# CVE-2026-40394: Denial of service in Varnish Cache

> A workspace overflow in Varnish Cache during HTTP/2 upgrades allows attackers to cause a daemon panic and denial of service.

- **Severity:** Medium 4.0 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/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:** No
- **Published:** 12 Apr 2026
- **Updated:** 13 Apr 2026
- **Also known as:** GHSA-26pg-g54q-jv62, UBUNTU-CVE-2026-40394

## What is CVE-2026-40394?

CVE-2026-40394 is a medium-severity denial of service vulnerability in varnish-software Varnish Cache, published on 12 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Varnish Cache 9 before 9.0.1 and Varnish Enterprise before 6.0.16r11 allows a "workspace overflow" denial of service (daemon panic) for certain amounts of prefetched data. The setup of an HTTP/2 session starts with a speculative HTTP/1 transport, and upon upgrading to h2 the HTTP/1 request is repurposed as stream zero. During the upgrade, a buffer allocation is made to reserve space to send frames to the client. This allocation would split the original workspace, and depending on the amount of prefetched data, the next fetch could perform a pipelining operation that would run out of workspace.

Weakness: CWE-670 (Always-Incorrect Control Flow Implementation)

## Which versions are affected by CVE-2026-40394?

It affects 2 products.


Products: Varnish-Software Varnish Enterprise, Vinyl-Cache Vinyl Cache.

## How do I fix CVE-2026-40394?

No fix has been published for CVE-2026-40394 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-40394 being exploited?

There is no known exploitation of CVE-2026-40394 in the wild and no public exploit code.

## Other vulnerabilities in varnish-software Varnish Cache

- [CVE-2023-41104](https://insights.sbomify.com/CVE-2023-41104/): Memory corruption in n/a (medium 6.5)
- [CVE-2026-34475](https://insights.sbomify.com/CVE-2026-34475/): Varnish Cache before 8.0.1 and Varnish Enterprise before 6.0.16r12, in certain unchecked req.url scenarios, mishandle URLs with a path of / (medium 5.4)
- [CVE-2025-30346](https://insights.sbomify.com/CVE-2025-30346/): Other in Varnish Cache (medium 5.4)
- [CVE-2026-40395](https://insights.sbomify.com/CVE-2026-40395/): Denial of service in Varnish Enterprise (medium 4.0)
- [CVE-2025-30347](https://insights.sbomify.com/CVE-2025-30347/): Information disclosure in Varnish Enterprise (medium 4.0)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-40394 and fix it if it is affected.

What it is: CVE-2026-40394 is a medium-severity denial of service vulnerability in varnish-software Varnish Cache, published on 12 Apr 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-2026-40394 in the wild and no public exploit code.

Affected product: varnish-software Varnish Cache. 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-2026-40394/ (Markdown: https://insights.sbomify.com/CVE-2026-40394/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-40394
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-40394
- OSV: https://osv.dev/vulnerability/CVE-2026-40394

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-40394/
Summaries are generated and may contain mistakes; the official description above is the record.