CVE-2026-34475
Published: 27 March 2026
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 10.1th 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).
Deeper analysis
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.
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-16801
Vulnerability details
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.
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise TechniquesAI
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.
CVEs Like This One
Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
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.
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.
Mitigates remote attacks on the public-facing Varnish Cache proxy by monitoring and controlling boundary communications to filter anomalous HTTP/1.1 requests.