High [CVE-2026-44690] Cache poisoning via insufficient RRSIG.Labels validation and premature cache writes

In NLnet Labs Unbound 1.7.0 up to and including 1.25.1, insufficient validation of the RRSIG.Labels field combined with premature cache writes during RFC 8198 aggressive NSEC processing leads to cache poisoning that permits a malicious actor controlling a single delegated zone to poison arbitrary sibling zones under NSEC-signed parent domains.

A malicious actor with one registered domain under an NSEC-signed TLD can serve malicious insecure DNS responses for unrelated sibling domains (sharing the same parent zone).

Arbitrary delegations that do not exist under the parent domain and are covered by the parent's NSEC chain can be brought into insecure existence by fraudulent wildcard DS records (less labels than expected, unknown algorithm) from the malicious sibling domain. A flaw was found in Unbound.

The primary impact is the integrity of DNS resolution, potentially leading to users being directed to malicious websites or services. This Important flaw in Unbound allows a remote attacker to perform DNS cache poisoning.

By controlling a delegated zone, a malicious actor can manipulate DNS resolution for arbitrary sibling zones under NSEC-signed parent domains, potentially redirecting users to malicious destinations. This poses a significant integrity risk to systems relying on Unbound for DNS resolution.