CVE-2026-56444
MEDIUMDescription
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.
How to fix
TridentStack Control can deploy fixes like this automatically across your Windows, macOS, and Linux fleet. See how it works
Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.
CVSS v3 Vector
Exploitability
Impact
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
Exploit Intelligence
Low risk: more likely to be exploited than 17% of all known CVEs.
References
Related Vulnerabilities
Other CWE-772 vulnerabilities, ordered by exploit likelihood. View all
| CVE | Severity | CVSS | EPSS | Exploited | Fix |
|---|---|---|---|---|---|
| CVE-2003-0132 | Medium | 5.0 | 87% | - | Fix |
| CVE-2017-14495 | High | 7.5 | 84% | - | Fix |
| CVE-2021-22883 | High | 7.5 | 74% | - | Fix |
| CVE-2024-2398 | High | 8.6 | 36% | - | Fix |
| CVE-2020-9375 | High | 7.5 | 27% | - | - |
| CVE-1999-1127 | High | 7.5 | 18% | - | - |
Common questions
How do I fix CVE-2026-56444?
Upgrade unbound on debian unstable to 1.25.2-1 or later.
Is CVE-2026-56444 being actively exploited?
Not that we know of. CVE-2026-56444 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 0.26% is the estimated probability that it will be exploited in the next 30 days. That is higher than 17% of all scored CVEs.
How severe is CVE-2026-56444?
CVE-2026-56444 has a CVSS v3 base score of 5.9, rated moderate. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.
What does CVE-2026-56444 affect?
Published advisories record a fix for unbound (Debian). Only products with a sourced advisory are listed, so treat this as what we can cite rather than a complete inventory.
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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-07-24.