CVE-2026-34475

Medium

Published: 27 March 2026

Published

27 March 2026

Modified

22 April 2026

KEV Added

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Patch

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CVSS Score 5.4 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:L/I:L/A:N

EPSS Score 0.0007 20.9th percentile

Risk Priority 11 60% EPSS · 20% KEV · 20% CVSS

Summary

CVE-2026-34475 is a medium-severity Incorrect Behavior Order: Validate Before Canonicalize (CWE-180) vulnerability in Varnish-Software Varnish Enterprise. Its CVSS base score is 5.4 (Medium).

Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked at the 20.9th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.

The strongest mitigations our analysis identified are NIST 800-53 SI-10 (Information Input Validation) and SI-2 (Flaw Remediation).

Threat & Defense at a Glance

What attackers do: exploitation maps to Exploit Public-Facing Application (T1190) and 1 other technique. What defenders deploy: see the NIST 800-53 controls recommended below.

Threat & Defense Details

Mitigating Controls (NIST 800-53 r5)AI

SI-2 Flaw Remediation good match

prevent

Directly addresses CVE-2026-34475 by mandating timely flaw remediation through patching Varnish Cache to version 8.0.1 or later, eliminating the root-path URL mishandling.

SI-10 Information Input Validation good match

prevent

Prevents exploitation of unchecked req.url scenarios by validating HTTP/1.1 request inputs, blocking crafted root-path URLs that lead to cache poisoning or authentication bypass.

SC-7 Boundary Protection partial match

preventdetect

Mitigates remote attacks on the public-facing Varnish Cache proxy by monitoring and controlling boundary communications to filter anomalous HTTP/1.1 requests.

MITRE ATT&CK Enterprise TechniquesAI

T1190 Exploit Public-Facing Application Initial Access

Adversaries may attempt to exploit a weakness in an Internet-facing host or system to initially access a network.

attack.mitre.org →

T1189 Drive-by Compromise Initial Access

Adversaries may gain access to a system through a user visiting a website over the normal course of browsing.

attack.mitre.org →

Why these techniques?

CVE directly describes remote exploitation of a public-facing Varnish HTTP cache/proxy (T1190) that enables web cache poisoning to serve malicious responses to users (T1189) plus authentication bypass to backend resources.

Confidence: HIGH · MITRE ATT&CK Enterprise v18.1

NVD Description

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, potentially leading to cache poisoning or authentication bypass.

Deeper analysisAI

CVE-2026-34475 is a vulnerability in Varnish Cache before version 8.0.1 and Varnish Enterprise before 6.0.16r12. In certain unchecked req.url scenarios, the software mishandles URLs with a path of / for HTTP/1.1 requests, potentially leading to cache poisoning or authentication bypass. The issue is classified under CWE-180 and carries a CVSS v3.1 base score of 5.4 (AV:N/AC:H/PR:N/UI:N/S:C/C:L/I:L/A:N), indicating medium severity with network accessibility, high attack complexity, no privileges or user interaction required, and a changed scope affecting low confidentiality and integrity.

Remote, unauthenticated attackers can exploit this vulnerability over the network by crafting specific HTTP/1.1 requests that trigger the mishandling of root-path URLs. Exploitation requires sophisticated manipulation due to the high attack complexity but could enable cache poisoning, where malicious responses are stored and served to subsequent users, or authentication bypass, allowing unauthorized access to backend resources without valid credentials.

Mitigation involves upgrading to Varnish Cache 8.0.1 or later and Varnish Enterprise 6.0.16r12 or later. Further details on patches and workarounds are provided in the vendor advisory at https://vinyl-cache.org/security/VSV00018.html.