CVE-2026-44687 in Unbound
Summary
by MITRE • 07/22/2026
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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Analysis
by VulDB Data Team • 07/23/2026
This vulnerability exists in NLnet Labs Unbound DNS resolver versions 1.13.2 through 1.25.1, specifically affecting stub or forward zones that are positioned below an intermediate label within a DNSSEC signed zone. The issue stems from a critical off-by-one error in the harden-below-nxdomain functionality, which operates by default and fundamentally undermines the intended configuration of these zones. When a query is made for a domain name that falls between the stub/forward zone apex and its DNSSEC parent zone, the resolver's logic fails to properly account for label stripping during cache traversal.
The technical flaw manifests through an improper implementation of the delegation point guard mechanism that is designed to prevent NXDOMAIN synthesis above stub/forward zones. This guard performs its validation test on the domain name before the label is stripped from the query, creating a condition where an iteration can occur with the domain name matching exactly the configured stub/forward zone apex. Once this match occurs, the system proceeds to strip one additional label and then probes the cache at the immediate public parent of the apex. When that parent has a cached DNSSEC-secure NXDOMAIN response, particularly in scenarios involving private namespaces nested two or more labels under signed public names, the resolver incorrectly returns this cached response instead of contacting the configured stub/forward upstream server.
This vulnerability directly maps to CWE-129 and CWE-130 within the Common Weakness Enumeration framework, representing issues with input validation and improper handling of boundary conditions in DNS resolution logic. The operational impact is severe as it allows attackers to bypass configured stub or forward zone settings entirely, potentially redirecting queries through unintended paths or preventing access to specific services that rely on these configurations. The attack vector specifically requires targeting the intermediate label between the stub/forward zone apex and its DNSSEC parent, making it particularly insidious for organizations with complex DNS infrastructures where private zones are nested under public signed domains.
The implications extend beyond simple configuration bypasses into potential security consequences when considering the ATT&CK framework's T1071.3 technique related to application layer protocol tunneling and command and control communications. Organizations relying on Unbound for DNS resolution, particularly those implementing stub or forward zones for specific upstream servers, face a critical risk where their network traffic could be silently redirected or blocked based on this vulnerability. The default enablement of harden-below-nxdomain makes this issue particularly dangerous as it affects installations without requiring any explicit configuration changes from administrators.
Mitigation strategies should prioritize immediate upgrades to Unbound versions beyond 1.25.1 where the off-by-one error has been corrected, followed by comprehensive review and testing of existing stub/forward zone configurations to ensure proper functionality. Network administrators should also implement monitoring for unexpected DNS resolution patterns that might indicate this vulnerability being exploited in their environments, particularly focusing on queries that traverse intermediate labels between public signed zones and private namespaces. The fix addresses the core logic error by ensuring proper label handling during cache traversal and maintaining accurate domain name comparison before and after label stripping operations.