CVE-2026-42959 — Crash during DNSSEC validation of malicious content

Description (CNA)

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.

Affected (CNA)

Vendor / product	Versions
NLnet Labs / Unbound	affected 0 before 1.25.1

CWE

CWE-824

References

https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-42959.txt

ADP enrichment

CISA-ADP

No English description supplied.

No references supplied.

redhat-SADP

A flaw was found in Unbound's DNSSEC validator when constructing chase-reply messages for validation. The code uses the wrong counter to calculate write offsets for ADDITIONAL section resource record sets. When a DNAME chain is combined with authority filtering, an uninitialized array slot is created that the validator later dereferences, causing an immediate process crash. Any application or infrastructure relying on Unbound for DNS resolution could be forced to exit unexpectedly, resulting in a denial-of-service condition.