Vulnerability · Published Apr 12, 2026 · Assigned by mitre
CVE-2026-40395
Medium 4.0
In plain English
A medium-severity resource allocation without limits in Varnish Enterprise from varnish-software. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 15% of all known vulnerabilities).
Official 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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Ubuntu:24.04:LTS | varnish | no fix listed | UBUNTU-CVE-2026-40395 |
| Ubuntu:25.10 | varnish | no fix listed | UBUNTU-CVE-2026-40395 |
| Ubuntu:26.04:LTS | varnish | no fix listed | UBUNTU-CVE-2026-40395 |
| Ubuntu:26.10 | varnish | no fix listed | UBUNTU-CVE-2026-40395 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| varnish-software | Varnish Enterprise | 6.0.9r5 – before 6.0.16r12 |
Weakness: CWE-770 · Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:N/A:L