# CVE-2026-19538: Bypass of BLOCKED ACL items on proxy protocol port over TCP or TLS

> Attackers can bypass BLOCKED ACL restrictions on the proxy protocol port by sending duplicate queries over persistent TCP or TLS connections.

- **Severity:** High 8.2 (`CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 26 Aug 2026
- **Updated:** 26 Aug 2026
- **Also known as:** DEBIAN-CVE-2026-19538, GHSA-q27j-h4qm-x5f7, UBUNTU-CVE-2026-19538

## What is CVE-2026-19538?

CVE-2026-19538 is a high-severity vulnerability in NLnet Labs NSD, published on 26 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

The BLOCKED access control list items that are evaluated to deny access on the the proxy protocol port can be bypassed completely when connecting over TCP or TLS and sending the query twice on connection that is kept open.

Weakness: CWE-672 (Operation on a Resource after Expiration or Release), CWE-290 (Authentication Bypass by Spoofing)

## Which versions are affected by CVE-2026-19538?

It affects 1 product.


Products: Nlnetlabs Nsd.

## How do I fix CVE-2026-19538?

Apply the vendor's patch.

## Is CVE-2026-19538 being exploited?

There is no known exploitation of CVE-2026-19538 in the wild and no public exploit code.

## Other vulnerabilities in NLnet Labs NSD

- [CVE-2026-12244](https://insights.sbomify.com/CVE-2026-12244/): Heap overflow and crash with crafted SVCB RR (high 8.7)
- [CVE-2026-12245](https://insights.sbomify.com/CVE-2026-12245/): Denial of DNS over TLS service by any DoT client (high 8.7)
- [CVE-2026-18664](https://insights.sbomify.com/CVE-2026-18664/): Wrong interpretation of ACL ranges (high 8.2)
- [CVE-2026-19401](https://insights.sbomify.com/CVE-2026-19401/): Remote UDP DoS by sending multiple DNS Cookie options (high 8.2)
- [CVE-2026-12490](https://insights.sbomify.com/CVE-2026-12490/): Bypass of client certificate verification with transfer over TLS (high 8.2)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-19538 and fix it if it is affected.

What it is: CVE-2026-19538 is a high-severity vulnerability in NLnet Labs NSD, published on 26 Aug 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-2026-19538 in the wild and no public exploit code.

Affected product: NLnet Labs NSD. 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-2026-19538/ (Markdown: https://insights.sbomify.com/CVE-2026-19538/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-19538
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-19538
- OSV: https://osv.dev/vulnerability/CVE-2026-19538

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-19538/
Summaries are generated and may contain mistakes; the official description above is the record.