CVE-2026-39155 Knot DNS vulnerability
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.
Browse Denial of Service security risksQuick answer
CZ.NIC Knot DNS should be reviewed and updated if it matches the affected versions. The recommended fix is to apply the vendor-supported patched version or the mitigation steps below, then retest the public website with Fixnx.
Who is affected
Affected versions
- n/a
Fixed versions
- Apply the latest vendor-supported patched version.
How to fix it
CVE-2026-39155 affects Knot DNS and involves denial of service. A remote or untrusted peer may consume excessive resources or crash the affected service. Prioritize deployments that process untrusted input, expose the affected feature, or hold sensitive data. No confirmed fixed version is recorded for CVE-2026-39155; follow the linked vendor advisory, apply its mitigation, and update Knot DNS as soon as a supported patched release is available.
- Inventory every deployment, device, package, plugin, container, build, and environment that uses Knot DNS.
- Compare the installed package, build, firmware, or commit with the affected range in the linked vendor and NVD advisories.
- No confirmed fixed version is recorded for CVE-2026-39155; follow the linked vendor advisory, apply its mitigation, and update Knot DNS as soon as a supported patched release is available.
- Until patched, limit untrusted files and peers, apply resource limits, and isolate the affected parser or service.
- Review configuration, roles, tokens, network paths, and exposed endpoints connected to Knot DNS; remove access that is not required.
- Review application, system, proxy, and audit logs for requests and behavior linked to the affected feature and CVE.
- If exploitation is suspected, isolate affected assets, preserve evidence, rotate exposed credentials or keys, restore trusted data, and rebuild compromised systems before returning them to service.
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Verify the fix
- Confirm every affected asset now runs the fixed vendor-supported release or has the documented mitigation for CVE-2026-39155 in place.
- Use a safe regression test to confirm the affected denial of service path no longer produces the vulnerable behavior.
- Confirm temporary access controls, endpoint restrictions, network blocks, and least-privilege settings remain effective after the update.
- Review post-remediation logs and rerun Fixnx plus the relevant package, dependency, firmware, browser, or product-specific security check.
- Document affected assets, versions, changes, validation evidence, reviewed logs, and any credential rotation, cleanup, or replacement completed.
Related categories
Related security risks
More published guidance from the same primary category.
Trusted references
FAQ
What is affected by CVE-2026-39155?
CZ.NIC Knot DNS versions listed as affected should be reviewed: n/a.
What should I fix first?
Start with internet-facing sites, admin panels, login flows, plugins, themes, modules, packages, and systems that process user-controlled input or sensitive data.
How do I confirm the fix worked?
Apply the patched version or mitigation, clear caches where relevant, retest the affected workflow, and run a new Fixnx scan to verify public website exposure signals.
How are Fixnx security risk categories chosen?
Fixnx keeps one canonical risk page and assigns only broad, relevant categories such as ecosystem, technology area, or vulnerability class.