CVE-2026-40394 Denial of service in Varnish Cache

Medium 4.0 Published 12 Apr 2026 Updated 13 Apr 2026

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

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: Not needed
Fix: None listed

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

Which versions are affected by CVE-2026-40394?

It affects 2 products.

Products include 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.

Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.

Is CVE-2026-40394 being exploited?

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

Sources

Also known as GHSA-26pg-g54q-jv62, UBUNTU-CVE-2026-40394.