Executive brief
Varnish Enterprise, a high-performance web accelerator and caching server, is vulnerable to a denial-of-service attack. By sending a specially crafted web request with an excessive number of headers, a malicious actor can cause the server to crash. This results in a service outage, preventing legitimate users from accessing the websites or applications protected by the Varnish server.
Technical details
A workspace overflow vulnerability exists in Varnish Enterprise's vmod_headerplus module. The headerplus.write_req0() function is used to update the underlying req0 object (the original read-only request) before transitioning between VCL (Varnish Configuration Language) labels, a common pattern in shared VCL deployments managed by Varnish Controller. If an incoming request is modified to contain more header fields than the req0 workspace can accommodate, the function triggers a workspace overflow. This results in a daemon panic and immediate crash of the Varnish server. The issue is resolved in version 6.0.16r12.
Affected products
- Varnish Software Varnish Enterprise 6.0.9r5 through 6.0.16r11
Timeline
- 2026-04-09: advisory: Vendor advisory published by Varnish Software
- 2026-04-12: disclosed: CVE-2026-40395 published
- 2026-04-12: patched: Fixed in Varnish Enterprise 6.0.16r12