CVE-2026-40396

CVE-2026-40396 is a medium-severity vulnerability in Vinyl-cache Vinyl Cache with a CVSS 3.x base score of 4.0. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-670.

Key facts

Description

Varnish Cache 9 before 9.0.1 allows a "workspace overflow" denial of service (daemon panic) after timeout_linger. A malicious client could send an HTTP/1 request, wait long enough until the session releases its worker thread (timeout_linger) and resume traffic before the session is closed (timeout_idle) sending more than one request at once to trigger a pipelining operation between requests. This vulnerability affecting Varnish Cache 9.0.0 emerged from a port of the Varnish Enterprise non-blocking architecture for HTTP/2. New code was needed to adapt to a more recent workspace API that formalizes the pipelining operation. In addition to the workspace change on the Varnish Cache side, other differences created merge conflicts, like partial support for trailers in Varnish Enterprise. The conflict resolution missed one code path configuring pipelining to perform a complete workspace rollback, losing the guarantee that prefetched data would fit inside workspace_client during the transition from one request to the next. This can result in a workspace overflow, triggering a panic and crashing the Varnish server.

Frequently asked questions

What is CVE-2026-40396?
Varnish Cache 9 before 9.0.1 allows a "workspace overflow" denial of service (daemon panic) after timeout_linger. A malicious client could send an HTTP/1 request, wait long enough until the session releases its worker thread (timeout_linger) and resume traffic before the session is closed (timeout_idle) sending more than one request at once to trigger a pipelining operation between requests. This vulnerability affecting Varnish Cache 9.0.0 emerged from a port of the Varnish Enterprise non-blocking architecture for HTTP/2. New code was needed to adapt to a more recent workspace API that formalizes the pipelining operation. In addition to the workspace change on the Varnish Cache side, other differences created merge conflicts, like partial support for trailers in Varnish Enterprise. The conflict resolution missed one code path configuring pipelining to perform a complete workspace rollback, losing the guarantee that prefetched data would fit inside workspace_client during the transition from one request to the next. This can result in a workspace overflow, triggering a panic and crashing the Varnish server.
How severe is CVE-2026-40396?
CVE-2026-40396 has a CVSS 3.x base score of 4.0, rated medium severity. It is exploitable over network with high attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability low.
Is CVE-2026-40396 being actively exploited?
It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (27th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-40396?
CVE-2026-40396 affects Vinyl-cache Vinyl Cache. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2026-40396?
Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.
Does CVE-2026-40396 have an EU (EUVD) identifier?
Yes. CVE-2026-40396 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-21742.
When was CVE-2026-40396 published?
CVE-2026-40396 was published on 2026-04-12 and last updated on 2026-06-17.

References

Affected products (1)

More vulnerabilities in Vinyl-cache Vinyl Cache

All CVEs affecting Vinyl-cache Vinyl Cache →

Other CWE-670 vulnerabilities

Browse all CWE-670 vulnerabilities →