Varnish Cache: Incorrect Behavior Order: Validate Before Canonicalize
CVE-2026-34475 is a critical-severity incorrect behavior order: validate before canonicalize vulnerability in Varnish Cache from Varnish-software, with a CVSS 3.1 base score of 9.8 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, without any action from a user. The record lists versions 6.0.17 LTS, 8.0.1 as the first without the flaw, so updating Varnish Cache to one of those releases or later removes the exposure. There is no confirmed exploitation so far. EPSS estimates a 0.21% chance of exploitation within 30 days, which is low compared with other CVEs.
Critical severity: a successful attack would have severe impact.
What to do
- Update Varnish Cache to 6.0.17 LTS or 8.0.1, whichever matches your release line, or a later version.
- Update Vinyl Cache to 8.0.1 or a later version.
- Read the advisory on vinyl-cache.org for workarounds and exact fixed builds.
- Until you can update, restrict network access to the affected service so it is not reachable from untrusted networks.
Affected products and versions
| Vendor | Product | Affected versions | Fixed in |
|---|---|---|---|
| Varnish-software | Varnish Cache | before 6.0.17 LTS; 7.0.0 up to, but not including, 8.0.1 | 6.0.17 LTS, 8.0.1 |
| Varnish-software | Varnish Enterprise | 6.0.15 and earlier; 6.0.16 | Not stated |
| Vinyl-cache | Vinyl Cache | before 8.0.1 | 8.0.1 |
Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.
How the severity score breaks down
- Attack vector
- NetworkCan be exploited remotely over a network, including the internet.
- Attack complexity
- LowNo special conditions are needed; the attack works reliably.
- Privileges required
- NoneNo account or login is needed.
- User interaction
- NoneNo action by a victim is required.
- Scope
- UnchangedImpact stays within the vulnerable component.
- Confidentiality
- HighAll data handled by the component can be read.
- Integrity
- HighData or code can be modified at will.
- Availability
- HighThe component can be taken down completely.
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H. Scored by NVD.
| Scoring system | Score | Severity | Scored by |
|---|---|---|---|
| CVSS 3.1 | 5.4 | Medium | CNA (mitre.org) |
| CVSS 3.1 | 9.8 | Critical | NVD |
Official 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.
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Incorrect Behavior Order: Validate Before Canonicalize
The product validates input before it is canonicalized.
How developers prevent this class of flaw
- Inputs should be decoded and canonicalized to the application's current internal representation before being validated (CWE-180). Make sure that the application does not decode the same input twice (CWE-174). Such errors could be used to bypass allowlist validation schemes by introducing dangerous inputs after they have been checked.
More Incorrect Behavior Order: Validate Before Canonicalize vulnerabilities (CWE-180)
Timeline
- CVE record published.
- CISA published its SSVC assessment (exploitation: none, automatable: no, technical impact: partial).
- Record last changed materially (score, affected products, status or references).
References
Advisories
- vinyl-cache.org/security/VSV00018.htmlMitigation
Other vulnerabilities in Varnish Cache
- 7.5 High severity
- 7.5 High severity
- 7.5 High severity
- 7.5 High severity
- 9.1 Critical severity
Recent Incorrect Behavior Order: Validate Before Canonicalize vulnerabilities
- 9.8 Critical severity
- 7.1 High severity
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