Unbound DNSSEC Validator DoS via Off-by-Counter in ADDITIONAL Section 1.25.0
CVE-2026-42959 Published on May 20, 2026
Crash during DNSSEC validation of malicious content
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.
Timeline
Issue reported by Qifan Zhang
NLnet Labs shares patch 14 days later.
Qifan Zhang verifies patch
Fixes released with version 1.25.1 21 days later.
Weakness Type
Access of Uninitialized Pointer
The program accesses or uses a pointer that has not been initialized.
Products Associated with CVE-2026-42959
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Affected Versions
NLnet Labs Unbound:- Before 1.25.1 is affected.