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CVE-2026-40395

4.0 · MEDIUM

Varnish Enterprise before 6.0.16r12 allows a "workspace overflow" denial of service (daemon panic) for shared VCL.

Published Apr 12, 2026CWE-770EPSS 0.40%

Overview

CVE-2026-40395 is a medium-severity vulnerability. It was published on April 12, 2026 and has a CVSS 3.1 base score of 4.0 (MEDIUM).

This vulnerability has a CVSS 3.1 base score of 4.0, rated MEDIUM. It can be exploited remotely over the network. No authentication or special privileges are required for exploitation.

Technical description

Varnish Enterprise before 6.0.16r12 allows a "workspace overflow" denial of service (daemon panic) for shared VCL. The headerplus.write_req0() function from vmod_headerplus updates the underlying req0, which is normally the original read-only request from which req is derived (readable and writable from VCL). This is useful in the active VCL, after amending req, to prepare a refined req0 before switching to a different VCL with the return (vcl(<label>)) action. This is for example how the Varnish Controller operates shared VCL deployments. If the amended req contained too many header fields for req0, this would have resulted in a workspace overflow that would in turn trigger a panic and crash the Varnish Enterprise server. This could be used as a Denial of Service attack vector by malicious clients.

Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

References

Refdocs.varnish-software.com/security/VEV00003/

How CyberStrike helps

Frequently asked questions

What is CVE-2026-40395?

CVE-2026-40395 is a medium-severity vulnerability. It was published on April 12, 2026 and has a CVSS 3.1 base score of 4.0 (MEDIUM).

How severe is CVE-2026-40395?

This vulnerability has a CVSS 3.1 base score of 4.0, rated MEDIUM. It can be exploited remotely over the network. No authentication or special privileges are required for exploitation.

How do I fix or remediate CVE-2026-40395?

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

How can CyberStrike help with CVE-2026-40395?

CyberStrike is an open-source, AI-augmented pentesting engine. Rather than flag a version number, it tests whether an issue like CVE-2026-40395 is actually reachable in your own applications and APIs, then verifies each finding before reporting it. That tells you if the weakness is a real, exploitable risk in your environment or a false positive you can safely deprioritize.

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