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CVE-2026-42959 is a denial-of-service vulnerability in NLnet Labs Unbound up to and including version 1.25.0. The flaw resides in the DNSSEC validator while constructing chase-reply messages for validation. The implementation uses the wrong counter when calculating write offsets for rrsets in the ADDITIONAL section. Under specific malformed reply conditions, DNAME duplication can increase the ANSWER section count while authority filtering decreases the AUTHORITY section count, leaving an uninitialized array slot. During later validation processing, Unbound dereferences this uninitialized pointer, causing an immediate crash of the resolver process. The issue can be triggered by malicious upstream replies associated with a DNSSEC-signed domain and is fixed in Unbound 1.25.1 by using the proper counters for write-offset calculation.
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