How severe is CVE-2026-42959?

CVE-2026-42959 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability high.

Is CVE-2026-42959 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (51st percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-42959?

CVE-2026-42959 affects Nlnetlabs Unbound. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2026-42959?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.

Does CVE-2026-42959 have an EU (EUVD) identifier?

Yes. CVE-2026-42959 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-31084.

When was CVE-2026-42959 published?

CVE-2026-42959 was published on 2026-05-20 and last updated on 2026-06-30.

CVE-2026-42959

CVE-2026-42959 is a high-severity vulnerability in Nlnetlabs Unbound with a CVSS 3.x base score of 7.5. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-824.

Key facts

Severity: High (CVSS 3.x base score 7.5)
CVSS v4: 8.7
EPSS exploit prediction: 1% (51st percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-31084
Weakness: CWE-824
Affected product: Nlnetlabs Unbound
Published: 2026-05-20
Last modified: 2026-06-30

Description

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.

Frequently asked questions

What is 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. 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 severe is CVE-2026-42959?: CVE-2026-42959 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability high.
Is CVE-2026-42959 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (51st percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-42959?: CVE-2026-42959 affects Nlnetlabs Unbound. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2026-42959?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
Does CVE-2026-42959 have an EU (EUVD) identifier?: Yes. CVE-2026-42959 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-31084.
When was CVE-2026-42959 published?: CVE-2026-42959 was published on 2026-05-20 and last updated on 2026-06-30.

References

Affected products (1)

cpe:2.3:a:nlnetlabs:unbound:*:*:*:*:*:*:*:*

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