CVE-2026-40394: Varnish Cache 9 before 9.0.1 and Varnish Enterprise before 6.0.16r11 allows a "workspace overflow" denial of s

Routine

MEDIUM 4.0REMOTEUPGRADEDOWNGRADE

CVE-2026-40394 is a medium-severity flaw (CVSS 4.0) affecting varnish enterprise, vinyl cache. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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 or

Why this priority

Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	4.0 (Medium)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	0.2% (top 85%)
CISA exploited list	No
Weakness	CWE-670
Published	2026-04-12
Last modified	2026-10-26

Publisher (CNA): MITRE · Vendor: varnish-software · vinyl-cache

Last updated 2026-10-26 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-04-12 20:16 UTCPublished CVE record published
2026-10-26 21:10 UTCUpdated Record last modified

varnish-software varnish enterprise UPGRADE

Affected: <= 6.0.15 · 6.0.16 r1 · 6.0.16 r10 · 6.0.16 r2 · 6.0.16 r3 · 6.0.16 r4 · 6.0.16 r5 · 6.0.16 r6

Fixed in: newer than 6.0.15

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy varnish-enterprise || rpm -q varnish-enterprise || varnish-enterprise --version

2. UPGRADE to newer than 6.0.15 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade varnish-enterprise

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade varnish-enterprise

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

vinyl-cache vinyl cache DOWNGRADE

Affected: 9.0.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 9.0.0

1. Check installed version

apt-cache policy vinyl-cache || rpm -q vinyl-cache || vinyl-cache --version

2. No fixed release: DOWNGRADE to a release before 9.0.0 (Debian / Ubuntu)

apt-cache madison vinyl-cache sudo apt install vinyl-cache=<VERSION_BEFORE_9.0.0> sudo apt-mark hold vinyl-cache

2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list vinyl-cache sudo dnf downgrade vinyl-cache-<VERSION_BEFORE_9.0.0> sudo dnf versionlock add vinyl-cache

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-40394?

varnish enterprise: upgrade to newer than 6.0.15; vinyl cache: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2026-40394 being exploited?

CVE-2026-40394 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-40394 affect?

CVE-2026-40394 affects varnish enterprise, vinyl cache. Severity is medium (CVSS 4.0). 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 tran

How urgent is CVE-2026-40394?

Priority 23/100. Remotely exploitable, no login, no user action

Software pages

Varnish vulnerabilities

Related CVEs

CVE-2026-34475 MEDIUM 5.4 UPGRADE UPGRADE Varnish Cache before 8.0.1 and Varnish Enterprise before 6.0.16r12, in certain unchecked req.url scenarios, mi
CVE-2026-40395 MEDIUM 4.0 UPGRADE Varnish Enterprise before 6.0.16r12 allows a "workspace overflow" denial of service (daemon panic) for shared

References

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