CVE-2026-40395 Details
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.
A denial-of-service vulnerability has been identified in Varnish Enterprise versions 6.0.9r5 prior to 6.0.16r11. The issue arises in shared VCL deployments, where the 'headerplus.write_req0()' function from 'vmod_headerplus' can cause a 'workspace overflow'. This function updates the 'req0' with header fields from 'req', which is normally read-only. If too many headers are added, it can lead to a workspace overflow, triggering a daemon panic and crashing the Varnish server. This vulnerability can be exploited by malicious clients to cause a denial-of-service condition.
Users are advised to upgrade Varnish Enterprise to version 6.0.16r12 or later and ensure that Varnish is restarted.
Metrics
CVSS 4.0 Severity and Vector Strings:
No CVSS 4.0 data is available for this CVE.
CVSS 3.x Severity and Vector Strings:
No data available for CVSS Version 2.0 on this CVE.
CISA-ADP
Assessed Apr 13, 2026References to Advisories, Solutions, and Tools
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| URL | Source(s) | Tag(s) |
|---|---|---|
| https://docs.varnish-software.com/security/VEV00003/ | [email protected] | Vendor Advisory |
Weakness Enumeration
| CWE-ID | CWE Name | Source |
|---|---|---|
| CWE-770 | Allocation of Resources Without Limits or Throttling | [email protected] |
Affected Products
| Product | Versions |
|---|---|
| varnish-software varnish enterprise | <= 6.0.15 6.0.16 r1 6.0.16 r10 6.0.16 r11 6.0.16 r2 6.0.16 r3 6.0.16 r4 6.0.16 r5 6.0.16 r6 6.0.16 r7 6.0.16 r8 6.0.16 r9 |
CPE
Remediation
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Change History
4 change records found show changes
| Date | Action | Recorded By |
|---|---|---|
| Jun 17, 2026 | CVE Modified | [email protected] |
| Jun 17, 2026 | CVE Modified | CISA-ADP |
| Apr 17, 2026 | Initial Analysis | [email protected] |
| Apr 12, 2026 | New CVE Received | [email protected] |