CVE-2026-40396
MediumCVSS 4.0Exploitation Probability (EPSS)
Low risk28th percentile - higher than 28% of all known CVEs
Summary
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 until the session releases its worker thread (timeout_linger), and resume traffic before the session is closed (timeout_idle) by sending more than one request at once to trigger pipelining and workspace overflow. The vulnerability stems from a porting error of the non-blocking architecture from Varnish Enterprise.
Risk Assessment
Potential panic and crash of the Varnish server triggered by a malicious client, leading to service unavailability. Requires crafted HTTP/1 traffic.
Recommendation
Update Varnish Cache to version 9.0.1 or later. Until updated, consider monitoring and rate-limiting unusual HTTP/1 traffic.
Other vulnerabilities in Varnish Cache
See all- CVE-2026-50052Low
In Vinyl Cache before 9.0.1 and Varnish Cache before 9.0.3, a deficiency in HTTP/2 request parsing can be exploited for backend request desync (request smuggling), leading to cache poisoning, authentication bypass, or information disclosure. The attack only works if HTTP/2 is enabled (disabled by default).
- CVE-2026-40394Medium
Varnish Cache 9 before 9.0.1 and Varnish Enterprise before 6.0.16r11 allow a "workspace overflow" denial of service (daemon panic) for certain amounts of prefetched data. During HTTP/2 upgrade, a buffer allocation splits the workspace, and the next fetch may run out of workspace.
- CVE-2026-34475Medium
Varnish Cache before 8.0.1 and Varnish Enterprise before 6.0.16r12, in certain unchecked req.url scenarios, mishandle URLs with a path of / for HTTP/1.1. This may lead to cache poisoning or authentication bypass.
Original NVD description (English source)
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.
Vulnerability data from NVD (NIST) · CISA KEV · EPSS