# CVE-2026-56444: Degradation of resolution service when 'discard-timeout' and 'serve-expired-client-timeout' are combined in unusual configuration

CVE-2026-56444 is fixed in unbound on 1 platform, first by DEBIAN-CVE-2026-56444, released July 28, 2026. It is not reported as exploited. Patch Radar priority: Routine.

- Web page: https://patchblox.com/security/cve/CVE-2026-56444
- Priority: Routine
- CVSS: 5.9 (Moderate)
- EPSS: 0.0037
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Jul 22, 2026
- Weakness: Missing Release of Resource after Effective Lifetime
- Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-56444

## 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.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | unbound Debian 13.5 (trixie) | 1.26.1-0+deb13u1 | [DEBIAN-CVE-2026-56444](https://patchblox.com/security/data/fix/linux/debian-cve-2026-56444.md) | Jul 28, 2026 |
| Linux | unbound Debian 14 | 1.25.2-1 | [DEBIAN-CVE-2026-56444](https://patchblox.com/security/data/fix/linux/debian-cve-2026-56444.md) | Jul 28, 2026 |
| Linux | unbound opensuse-leap 16.0 | 1.25.2-160000.1.1 | [OPENSUSE-SU-2026:21550-1](https://patchblox.com/security/data/fix/linux/opensuse-su-2026-21550-1.md) | Aug 10, 2026 |
| Linux | unbound sles 15.6 | 1.25.2-150600.23.19.1 | [SUSE-SU-2026:3885-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-3885-1.md) | Aug 31, 2026 |
| Linux | unbound sles 15.7 | 1.25.2-150600.23.19.1 | [SUSE-SU-2026:3885-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-3885-1.md) | Aug 31, 2026 |
| Linux | unbound sles 16.0 | 1.25.2-160000.1.1 | [SUSE-SU-2026:23226-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-23226-1.md) | Aug 10, 2026 |

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Data as of 2026-11-09T07:25:28Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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