CVE-2026-56444: nlnet labs unbound

Vulnerability summary

CVE-2026-56444 is a medium (CVSS 5.9) vulnerability in nlnet labs unbound. EPSS estimates a 0.36% chance of exploitation in the next 30 days, higher than 28.8% of scored vulnerabilities.

Timeline

1. 22-07-2026
   PublishedVulnerability first disclosed.
2. 22-07-2026
   Last modifiedVulnerability information updated.

Description

In NLnet Labs Unbound 1.20.0 up to and including 1.25.1, when Unbound is configured with 'serve-expired: yes' and 'serve-expired-client-timeout > discard-timeout > 0' (contrary to the suggested values), the discard-timeout branch during the serve expired logic drops an aged client reply without performing the correct accounting for the number of reply addresses for the query. Other identical branches outside of serve expired perform the correct decrement. Since the counter is never decremented in such scenario, it can reach the maximum limit and new clients for duplicate in-flight queries are silently dropped resulting in degradation of resolution service. A malicious actor can exploit the vulnerability by querying the resolver for a client-controlled slow-on-demand authoritative zone that can drive the counter past the threshold. Shipped defaults for 'serve-expired-client-timeout: 1800' and 'discard-timeout: 1900' make the branch unreachable.

• CWE-772Missing Release of Resource after Effective LifetimeThe product does not release a resource after its effective lifetime has ended, i.e., after the resource is no longer needed.