CVE-2026-44687
LowCVSS 3.7Exploitation Probability (EPSS)
Low risk15th percentile - higher than 15% of all known CVEs
Summary
In NLnet Labs Unbound 1.13.2 up to and including 1.25.1, an off-by-one error in the 'harden-below-nxdomain' logic allows stub or forward zones below a DNSSEC signed zone to be shadowed by a secure NXDOMAIN answer from the parent, bypassing the configuration.
Risk Assessment
An attacker can exploit this vulnerability to block queries to configured zones, leading to DNS service disruption and potential traffic hijacking.
Recommendation
Update Unbound to version 1.25.2 or later which includes the fix. As a workaround, disable the 'harden-below-nxdomain' option.
Original NVD description (English source)
In NLnet Labs Unbound 1.13.2 up to and including 1.25.1, stub or forward zones where the name is below an intermediate labed below a DNSSEC signed zone could be shadowed by the intermediate label's secure NXDOMAIN answer from the parent. This is caused by an off-by-one error in 'harden-below-nxdomain' logic; enabled by default. It effectively bypasses the configuration and the configured stub/forward zone is never contacted. 'harden-below-nxdomain' does an upward DNS cache walk together with a delegation point guard that does not allow NXDOMAIN synthesis above stub/forward zones. The guard tests the domain name but before stripping a label. This results in an iteration where the domain name equals the configured stub/forward zone apex that passes the guard, strips one more label, and probes the cache at the apex's immediate public parent. If that parent has a cached DNSSEC-secure NXDOMAIN, which it will for any private namespace nested two or more labels under a signed public name, the walk returns it and the configured stub/forward upstream is never contacted. This can only be triggered by the query for the intermediate label (between the stub/forward apex and the DNSSEC parent zone).