CVE-2026-44687 Off-by-one error in 'harden-below-nxdomain' logic can shadow a stub/forward zone by a legitimate parent's NXDOMAIN
An off-by-one error in Unbound's harden-below-nxdomain logic allows DNSSEC-signed parent zones to shadow configured stub or forward zones, bypassing intended routing.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-44687?
CVE-2026-44687 is a low-severity vulnerability in Nlnetlabs Unbound, published on 22 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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).
Weakness: CWE-193
Which versions are affected by CVE-2026-44687?
It affects 1 product.
Products include Nlnetlabs Unbound.
How do I fix CVE-2026-44687?
Upgrade to a fixed version.
Is CVE-2026-44687 being exploited?
There is no known exploitation of CVE-2026-44687 in the wild and no public exploit code.
Other vulnerabilities in Nlnetlabs Unbound
- CVE-2026-81642 Heap buffer overflow and possible Remote Code Execution when digesting DNSKEY
- CVE-2026-33278 Possible arbitrary code execution during DNSSEC validation
- CVE-2026-42944 Heap overflow with multiple NSID, COOKIE, PADDING EDNS options
- CVE-2026-42959 Crash during DNSSEC validation of malicious content
- CVE-2026-82717 CNAME synthesis could lead to heap corruption
Sources
Also known as ALPINE-CVE-2026-44687, AZL-92922, DEBIAN-CVE-2026-44687, ECHO-be32-8712-2a19, GHSA-x324-pxwv-r76h, UBUNTU-CVE-2026-44687.