Vulnerability in Varnish-Software Varnish Cache
CVE-2026-40394
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.
EPSS: 0.002 (15.0th percentile) — read the EPSS interpretation.
CVSS v3 metric
CVSS v3 base score 4.0 (Medium). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:N/A:L.
Affected products
- Varnish-software Varnish Cache — versions 9.0.0
- Varnish-software Varnish_enterprise — versions 6.0.16
- Vinyl-cache Vinyl_cache — versions 9.0.0
Weakness classification (CWE)
References
- cve@mitre.org (Vendor Advisory)
Frequently asked questions
- What is CVE-2026-40394?
- CVE-2026-40394 is a medium-severity vulnerability in Varnish-Software Varnish Cache, classified under Always-Incorrect Control Flow Implementation. CVSS score: 4.0/10. Published 2026-04-12.
- How severe is CVE-2026-40394?
- Medium severity. CVSS v3 base score is 4.0 out of 10.