CVE-2026-42959

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.

The Red Hat Product Security team has assessed the severity of this vulnerability as Important, given that it can be remotely triggered without authentication or user interaction via a single crafted DNS query. Successful exploitation allows an attacker to crash the Unbound resolver process, disrupting DNS resolution for all dependent services. The vulnerability's root cause is the use of incorrect counters to calculate write offsets, leading to dereference of an uninitialized pointer and subsequent denial-of-service.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.