CVE-2026-44687: CWE-193: Off-by-one Error in NLnet Labs Unbound
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).
AI Analysis
Technical Summary
CVE-2026-44687 is an off-by-one error vulnerability in NLnet Labs Unbound DNS resolver software affecting versions 1.13.2 through 1.25.1. The flaw exists in the 'harden-below-nxdomain' feature, which performs an upward DNS cache walk with a delegation point guard to prevent NXDOMAIN synthesis above stub/forward zones. Due to incorrect label stripping in the guard test, queries for intermediate labels between stub/forward zone apexes and DNSSEC signed parent zones can be incorrectly answered with a cached DNSSEC-secure NXDOMAIN from the parent. This causes the resolver to bypass contacting the configured stub/forward upstream servers, effectively ignoring the intended configuration for those zones.
Potential Impact
The vulnerability can cause DNS queries for certain intermediate labels in stub or forward zones to be answered incorrectly with a DNSSEC-secure NXDOMAIN from the parent zone cache. This results in the stub/forward zone upstream servers never being contacted, potentially disrupting DNS resolution for those zones. The CVSS score is 3.7 (low severity), indicating limited impact with no confidentiality, integrity, or user interaction effects, and requires high attack complexity.
Mitigation Recommendations
Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. No official fix or temporary workaround is provided in the available data. Users should monitor NLnet Labs advisories for updates. Until a fix is available, consider disabling 'harden-below-nxdomain' if feasible and safe in your environment, or carefully evaluate DNS configurations to mitigate the impact.
CVE-2026-44687: CWE-193: Off-by-one Error in NLnet Labs Unbound
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).
CVSS v3.1
Score 3.7low
Affected software
pkg:github/nlnetlabs/unboundRun on your own infrastructure? Check whether these packages are installed with threat-finder — our free open-source scanner.
Weaknesses
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
CVE-2026-44687 is an off-by-one error vulnerability in NLnet Labs Unbound DNS resolver software affecting versions 1.13.2 through 1.25.1. The flaw exists in the 'harden-below-nxdomain' feature, which performs an upward DNS cache walk with a delegation point guard to prevent NXDOMAIN synthesis above stub/forward zones. Due to incorrect label stripping in the guard test, queries for intermediate labels between stub/forward zone apexes and DNSSEC signed parent zones can be incorrectly answered with a cached DNSSEC-secure NXDOMAIN from the parent. This causes the resolver to bypass contacting the configured stub/forward upstream servers, effectively ignoring the intended configuration for those zones.
Potential Impact
The vulnerability can cause DNS queries for certain intermediate labels in stub or forward zones to be answered incorrectly with a DNSSEC-secure NXDOMAIN from the parent zone cache. This results in the stub/forward zone upstream servers never being contacted, potentially disrupting DNS resolution for those zones. The CVSS score is 3.7 (low severity), indicating limited impact with no confidentiality, integrity, or user interaction effects, and requires high attack complexity.
Mitigation Recommendations
Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. No official fix or temporary workaround is provided in the available data. Users should monitor NLnet Labs advisories for updates. Until a fix is available, consider disabling 'harden-below-nxdomain' if feasible and safe in your environment, or carefully evaluate DNS configurations to mitigate the impact.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- NLnet Labs
- Date Reserved
- 2026-06-22T12:35:21.340Z
- Cvss Version
- 3.1
- State
- PUBLISHED
- Remediation Level
- null
Threat ID: 6a60c4219c2644c7f80e9137
Added to database: 07/22/2026, 13:22:41 UTC
Last enriched: 07/22/2026, 13:41:00 UTC
Last updated: 07/22/2026, 14:52:44 UTC
Views: 5
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