Exploiting CVE-2026-40396 requires specific conditions, leading to a moderate security impact.
Status: Received on 12 Apr 2026, 20:16 UTC
Published on: 12 Apr 2026, 20:16 UTC
CVSS Release: version 3
cve@mitre.org
Secondary
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:N/A:L
CVE-2026-40396: 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.
The exploitability of CVE-2026-40396 depends on two key factors: attack complexity (the level of effort required to execute an exploit) and privileges required (the access level an attacker needs).
CVE-2026-40396 presents a challenge to exploit due to its high attack complexity, but the absence of privilege requirements still makes it a viable target for skilled attackers. A thorough security review is advised.
A lower complexity and fewer privilege requirements make exploitation easier. Security teams should evaluate these aspects to determine the urgency of mitigation strategies, such as patch management and access control policies.
Attack Complexity (AC) measures the difficulty in executing an exploit. A high AC means that specific conditions must be met, making an attack more challenging, while a low AC means the vulnerability can be exploited with minimal effort.
Privileges Required (PR) determine the level of system access necessary for an attack. Vulnerabilities requiring no privileges are more accessible to attackers, whereas high privilege requirements limit exploitation to authorized users with elevated access.
Above is the CVSS Sub-score Breakdown for CVE-2026-40396, illustrating how Base, Impact, and Exploitability factors combine to form the overall severity rating. A higher sub-score typically indicates a more severe or easier-to-exploit vulnerability.
Below is the Impact Analysis for CVE-2026-40396, showing how Confidentiality, Integrity, and Availability might be affected if the vulnerability is exploited. Higher values usually signal greater potential damage.
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