Vulnerability · Published Apr 12, 2026 · Assigned by mitre
CVE-2026-40394
Medium 4.0
In plain English
A medium-severity security flaw in Varnish Cache from varnish-software. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 15% of all known vulnerabilities).
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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian:12 | varnish | no fix listed | DEBIAN-CVE-2026-40394 |
| Debian:13 | varnish | no fix listed | DEBIAN-CVE-2026-40394 |
| Debian:14 | varnish | no fix listed | DEBIAN-CVE-2026-40394 |
| Ubuntu:24.04:LTS | varnish | no fix listed | UBUNTU-CVE-2026-40394 |
| Ubuntu:25.10 | varnish | no fix listed | UBUNTU-CVE-2026-40394 |
| Ubuntu:26.04:LTS | varnish | no fix listed | UBUNTU-CVE-2026-40394 |
| Ubuntu:26.10 | varnish | no fix listed | UBUNTU-CVE-2026-40394 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| varnish-software | Varnish Cache | 9.0.0 – before 9.0.1 |
Weakness: CWE-670 · Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:N/A:L