---
id: CVE-2026-104421
title: >-
  Zebra before 6.2.1 contains an incomplete cleanup vulnerability that allows
  unauthenticated peers to block downloading of valid blocks by leaving rejected
  hashes in SentHashes
summary: >-
  Zebra before 6.2.1 contains an incomplete cleanup vulnerability that allows
  unauthenticated peers to block downloading of valid blocks by leaving rejected
  hashes in SentHashes. Attackers can send a contextually invalid block sharing
  an h…
severity: medium
cvss: 5.3
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L'
cwe:
  - CWE-459
vendor: ZcashFoundation
product: zebra
affected:
  - zebra < 6.2.1
published: '2026-10-02'
updated: '2026-10-02'
sourceUpdated: '2026-10-02T13:17:42.550'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-104421'
references:
  - url: >-
      https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-x93j-mj2f-q338
    label: disclosure@vulncheck.com
  - url: >-
      https://www.vulncheck.com/advisories/zebra-before-6.2.1-block-download-denial-of-service-via-knownblock-senthashes-lockout
    label: disclosure@vulncheck.com
tags:
  - nvd
  - cve.org
ssvc:
  exploitation: none
  automatable: 'yes'
  technicalImpact: partial
  timestamp: '2026-10-02T12:27:43.846358Z'
ingestedAt: '2026-10-02T12:17:44.329Z'
---

## Overview

Zebra before 6.2.1 contains an incomplete cleanup vulnerability that allows unauthenticated peers to block downloading of valid blocks by leaving rejected hashes in SentHashes. Attackers can send a contextually invalid block sharing an honest block's header hash, causing Request::KnownBlock to skip the honest block and keep nodes behind the tip.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.