# CVE-2026-50251: Attacker supplied '0.0.0.0'/'::' glue triggers defensive full-cache flush

> Attackers can trigger indefinite cache flushes in Unbound by exploiting glue record handling, causing denial of service when unwanted-reply-threshold is enabled.

- **Severity:** Medium 5.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L`)
- **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-2026-50251, AZL-92979, DEBIAN-CVE-2026-50251, ECHO-b852-f6a0-98dc, GHSA-rj42-r8f9-w75j, UBUNTU-CVE-2026-50251

## What is CVE-2026-50251?

CVE-2026-50251 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

In NLnet Labs Unbound up to and including version 1.25.1, when 'unwanted-reply-threshold' is enabled (set to any value greater than zero), glue records of 0.0.0.0/::0 can short-circuit Unbound, on systems that can direct such traffic, by issuing DNS queries and receiving seemingly unwanted replies since the remote IP does not match the original source IP of 0.0.0.0/::0. This behavior keeps on looping for the glue records and pushing the counter to the configured 'unwanted-reply-threshold' that triggers a defensive cache clear. A malicious actor who controls a delegation that returns in-bailiwick glue of 0.0.0.0/::0 can drive the counter to the limit of 'unwanted-reply-threshold' to the threshold and trigger a cache clean of the message and rrset caches; at will, indefinitely, without sending a single spoofed packet. The iterator uses the 0.0.0.0/::0 glue, and a system that can route this (e.g., Linux kernel routes the datagram over loopback), Unbound's own listener answers from 127.0.0.1. Because of the mismatch of 0.0.0.0 and 127.0.0.1, in this example, Unbound accounts the reply as an unwanted (probably spoofed) answer. The counter resets to zero on every cache flush, so the attack loops forever.

Weakness: CWE-184 (Incomplete List of Disallowed Inputs)

## Which versions are affected by CVE-2026-50251?

It affects 1 product.


Products: Nlnetlabs Unbound.

## How do I fix CVE-2026-50251?

Apply the vendor's patch.

## Is CVE-2026-50251 being exploited?

There is no known exploitation of CVE-2026-50251 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-2026-50251 and fix it if it is affected.

What it is: CVE-2026-50251 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-2026-50251 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-2026-50251/ (Markdown: https://insights.sbomify.com/CVE-2026-50251/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-50251
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-50251
- OSV: https://osv.dev/vulnerability/CVE-2026-50251

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-50251/
Summaries are generated and may contain mistakes; the official description above is the record.