CyberRota Analysis
AI-GeneratedUnbound versions 1.13.2 through 1.25.1 are vulnerable due to an off-by-one error in the 'harden-below-nxdomain' logic, which can lead to the shadowing of stub or forward zones by secure NXDOMAIN responses from parent zones. This vulnerability effectively prevents the configured stub/forward zone from being contacted, potentially disrupting DNS resolution for affected configurations. Organizations using these versions of Unbound, particularly those relying on stub or forward zones within DNSSEC environments, should prioritize addressing this issue to maintain proper DNS functionality.
Original NVD Description
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).
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