CVE-2026-40395: Varnish-Software Varnish Enterprise

High severity, CVSS 7.5. EPSS: 0.4% chance of exploitation in the next 30 days.

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.

Affected products

• Varnish-Software Varnish Enterprise: up to and including 6.0.15; version 6.0.16 only

Published 2026-04-12. Last modified 2026-06-17.