VulnerabilityPublished 12 Apr 2026

CVE-2026-40396: Varnish Cache · Always-Incorrect Control Flow Implementation

Affects Varnish CacheSource CVE Database V5

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Not known to be exploited · CVSS 4.0 Medium : upgrade to 9.0.1 when convenient.

Data as of 29 Oct 2026, 19:04 UTC

CVSS 3.1 base score

4.0Medium

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.3% chance of exploitation in 30 days – higher than 28% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 9.0.1

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:N/A:L

Attack Vector: Network
Attack Complexity: High
Privileges Required: None
User Interaction: None
Scope: Changed
Confidentiality: None
Integrity: None
Availability: Low

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: High: A successful attack depends on conditions beyond the attacker's control. That is, a successful attack cannot be accomplished at will, but requires the attacker to invest in some measurable amount of effort in preparation or execution against the vulnerable component before a successful attack can be expected. For example, a successful attack may require an attacker to: gather knowledge about the environment in which the vulnerable target/component exists; prepare the target environment to improve exploit reliability; or inject themselves into the logical network path between the target and the resource requested by the victim in order to read and/or modify network communications (e.g., a man in the middle attack).
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Changed: An exploited vulnerability can affect resources beyond the security scope managed by the security authority of the vulnerable component. In this case, the vulnerable component and the impacted component are different and managed by different security authorities.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: Low: Performance is reduced or there are interruptions in resource availability. Even if repeated exploitation of the vulnerability is possible, the attacker does not have the ability to completely deny service to legitimate users. The resources in the impacted component are either partially available all of the time, or fully available only some of the time, but overall there is no direct, serious consequence to the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 12 Apr 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 29 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade Varnish Cache to 9.0.1.

Affected software

Product	Affected	Fixed in
varnish-softwareVarnish Cache	Affected >=9.0.0 <9.0.1	Fixed in 9.0.1
GitHubvarnish/varnish`pkg:github/varnish/varnish`Other threats	Affected =9.0.0	Fixed in–

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

If you cannot patch yet, consider restricting access to Varnish Cache 9.0.0 instances to trusted clients and monitor for unusual request patterns that may indicate attempts to exploit pipelining behavior.

Description

Varnish Cache version 9.0.0 contains a vulnerability that allows a denial of service via a workspace overflow leading to a daemon panic. This occurs when a malicious client sends an HTTP/1 request, waits for the session to release its worker thread (timeout_linger), and then resumes traffic before the session closes (timeout_idle), sending multiple requests simultaneously to trigger pipelining. The issue stems from incomplete handling of workspace rollback during pipelining, causing prefetched data to exceed workspace limits and crash the server.

Impact

The vulnerability causes a denial of service by crashing the Varnish Cache server daemon. There is no indication of confidentiality or integrity impact. Exploitation requires network access and precise timing to trigger pipelining between requests. No known exploits are reported in the wild.

Summary

CVE-2026-40396 is a medium severity vulnerability in Varnish Cache 9.0.0 where improper control flow in the pipelining implementation causes a workspace overflow. This overflow triggers a daemon panic (crash) when a client exploits timing windows between timeout_linger and timeout_idle by sending multiple HTTP/1 requests in a pipelined manner. The root cause is a missed code path during merge conflict resolution adapting Varnish Enterprise's non-blocking HTTP/2 architecture to Varnish Cache, resulting in a failure to perform a complete workspace rollback and losing the guarantee that prefetched data fits within workspace_client. This leads to a denial of service condition.

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 20 Apr 2026, 06:21 UTC

WeaknessesCWE-670

Technical details

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