CVE-2026-39155

Deferred Deferred - Pending Action

Knot DNS NSEC Owner Name Computation Flaw

Publication date: 2026-07-23

Last updated on: 2026-07-30

Assigner: MITRE

Description

Knot DNS before 3.4.10 and 3.5.x before 3.5.4 contains a vulnerability in mod-onlinesign where the next NSEC owner name can be computed incorrectly. This can create an overly broad authenticated denial interval, allowing downstream validating resolvers using aggressive negative caching to synthesize negative answers for legitimate names and causing resolver-side denial of service.

CVSS Scores

EPSS Scores

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Meta Information

Published

2026-07-23

Last Modified

2026-07-30

Generated

2026-08-13

AI Q&A

2026-07-24

EPSS Evaluated

2026-08-11

NVD

CVE-2026-39155

Affected Vendors & Products

Vendor	Product	Version / Range
knot_dns	knot_dns	to 3.4.10 (exc)
knot_dns	knot_dns	to 3.5.4 (exc)

Helpful Resources

Exploitability

CWE

KEV

CWE ID	Description
CWE-345	The product does not sufficiently verify the origin or authenticity of data, in a way that causes it to accept invalid data.

Attack-Flow Graph

Executive Summary

Knot DNS before versions 3.4.10 and 3.5.x before 3.5.4 has a flaw in the mod-onlinesign module where the next NSEC owner name is computed incorrectly. This leads to an overly broad authenticated denial interval, which can cause downstream validating resolvers with aggressive negative caching to generate false negative responses for valid names, resulting in a denial of service at the resolver level.

Detection Guidance

This vulnerability affects Knot DNS versions before 3.4.10 and 3.5.x before 3.5.4. To detect it, check your Knot DNS version using 'knotd --version'. If you are running an affected version, update to 3.4.10 or 3.5.4 or later immediately.

Impact Analysis

If you use a vulnerable version of Knot DNS, your DNS resolver might incorrectly block access to legitimate domain names due to synthesized negative answers. This can disrupt network services, prevent users from reaching valid websites, and cause widespread connectivity issues.

Compliance Impact

This vulnerability does not directly affect compliance with GDPR, HIPAA, or similar standards as it pertains to DNS resolver functionality and denial of service rather than data protection or privacy controls.

Mitigation Strategies

Upgrade Knot DNS to version 3.4.10 or later for the 3.4.x branch, or to version 3.5.4 or later for the 3.5.x branch to address the vulnerability in mod-onlinesign.

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