# CVE-2026-109125: Possible cache poisoning vulnerability via a cache side-channel exposed by 'unwanted-reply-threshold'


- **Severity:** Medium 5.3 (`CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Fix available:** Yes
- **Published:** 21 Oct 2026
- **Updated:** 29 Oct 2026
- **Also known as:** AZL-110528, DEBIAN-CVE-2026-109125, ECHO-b3bc-a96b-2f61, GHSA-66rc-377g-m4m4, UBUNTU-CVE-2026-109125

## What is CVE-2026-109125?

CVE-2026-109125 is a medium-severity vulnerability in NLnet Labs Unbound, published on 21 Oct 2026. 

### Official description

In NLnet Labs Unbound up to and including version 1.26.1, an observable  cache-level side-channel can be exposed with 'unwanted-reply-threshold > 0' configurations (not a default configuration). If a response-like UDP datagram reaches the upstream source port currently used by a recursive query, but its TXID does not match any pending query, Unbound counts the packet as an unwanted reply. When the accumulated count reaches the configured threshold, Unbound clears the worker's RRset cache and message cache. This cache clear action does not destroy the current upstream UDP pending query. Therefore, an external observer can distinguish whether a candidate port matches the current upstream source port by checking whether an existing cached item has been cleared. This is a cache side-channel vulnerability that can be used to probe the upstream source port. Although the attack is not feasible on default configurations and the suggested value (10 million) makes it an ureliable attack on a public resolver, in scenarios where the attacker can keep a concurrent query window alive, the attacker can first filter the source port and then perform birthday-style guessing against the TXID, making cache poisoning a feasible attack path.

Weakness: CWE-205 (Observable Behavioral Discrepancy)

## Which versions are affected by CVE-2026-109125?


## How do I fix CVE-2026-109125?

Upgrade to a fixed version.

## Is CVE-2026-109125 being exploited?

There is no known exploitation of CVE-2026-109125 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-109125 and fix it if it is affected.

What it is: CVE-2026-109125 is a medium-severity vulnerability in NLnet Labs Unbound, published on 21 Oct 2026. 
Exploitation: There is no known exploitation of CVE-2026-109125 in the wild and no public exploit code.

Affected product: NLnet Labs 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-2026-109125/ (Markdown: https://insights.sbomify.com/CVE-2026-109125/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-109125
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-109125
- OSV: https://osv.dev/vulnerability/CVE-2026-109125

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-109125/
Summaries are generated and may contain mistakes; the official description above is the record.