operation; the node already rejects invalid blocks but cannot restore the ban mechanism by configuration. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-104420 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-104420?** 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 6.3.0

None that preserve normal operation; the node already rejects invalid blocks but cannot restore the ban mechanism by configuration.

## Technical details

The inbound gossip verifier returns a RouterError on consensus-invalid blocks, but the cleanup step downcasts to VerifyBlockError, which fails and discards the misbehavior score. An attacker can repeatedly supply invalid gossiped blocks and evade the peer ban that occurs at a score of 100, though each attempt remains bounded by one-block-in-transit limits and verification timeouts. The sync download path handles this correctly; the gossip path is the only affected code path.

## Affected products

- Zcash Foundation Zebra before 6.3.0, from 2.3.0 to 6.2.3

## Timeline

- 2026-08-11: disclosed
- 2026-10-02: advisory

## References

- [https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-8hh2-hrf2-cqf4](https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-8hh2-hrf2-cqf4)
- [https://www.vulncheck.com/advisories/zebra-before-6.3.0-peer-misbehavior-ban-bypass-via-gossiped-blocks](https://www.vulncheck.com/advisories/zebra-before-6.3.0-peer-misbehavior-ban-bypass-via-gossiped-blocks)

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