What’s Vulnerable › Vulnerabilities

CVE-2026-104421: Zebra before 6.2.1 Block Download Denial of Service via KnownBlock SentHashes Lockout

• Severity: medium (CVSS 5.3 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.3% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-10-02 · updated 2026-10-02
• Weakness: CWE-459

Description

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.

Affected products

• ZcashFoundation zebra: before 6.2.1 — fixed in 6.2.1

How to fix

Upgrade to a fixed version: ZcashFoundation zebra 6.2.1. Check the vendor advisory for mitigations.

References

• https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-x93j-mj2f-q338
• https://www.vulncheck.com/advisories/zebra-before-6.2.1-block-download-denial-of-service-via-knownblock-senthashes-lockout

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