# CVE-2021-40083: Denial of service in n/a

> A remote attacker can cause a denial of service in Knot Resolver by triggering an assertion failure via crafted NSEC3 records.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 25 Aug 2021
- **Updated:** 4 Aug 2024
- **Also known as:** DEBIAN-CVE-2021-40083, GHSA-wvjr-27w9-ff2r

## What is CVE-2021-40083?

CVE-2021-40083 is a high-severity denial of service vulnerability in n/a, published on 25 Aug 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Knot Resolver before 5.3.2 is prone to an assertion failure, triggerable by a remote attacker in an edge case (NSEC3 with too many iterations used for a positive wildcard proof).

Weakness: CWE-617 (Reachable Assertion)

## Which versions are affected by CVE-2021-40083?

It affects 1 product.


Products: Nic Knot Resolver.

## How do I fix CVE-2021-40083?

Upgrade to a fixed version.

## Is CVE-2021-40083 being exploited?

There is no known exploitation of CVE-2021-40083 in the wild and no public exploit code.

## Other vulnerabilities in n/a

- [CVE-2023-46317](https://insights.sbomify.com/CVE-2023-46317/): Denial of service in n/a (high 7.5)
- [CVE-2023-26249](https://insights.sbomify.com/CVE-2023-26249/): Denial of service in n/a (high 7.5)
- [CVE-2018-1110](https://insights.sbomify.com/CVE-2018-1110/): Denial of service in knot-resolver (high 7.5)
- [CVE-2020-12667](https://insights.sbomify.com/CVE-2020-12667/): Denial of service in n/a (high 7.5)
- [CVE-2018-10920](https://insights.sbomify.com/CVE-2018-10920/): Other in knot-resolver (high 7.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-40083 and fix it if it is affected.

What it is: CVE-2021-40083 is a high-severity denial of service vulnerability in n/a, published on 25 Aug 2021. 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-2021-40083 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-2021-40083/ (Markdown: https://insights.sbomify.com/CVE-2021-40083/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-40083
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-40083
- OSV: https://osv.dev/vulnerability/CVE-2021-40083

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-40083/
Summaries are generated and may contain mistakes; the official description above is the record.