CVE-2026-42959

HighUpdated

Published: 20 May 2026

Published

20 May 2026

Modified

30 June 2026

KEV Added

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Patch

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CVSS Score v4 8.7 CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:Red

EPSS Score 0.0078 51.3th percentile

Risk Priority 55 floored blend · peak EPSS

Summary

CVE-2026-42959 is a high-severity Access of Uninitialized Pointer (CWE-824) vulnerability in Nlnetlabs Unbound. Its CVSS base score is 8.7 (High).

Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked in the top 48.7% of CVEs by exploit likelihood; it is not currently listed in the CISA KEV catalog.

EU & UK References

🇪🇺 ENISA EUVD: EUVD-2026-31084

Vulnerability details

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.

CWE(s): CWE-824

Related Threats

MITRE ATT&CK Enterprise TechniquesAI

T1190 Exploit Public-Facing Application Initial Access

Adversaries may attempt to exploit a weakness in an Internet-facing host or system to initially access a network.

attack.mitre.org →

T1499.004 Application or System Exploitation Impact

Adversaries may exploit software vulnerabilities that can cause an application or system to crash and deny availability to users.

attack.mitre.org →

Why these techniques?

DoS crash via uninitialized pointer in DNSSEC validator triggered by malicious upstream replies to public-facing Unbound resolver (T1190 + T1499.004).

Confidence: HIGH · MITRE ATT&CK Enterprise v19.0