Varnish Cache 9 before 9.0.1 allows a "workspace overflow" denial of service (daemon panic) after timeout_linger. A malicious client could send an HTTP/1 request, wait long enough until the session releases its worker thread (timeout_linger) and resume traffic before the session is closed (timeout_idle) sending more than one request at once to trigger a pipelining operation between requests. This vulnerability affecting Varnish Cache 9.0.0 emerged from a port of the Varnish Enterprise non-blocking architecture for HTTP/2. New code was needed to adapt to a more recent workspace API that formalizes the pipelining operation. In addition to the workspace change on the Varnish Cache side, other differences created merge conflicts, like partial support for trailers in Varnish Enterprise. The conflict resolution missed one code path configuring pipelining to perform a complete workspace rollback, losing the guarantee that prefetched data would fit inside workspace_client during the transition from one request to the next. This can result in a workspace overflow, triggering a panic and crashing the Varnish server.
History

Mon, 13 Apr 2026 17:15:00 +0000

Type Values Removed Values Added
Metrics ssvc

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


Mon, 13 Apr 2026 14:30:00 +0000

Type Values Removed Values Added
Title Workspace Overflow Leading to Denial of Service in Varnish Cache 9.0.0

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

Type Values Removed Values Added
Description Varnish Cache 9 before 9.0.1 allows a "workspace overflow" denial of service (daemon panic) after timeout_linger. A malicious client could send an HTTP/1 request, wait long enough until the session releases its worker thread (timeout_linger) and resume traffic before the session is closed (timeout_idle) sending more than one request at once to trigger a pipelining operation between requests. This vulnerability affecting Varnish Cache 9.0.0 emerged from a port of the Varnish Enterprise non-blocking architecture for HTTP/2. New code was needed to adapt to a more recent workspace API that formalizes the pipelining operation. In addition to the workspace change on the Varnish Cache side, other differences created merge conflicts, like partial support for trailers in Varnish Enterprise. The conflict resolution missed one code path configuring pipelining to perform a complete workspace rollback, losing the guarantee that prefetched data would fit inside workspace_client during the transition from one request to the next. This can result in a workspace overflow, triggering a panic and crashing the Varnish server.
First Time appeared Varnish-software
Varnish-software varnish Cache
Weaknesses CWE-670
CPEs cpe:2.3:a:varnish-software:varnish_cache:*:*:*:*:*:*:*:*
Vendors & Products Varnish-software
Varnish-software varnish Cache
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'}


cve-icon MITRE

Status: PUBLISHED

Assigner: mitre

Published: 2026-04-12T19:23:03.408Z

Updated: 2026-04-13T15:35:43.858Z

Reserved: 2026-04-12T19:23:00.553Z

Link: CVE-2026-40396

cve-icon Vulnrichment

Updated: 2026-04-13T15:35:38.198Z

cve-icon NVD

Status : Awaiting Analysis

Published: 2026-04-12T20:16:19.057

Modified: 2026-04-13T15:01:43.663

Link: CVE-2026-40396

cve-icon Redhat

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