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.
Published: 2026-04-12
Score: 4 Medium
EPSS: < 1% Very Low
KEV: No
Impact: Denial of Service
Action: Apply Patch
AI Analysis

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Remediation

No vendor fix or workaround currently provided.

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Tracking

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Advisories

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History

Fri, 17 Apr 2026 14:45:00 +0000

Type Values Removed Values Added
CPEs cpe:2.3:a:varnish-software:varnish_enterprise:6.0.16:r10:*:*:*:*:*:*
cpe:2.3:a:varnish-software:varnish_enterprise:6.0.16:r11:*:*:*:*:*:*
cpe:2.3:a:varnish-software:varnish_enterprise:6.0.16:r1:*:*:*:*:*:*
cpe:2.3:a:varnish-software:varnish_enterprise:6.0.16:r2:*:*:*:*:*:*
cpe:2.3:a:varnish-software:varnish_enterprise:6.0.16:r3:*:*:*:*:*:*
cpe:2.3:a:varnish-software:varnish_enterprise:6.0.16:r4:*:*:*:*:*:*
cpe:2.3:a:varnish-software:varnish_enterprise:6.0.16:r5:*:*:*:*:*:*
cpe:2.3:a:varnish-software:varnish_enterprise:6.0.16:r6:*:*:*:*:*:*
cpe:2.3:a:varnish-software:varnish_enterprise:6.0.16:r7:*:*:*:*:*:*
cpe:2.3:a:varnish-software:varnish_enterprise:6.0.16:r8:*:*:*:*:*:*
cpe:2.3:a:varnish-software:varnish_enterprise:6.0.16:r9:*:*:*:*:*:*

Mon, 13 Apr 2026 16:15:00 +0000

Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'no', 'Exploitation': 'none', 'Technical Impact': 'partial'}, 'version': '2.0.3'}


Mon, 13 Apr 2026 12:15:00 +0000

Type Values Removed Values Added
Title Varnish: Varnish Enterprise: Denial of Service via workspace overflow
References
Metrics threat_severity

None

threat_severity

Moderate


Sun, 12 Apr 2026 19:30:00 +0000

Type Values Removed Values Added
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.
First Time appeared Varnish-software
Varnish-software varnish Enterprise
Weaknesses CWE-770
CPEs cpe:2.3:a:varnish-software:varnish_enterprise:*:*:*:*:*:*:*:*
Vendors & Products Varnish-software
Varnish-software varnish Enterprise
References
Metrics cvssV3_1

{'score': 4, 'vector': 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:N/A:L'}


Subscriptions

Varnish-software Varnish Enterprise
cve-icon MITRE

Status: PUBLISHED

Assigner: mitre

Published:

Updated: 2026-04-13T15:45:30.791Z

Reserved: 2026-04-12T19:21:08.847Z

Link: CVE-2026-40395

cve-icon Vulnrichment

Updated: 2026-04-13T15:45:27.838Z

cve-icon NVD

Status : Analyzed

Published: 2026-04-12T20:16:18.893

Modified: 2026-04-17T14:37:34.147

Link: CVE-2026-40395

cve-icon Redhat

Severity : Moderate

Publid Date: 2026-04-12T19:21:09Z

Links: CVE-2026-40395 - Bugzilla

cve-icon OpenCVE Enrichment

Updated: 2026-04-13T12:54:01Z

Weaknesses