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Varnish EnterpriseApplication · Varnish Software

CVE-2026-40394

HIGH · 7.5 CVSS v3.1 Published 2026-04-12
Fix available
A fix is available. Upgrade to after 6.0.15 or later.
See remediation →
84/100
Remediation priority · High
Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

Varnish Cache versions before 9.0.1 and Varnish Enterprise before 6.0.16r11 suffer from a workspace overflow vulnerability during HTTP/2 session upgrades. When an HTTP/1 request is repurposed as stream zero during h2 upgrade, a buffer allocation splits the original workspace. With certain amounts of prefetched data, subsequent pipelining operations exhaust the workspace, triggering daemon panic and causing denial of service.

MitigationUpgrade to Varnish Cache 9.0.1, Varnish Enterprise 6.0.16r11, or later versions to address the workspace allocation vulnerability during HTTP/2 upgrade.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Varnish EnterpriseApplication
Affected:<= 6.0.15= 6.0.16
Vinyl CacheApplication
Affected:= 9.0.0

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Check installed Varnish version
    Run 'varnishd -V' to display the Varnish daemon version
    Affected if Version is Varnish Cache below 9.0.1, or Varnish Enterprise at or below 6.0.16r10, or exactly 9.0.0 for Vinyl Cache (if applicable)
  2. Verify HTTP/2 protocol support is enabled
    Check varnishd startup options for h2 parameter (e.g., 'varnishd -C' shows compiled VCL, or inspect init scripts/ systemd unit for -p h2=On or similar)
    Affected if HTTP/2 support (h2) is enabled in the Varnish configuration or daemon arguments
  3. Confirm h2 upgrade configuration
    Examine VCL configuration files for 'h2' parameter settings or 'http2' module usage with 'varnishd -C' to view compiled VCL
    Affected if HTTP/2 upgrade capability is present in the configuration (even if not actively used, the vulnerable code path exists when h2 is compiled in)
  4. Check for active HTTP/1 pipelining usage
    Review access patterns or logs for HTTP/1 pipelined requests that may interact with upgraded HTTP/2 sessions
    Affected if HTTP/1 pipelining is used in conjunction with HTTP/2 upgrade attempts, which would trigger the workspace exhaustion condition

Your environment is affected if you run a vulnerable Varnish version (before 9.0.1 for Cache, before 6.0.16r11 for Enterprise, or exactly 9.0.0 for Vinyl Cache) AND have HTTP/2 support enabled, as the workspace overflow occurs during h2 upgrade with pipelined requests.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to a release after 6.0.15
Interim mitigation

Upgrade to Varnish Cache 9.0.1, Varnish Enterprise 6.0.16r11, or later versions to address the workspace allocation vulnerability during HTTP/2 upgrade.

Recommended fix High confidence

Varnish Enterprise: 6.0.16r11 or later; Varnish Cache 9: 9.0.1 or later

  1. 1. Identify the current Varnish installation version using 'varnishd -V' or checking the package manager
  2. 2. For Varnish Enterprise: Upgrade to version 6.0.16r11 or later
  3. 3. For Varnish Cache (Vinyl) 9: Upgrade to version 9.0.1 or later
  4. 4. Backup all Varnish configuration files (vcl files, param files) before upgrading
  5. 5. Review the Varnish upgrade documentation for your specific distribution for proper upgrade procedures
  6. 6. After upgrade, restart the Varnish service to load the fixed version
  7. 7. Verify the upgrade was successful by checking the version with 'varnishd -V'
  8. 8. Test HTTP/2 connections to confirm the workspace overflow issue is resolved
Caveat Minor: Review release notes for any configuration or behavior changes between versions; ensure custom VCL syntax compatibility with the new release

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Varnish Enterprise Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing6.0 h
  • Review / QA2.0 h
20.0 hours of engineering $3,500
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-40394 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data