CVE-2026-40396
Varnish Cache HTTP/1 Pipeline Denial of Service
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.
INFO
Published Date :
April 12, 2026, 8:16 p.m.
Last Modified :
Oct. 26, 2026, 9:10 p.m.
Remotely Exploit :
Yes !
Source :
[email protected]
Affected Products
The following products are affected by CVE-2026-40396
vulnerability.
Even if cvefeed.io is aware of the exact versions of the
products
that
are
affected, the information is not represented in the table below.
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS | 134c704f-9b21-4f2e-91b3-4a467353bcc0 | |||||
| CVSS 3.1 | MEDIUM | 8254265b-2729-46b6-b9e3-3dfca2d5bfca | ||||
| CVSS 3.1 | MEDIUM | [email protected] | ||||
| CVSS 3.1 | HIGH | [email protected] |
Solution
- Update Varnish Cache to version 9.0.1 or later.
- Apply relevant vendor patches if available.
- Review workspace handling and pipelining configurations.
References to Advisories, Solutions, and Tools
Here, you will find a curated list of external links that provide in-depth
information, practical solutions, and valuable tools related to
CVE-2026-40396.
| URL | Resource |
|---|---|
| https://github.com/varnish/varnish/issues/15 | Issue Tracking |
| https://github.com/varnish/varnish/releases/tag/varnish-9.0.1 | Product |
CWE - Common Weakness Enumeration
While CVE identifies
specific instances of vulnerabilities, CWE categorizes the common flaws or
weaknesses that can lead to vulnerabilities. CVE-2026-40396 is
associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification
(CAPEC)
stores attack patterns, which are descriptions of the common attributes and
approaches employed by adversaries to exploit the CVE-2026-40396
weaknesses.
We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).
Results are limited to the first 15 repositories due to potential performance issues.
The following list is the news that have been mention
CVE-2026-40396 vulnerability anywhere in the article.
The following table lists the changes that have been made to the
CVE-2026-40396 vulnerability over time.
Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.
-
CVE Translated by [email protected]
Oct. 26, 2026
Action Type Old Value New Value Added Translation Title: Varnish Cache de varnish-software, Description: Varnish Cache 9 anterior a la 9.0.1 permite una ’sobrecarga de espacio de trabajo’ (workspace overflow) denegación de servicio (pánico del demonio) después de timeout_linger. Un cliente malicioso podría enviar una solicitud HTTP/1, esperar el tiempo suficiente hasta que la sesión libere su hilo de trabajo (timeout_linger) y reanudar el tráfico antes de que la sesión se cierre (timeout_idle) enviando más de una solicitud a la vez para activar una operación de pipelining entre solicitudes. Esta vulnerabilidad que afecta a Varnish Cache 9.0.0 surgió de una adaptación de la arquitectura no bloqueante de Varnish Enterprise para HTTP/2. Se necesitó código nuevo para adaptarse a una API de espacio de trabajo más reciente que formaliza la operación de pipelining. Además del cambio en el espacio de trabajo por parte de Varnish Cache, otras diferencias crearon conflictos de fusión, como el soporte parcial para trailers en Varnish Enterprise. La resolución de conflictos omitió una ruta de código que configuraba el pipelining para realizar una reversión completa del espacio de trabajo, perdiendo la garantía de que los datos precargados cabrían dentro de workspace_client durante la transición de una solicitud a la siguiente. Esto puede resultar en una sobrecarga del espacio de trabajo (workspace overflow), lo que desencadena un pánico y provoca la caída del servidor Varnish. -
CVE Modified by [email protected]
Jun. 17, 2026
Action Type Old Value New Value Added Affected [{'vendor': 'varnish-software', 'product': 'Varnish Cache', 'versions': [{'status': 'affected', 'version': '9.0.0', 'lessThan': '9.0.1', 'versionType': 'semver'}], 'defaultStatus': 'unaffected'}] -
CVE Modified by 134c704f-9b21-4f2e-91b3-4a467353bcc0
Jun. 17, 2026
Action Type Old Value New Value Added SSVC {'id': 'CVE-2026-40396', 'role': 'CISA Coordinator', 'options': [{'exploitation': 'none'}, {'automatable': 'no'}, {'technicalImpact': 'partial'}], 'version': '2.0.3', 'timestamp': '2026-04-13T15:35:30.690667Z'} -
Initial Analysis by [email protected]
Apr. 17, 2026
Action Type Old Value New Value Added CVSS V3.1 AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H Added CPE Configuration OR *cpe:2.3:a:vinyl-cache:vinyl_cache:9.0.0:*:*:*:*:*:*:* Added Reference Type MITRE: https://github.com/varnish/varnish/issues/15 Types: Issue Tracking Added Reference Type MITRE: https://github.com/varnish/varnish/releases/tag/varnish-9.0.1 Types: Product -
New CVE Received by [email protected]
Apr. 12, 2026
Action Type Old Value New Value Added 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. Added CVSS V3.1 AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:N/A:L Added CWE CWE-670 Added Reference https://github.com/varnish/varnish/issues/15 Added Reference https://github.com/varnish/varnish/releases/tag/varnish-9.0.1