CVE-2026-42959
MEDIUMDescription
NLnet Labs Unbound up to and including version 1.25.0 has a denial of service vulnerability in the DNSSEC validator that can lead to a crash given malicious upstream replies. When Unbound constructs chase-reply messages for validation, the code uses the wrong counter to calculate write offsets for ADDITIONAL section rrsets. DNAME duplication could increase the ANSWER section count and authority filtering could decrease the AUTHORITY section count and create an uninitialized array slot. Combining these two, the validator later dereferences this uninitialized pointer, causing an immediate process crash. An adversary controlling a DNSSEC-signed domain can trigger this bug with a single query by configuring a DNAME chain with unsigned CNAMEs and a response containing unsigned AUTHORITY records alongside signed ADDITIONAL glue records. Unbound 1.25.1 contains a patch with a fix to use the proper counters to calculate the write offsets.
How to fix
This vulnerability affects an unusually large number of packages. The highest-confidence fixes are shown above; the full list is longer. Check the referenced advisories for complete coverage.
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Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.
CVSS v3 Vector
Exploitability
Impact
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Exploit Intelligence
Moderate risk: more likely to be exploited than 53% of all known CVEs.
References
Related Vulnerabilities
Other CWE-824 vulnerabilities, ordered by exploit likelihood. View all
| CVE | Severity | CVSS | EPSS | Exploited | Fix |
|---|---|---|---|---|---|
| CVE-2018-9948 | Medium | 6.5 | 64% | - | - |
| CVE-2018-11803 | High | 7.5 | 58% | - | Fix |
| CVE-2022-21971 | High | 7.8 | 54% | KEV | Fix |
| CVE-2010-1818 | High | 9.3 | 43% | - | - |
| CVE-2015-1770 | High | 8.8 | 35% | KEV | - |
| CVE-2017-12561 | Critical | 9.8 | 31% | - | - |
Common questions
How do I fix CVE-2026-42959?
Published advisories record a fix for 30 affected products. The "How to fix" section on this page lists the fixed version and source advisory for each one, so apply the entry matching what you actually run.
Is CVE-2026-42959 being actively exploited?
Not that we know of. CVE-2026-42959 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 0.78% is the estimated probability that it will be exploited in the next 30 days. That is higher than 53% of all scored CVEs.
How severe is CVE-2026-42959?
CVE-2026-42959 has a CVSS v3 base score of 7.5, rated moderate. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.
What does CVE-2026-42959 affect?
Published advisories record a fix for unbound (Debian), python3-unbound (Rocky), python3-unbound (Red Hat / RHEL), python3-unbound-debuginfo (Rocky), and 26 more. Only products with a sourced advisory are listed, so treat this as what we can cite rather than a complete inventory.
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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-07-24.