# CVE-2026-44621: Libunbound applications configured with 'unwanted-reply-threshold' could eventually be abruptly terminated

> Applications using libunbound with unwanted-reply-threshold set may crash when processing malformed UDP replies, causing denial of service.

- **Severity:** Medium 5.9 (`CVSS:3.1/AV:N/AC:H/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:** 22 Jul 2026
- **Updated:** 22 Jul 2026
- **Also known as:** ALPINE-CVE-2026-44621, AZL-92943, DEBIAN-CVE-2026-44621, ECHO-0093-f01d-663c, GHSA-3m86-wjgw-cgqc, UBUNTU-CVE-2026-44621

## What is CVE-2026-44621?

CVE-2026-44621 is a medium-severity denial of service vulnerability in Nlnetlabs Unbound, published on 22 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

With NLnet Labs Unbound up to and including version 1.25.1, applications using libunbound and configured with 'unwanted-reply-threshold', could eventually be abruptly terminated if the threshold is reached and libunbound needs to call 'libworker_alloc_cleanup' since the function is absent from the function call allow list. When an application using libunbound sets 'unwanted-reply-threshold' to any non-zero value and the iterator queries an authoritative that replies with enough wrong-transaction-ID UDP datagrams to cross the threshold, the 'libworker_alloc_cleanup' will eventually be called. Since the function is absent from the function call allow list, this leads to a fatal exit of libunbound and eventual termination of the embedding application.Unbound itself is not affected since its relevant function 'worker_alloc_cleanup' is registed in the allow list and proceeds to perform the documented cache flush.

Weakness: CWE-754 (Improper Check for Unusual or Exceptional Conditions)

## Which versions are affected by CVE-2026-44621?

It affects 1 product.


Products: Nlnetlabs Unbound.

## How do I fix CVE-2026-44621?

Apply the vendor's patch.

## Is CVE-2026-44621 being exploited?

There is no known exploitation of CVE-2026-44621 in the wild and no public exploit code.

## Other vulnerabilities in Nlnetlabs Unbound

- [CVE-2026-81642](https://insights.sbomify.com/CVE-2026-81642/): Heap buffer overflow and possible Remote Code Execution when digesting DNSKEY (critical 9.1)
- [CVE-2026-33278](https://insights.sbomify.com/CVE-2026-33278/): Possible arbitrary code execution during DNSSEC validation (critical 9.1)
- [CVE-2026-42944](https://insights.sbomify.com/CVE-2026-42944/): Heap overflow with multiple NSID, COOKIE, PADDING EDNS options (high 8.7)
- [CVE-2026-42959](https://insights.sbomify.com/CVE-2026-42959/): Crash during DNSSEC validation of malicious content (high 8.7)
- [CVE-2026-82717](https://insights.sbomify.com/CVE-2026-82717/): CNAME synthesis could lead to heap corruption (high 8.4)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-44621 and fix it if it is affected.

What it is: CVE-2026-44621 is a medium-severity denial of service vulnerability in Nlnetlabs Unbound, published on 22 Jul 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-2026-44621 in the wild and no public exploit code.

Affected product: Nlnetlabs Unbound. 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-2026-44621/ (Markdown: https://insights.sbomify.com/CVE-2026-44621/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-44621
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-44621
- OSV: https://osv.dev/vulnerability/CVE-2026-44621

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-44621/
Summaries are generated and may contain mistakes; the official description above is the record.