CVE-2026-40394Network attack vector

Daemon crash via HTTP/2 workspace fragmentation

Published Apr 12, 2026 · Updated Apr 12, 2026

Denial of service in Varnish Cache 9.0.0 allows remote attackers to crash the daemon via crafted HTTP/2 session data. The HTTP/2 upgrade path repurposes the speculative HTTP/1 request as stream zero, then splits workspace_client for a send buffer, leaving some prefetched-data layouts without space for the next pipelining fetch. An unauthenticated network client must supply a specific amount of prefetched data, causing a panic that interrupts the caching service.

CVSS severity4.0
Medium
EPSS probability0.24%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Denial of service in Varnish Cache 9.0.0 allows remote attackers to crash the daemon via crafted HTTP/2 session data. The HTTP/2 upgrade path repurposes the speculative HTTP/1 request as stream zero, then splits workspace_client for a send buffer, leaving some prefetched-data layouts without space for the next pipelining fetch. An unauthenticated network client must supply a specific amount of prefetched data, causing a panic that interrupts the caching service.

The record

CVE
CVE-2026-40394
Published
Apr 12, 2026
Updated
Apr 12, 2026
Vendor
Varnish Software
Product
Varnish Cache
Classifications
CWE-670, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Apr 12, 2026CVE publishedPublication date reported by the CVE source.
  2. Apr 12, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=9.0.0 <9.0.1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Varnish Software · Varnish Cacheversion=9.0.0 <9.0.1

Published CVSS scores

4.0MITRECVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:N/A:L
7.5NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-670Always-Incorrect Control Flow Implementation
T1499.004Endpoint Denial of Service: Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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