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

> Unauthenticated attackers can cause denial of service in Zcash nodes by preventing valid block downloads through hash manipulation.

- **Severity:** Medium 6.9 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 2 Oct 2026
- **Updated:** 2 Oct 2026
- **Also known as:** GHSA-x93j-mj2f-q338, GHSA-xx63-6p6j-3wv7

## What is CVE-2026-104421?

CVE-2026-104421 is a medium-severity denial of service vulnerability in ZcashFoundation zebra, published on 2 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

Weakness: CWE-459 (Incomplete Cleanup)

## Which versions are affected by CVE-2026-104421?


## How do I fix CVE-2026-104421?

No fix has been published for CVE-2026-104421 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-104421 being exploited?

There is no known exploitation of CVE-2026-104421 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-104421 and fix it if it is affected.

What it is: CVE-2026-104421 is a medium-severity denial of service vulnerability in ZcashFoundation zebra, published on 2 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-104421 in the wild and no public exploit code.

Affected product: ZcashFoundation zebra. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-104421/ (Markdown: https://insights.sbomify.com/CVE-2026-104421/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-104421
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-104421
- OSV: https://osv.dev/vulnerability/CVE-2026-104421

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-104421/
Summaries are generated and may contain mistakes; the official description above is the record.