---
title: "CVE-2026-40396\n    | Ubuntu"
description: Ubuntu is an open source software operating system that runs from the
  desktop, to the cloud, to all your internet connected things.
url: https://ubuntu.com/security/CVE-2026-40396?format=md
keywords: index, follow
---

# CVE-2026-40396

Publication date 12 April 2026

Last updated 16 April 2026

---

Ubuntu priority

**Medium**

[Why this priority?](https://ubuntu.com/security/cves/about#priority )

## Cvss 3 Severity Score

**4.0 · Medium**

[Score breakdown](https://ubuntu.com/security/CVE-2026-40396?format=md#impact-score)

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## Description

Varnish Cache 9 before 9.0.1 allows a "workspace overflow" denial of
service (daemon panic) after timeout\_linger. A malicious client could send
an HTTP/1 request, wait long enough until the session releases its worker
thread (timeout\_linger) and resume traffic before the session is closed
(timeout\_idle) sending more than one request at once to trigger a
pipelining operation between requests. This vulnerability affecting Varnish
Cache 9.0.0 emerged from a port of the Varnish Enterprise non-blocking
architecture for HTTP/2. New code was needed to adapt to a more recent
workspace API that formalizes the pipelining operation. In addition to the
workspace change on the Varnish Cache side, other differences created merge
conflicts, like partial support for trailers in Varnish Enterprise. The
conflict resolution missed one code path configuring pipelining to perform
a complete workspace rollback, losing the guarantee that prefetched data
would fit inside workspace\_client during the transition from one request to
the next. This can result in a workspace overflow, triggering a panic and
crashing the Varnish server.

## Status

Show unmaintained releases

| Package | Ubuntu Release | Status |
| --- | --- | --- |
| varnish | 26.04 LTS resolute | Needs evaluation |
| 25.10 questing | Ignored end of life, was needs-triage |
| 24.04 LTS noble | Needs evaluation |
| 22.04 LTS jammy | Needs evaluation |
| 20.04 LTS focal | Needs evaluation |
| 18.04 LTS bionic | Needs evaluation |
| 16.04 LTS xenial | Needs evaluation |
| 14.04 LTS trusty | Needs evaluation |

---

* [How can I get the fixes?](https://ubuntu.com/security/cves/about#security)
* [What do statuses mean?](https://ubuntu.com/security/cves/about#statuses)

## Severity score breakdown

CVSS version:
CVSS v3.0

**Base score**

4.0 · Medium

* Base metrics

  | Parameter | Value |
  | --- | --- |
  | Attack vector | Network |
  | Attack complexity | High |
  | Privileges required | None |
  | User interaction | None |
  | Scope | Changed |
  | Confidentiality impact | None |
  | Integrity impact | None |
  | Availability impact | Low |
* Scores

  | Parameter | Value |
  | --- | --- |
  | Base score | 4.0 · Medium |
  | Exploitability score | - |
  | Impact score | - |

**Vector:** CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:N/A:L

## References

* [MITRE](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2026-40396)
* [NVD](https://nvd.nist.gov/vuln/detail/CVE-2026-40396)
* [Launchpad](https://launchpad.net/bugs/cve/CVE-2026-40396)
* [Debian](https://security-tracker.debian.org/tracker/CVE-2026-40396)

### Other references

* <https://www.cve.org/CVERecord?id=CVE-2026-40396>
* <https://github.com/varnish/varnish/issues/15>
* <https://github.com/varnish/varnish/releases/tag/varnish-9.0.1>
* <https://docs.varnish-software.com/security/VEV00002/>