# CVE-2026-39155: Denial of service in n/a

> Attackers can cause downstream resolver denial of service by exploiting incorrect NSEC owner name computation in Knot DNS online signing.

- **Severity:** Medium 6.5 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 23 Jul 2026
- **Updated:** 24 Jul 2026
- **Also known as:** DEBIAN-CVE-2026-39155, GHSA-hxf3-75wr-4mhp, UBUNTU-CVE-2026-39155

## What is CVE-2026-39155?

CVE-2026-39155 is a medium-severity denial of service vulnerability in n/a, published on 23 Jul 2026. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

Knot DNS before 3.4.10 and 3.5.x before 3.5.4 contains a vulnerability in mod-onlinesign where the next NSEC owner name can be computed incorrectly. This can create an overly broad authenticated denial interval, allowing downstream validating resolvers using aggressive negative caching to synthesize negative answers for legitimate names and causing resolver-side denial of service.

Weakness: CWE-345 (Insufficient Verification of Data Authenticity)

## Which versions are affected by CVE-2026-39155?


## How do I fix CVE-2026-39155?

Upgrade to a fixed version.

## Is CVE-2026-39155 being exploited?

There is no known exploitation of CVE-2026-39155 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-39155 and fix it if it is affected.

What it is: CVE-2026-39155 is a medium-severity denial of service vulnerability in n/a, published on 23 Jul 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-39155 in the wild and no public exploit code.

Affected product: n/a. 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-2026-39155/ (Markdown: https://insights.sbomify.com/CVE-2026-39155/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-39155
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-39155
- OSV: https://osv.dev/vulnerability/CVE-2026-39155

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-39155/
Summaries are generated and may contain mistakes; the official description above is the record.