Is CVE-2026-40395 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-40395 being exploited in the wild.

How severe is CVE-2026-40395?

Medium severity, with a CVSS score of 4, published 12 April 2026.

Which products and versions are affected?

Varnish Software Varnish Enterprise 6.0.9r5 through 6.0.16r11.

Is there a fix for CVE-2026-40395?

A patch was published on 2026-04-12: Fixed in Varnish Enterprise 6.0.16r12 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

How is CVE-2026-40395 attacked?

It is attacked over the network, in a network appliance.

Am I affected by CVE-2026-40395?

Check whether anything you run is named in the advisory. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

CVE-2026-40395: Varnish Enterprise denial of service in vmod_headerplus

CVE-2026-40395 · Severity: medium · CVSS 4 · Published 2026-04-12

Varnish Enterprise, a high-performance web accelerator and caching server, is vulnerable to a denial-of-service attack. By sending a specially crafted web request with an excessive number of headers, a malicious actor can cause the server to crash. This results in a service outage, preventing legitimate users from accessing the websites or applications protected by the Varnish server.

Quick answers

Is CVE-2026-40395 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-40395 being exploited in the wild.
How severe is CVE-2026-40395?: Medium severity, with a CVSS score of 4, published 12 April 2026.
Which products and versions are affected?: Varnish Software Varnish Enterprise 6.0.9r5 through 6.0.16r11.
Is there a fix for CVE-2026-40395?: A patch was published on 2026-04-12: Fixed in Varnish Enterprise 6.0.16r12 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-40395 attacked?: It is attacked over the network, in a network appliance.
Am I affected by CVE-2026-40395?: Check whether anything you run is named in the advisory. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

Technical details

A workspace overflow vulnerability exists in Varnish Enterprise's vmod_headerplus module. The headerplus.write_req0() function is used to update the underlying req0 object (the original read-only request) before transitioning between VCL (Varnish Configuration Language) labels, a common pattern in shared VCL deployments managed by Varnish Controller. If an incoming request is modified to contain more header fields than the req0 workspace can accommodate, the function triggers a workspace overflow. This results in a daemon panic and immediate crash of the Varnish server. The issue is resolved in version 6.0.16r12.

Affected products

Varnish Software Varnish Enterprise 6.0.9r5 through 6.0.16r11

Timeline

2026-04-09: advisory: Vendor advisory published by Varnish Software
2026-04-12: disclosed: CVE-2026-40395 published
2026-04-12: patched: Fixed in Varnish Enterprise 6.0.16r12

References

https://docs.varnish-software.com/security/VEV00003/

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