CVE-2026-40394: High severity Varnish Software Varnish Cache vulnerability
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.
Affected Software
Event History
Frequently Asked Questions
What is the severity of CVE-2026-40394?
CVE-2026-40394 is classified as a denial of service vulnerability that can lead to daemon panic.
How do I fix CVE-2026-40394?
To mitigate CVE-2026-40394, upgrade Varnish Cache to version 9.0.1 or Varnish Enterprise to version 6.0.16r12 or later.
What versions are affected by CVE-2026-40394?
CVE-2026-40394 affects Varnish Cache versions prior to 9.0.1 and Varnish Enterprise versions prior to 6.0.16r12.
What type of attack can CVE-2026-40394 enable?
CVE-2026-40394 allows for a 'workspace overflow' denial of service attack.
Is CVE-2026-40394 specific to any transport protocols?
Yes, CVE-2026-40394 involves an issue that arises during the setup of an HTTP/2 session that starts with HTTP/1 transport.