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Vulnerabilities

Vulnerability · CVE-2026-42959

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.

Published May 20, 2026 · catalog updated Sep 1, 2026 · NLnet Labs Unbound up

Check if you run it

No confirmed exploitation is on record for CVE-2026-42959. EPSS is 0.72%. Act only if a listed version of NLnet Labs Unbound up is installed.

Gaps on this record

  • The stored product name is a package slug. The name on this page is taken from the title (NLnet Labs Unbound up).
  • No version range is stored. A customer cannot check an installed version against this record.
  • No vendor fix or required action is stored. Compare the linked advisory before you change production.
Severity
High8.7
Known exploitation
Not in CISA/VulnCheck KEV
EPSS
0.72%
Remediation
Advisory link
Affected products
cpe nlnetlabs/unbound

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.

Are you affected?

Enter the version you run. The result uses only the ranges stored on this record.

The check returns affected, not affected, or incomplete. It does not guess a version that is missing from the record.

  • cpe nlnetlabs/unboundversion unspecified

How to fix CVE-2026-42959

What to do

Three questions, each answered from this record only. Confirm the vendor advisory before you patch.

Should you patch?

No confirmed exploitation is on record for CVE-2026-42959. EPSS is 0.72%. Act only if a listed version of NLnet Labs Unbound up is installed.

Which versions are affected?

Affected version ranges are not on this record. Compare the installed version with the linked vendor advisory before you change production.

What should an analyst verify?

CWE-824: Access of Uninitialized Pointer. EPSS 0.72%. There is no CISA KEV listing. No Exploit-DB or Metasploit reference is linked. Use the source links for the original advisory.

Code

The project this record applies to. Exploit listings stay in References.

No public source repository is identified for this product.

Timeline

Detection, updates, and resolution for this record.

  1. May 20, 2026 · NIST NVD

    Detected

    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 fi

  2. Sep 1, 2026 · NIST NVD

    Updated

    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 fi

Sources

  • CVE.orgVulnerability recordCVE-2026-42959

    Official CVE identifier

    cve.org
  • FIRST EPSSExploitation probabilityCVE-2026-42959

    30-day exploitation probability 0.7%

    first.org
  • FreeBSD SecurityVendor advisoryCVE-2026-42959

    CVE-2026-42959 is named by FreeBSD Security.

    freebsd.org
  • NIST NVDVulnerability recordCVE-2026-42959

    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

    nvd.nist.gov

References

Freshness

Last verified in this catalog Oct 18, 2026. Significant claims (KEV, EPSS, CVSS, affected ranges) come from the source rows above — open the original publisher for the authoritative advisory.