CVE WatchJul 22, 2026

CVE-2026-56444

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-time

CVSS

5.9

Medium

EPSS

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KEV

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Exploit Today

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0-100

Published: Jul 22, 2026 · Last modified: Jul 22, 2026 · CWE-772

EPSS · 30d

Not enough EPSS history yet.

Technical 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.

Official references

www.nlnetlabs.nlhttps://www.nlnetlabs.nl/downloads/unbound/CVE-2026-56444.txt

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