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.
Affected products
| Vendor | Product | Affected | Fixed |
|---|---|---|---|
| NLnet Labs | Unbound | 1.20.0 – < 1.25.2 | 1.25.2 |
Fixed in Linux distributions
Distributions backport fixes, so the version that fixes this vulnerability differs per release — upgrade to at least the version shown. From the distributions' security trackers and advisories.
Debian
Security tracker ↗| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian 14 | unbound | 1.25.2-1 | — |
| Debian 13 | unbound | 1.26.1-0+deb13u1 | — |
| Debian 12 | unbound | No fix yet | — |
Ubuntu
Security tracker ↗| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| Ubuntu 26.10 | unbound | No fix yet | — |
| Ubuntu 26.04 LTS | unbound | No fix yet | — |
| Ubuntu 24.04 LTS | unbound | No fix yet | — |
| Ubuntu 22.04 LTS | unbound | No fix yet | — |
Show 4 more (older releases, extended support, other products)Fewer releases
| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| Ubuntu Pro 20.04 LTS | unbound | No fix yet | — |
| Ubuntu Pro 18.04 LTS | unbound | No fix yet | — |
| Ubuntu Pro 16.04 LTS | unbound | No fix yet | — |
| Ubuntu Pro 14.04 LTS | unbound | No fix yet | — |
Red Hat
Security tracker ↗| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| Red Hat Hardened Images 1 | unbound +7 | 1.25.2-0.1.hum1 | RHSA-2026:43588 |
Rocky Linux
Errata ↗| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| Rocky Linux 10 | unbound +6 | 1.26.1-2.el10_3 | RLSA-2026:97121 |
| Rocky Linux 9 | unbound +4 | 1.26.1-1.el9_9 | RLSA-2026:97497 |
SUSE
Security tracker ↗| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| SLES 15 SP7 | unbound | 1.25.2-150600.23.19.1 | SUSE-SU-2026:3885-1 |
| SLES 15 SP6 | unbound | 1.25.2-150600.23.19.1 | SUSE-SU-2026:3885-1 |
| SLES 15 SP5 | unbound | 1.25.2-150100.10.28.1 | SUSE-SU-2026:3884-1 |
| SLES 15 SP4 | unbound | 1.25.2-150100.10.28.1 | SUSE-SU-2026:3884-1 |
Show 11 more (older releases, extended support, other products)Fewer releases
| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| SLES for SAP 16.0 | unbound | 1.25.2-160000.1.1 | SUSE-SU-2026:23226-1, SUSE-SU-2026:23215-1 |
| SLES for SAP 15 SP6 | unbound | 1.25.2-150600.23.19.1 | SUSE-SU-2026:3885-1 |
| SLES for SAP 15 SP5 | unbound | 1.25.2-150100.10.28.1 | SUSE-SU-2026:3884-1 |
| SLES for SAP 15 SP4 | unbound | 1.25.2-150100.10.28.1 | SUSE-SU-2026:3884-1 |
| SLES 16.0 | unbound | 1.25.2-160000.1.1 | SUSE-SU-2026:23226-1, SUSE-SU-2026:23215-1 |
| SLE Micro 6.2 | unbound | 1.25.2-160000.1.1 | SUSE-SU-2026:23050-1 |
| SLE Micro 6.1 | unbound | 1.25.2-slfo.1.1_1.1 | SUSE-SU-2026:23349-1 |
| SLE Micro 6.0 | unbound | 1.25.2-1.1 | SUSE-SU-2026:23360-1 |
| SLE Micro 5.5 | unbound | 1.25.2-150100.10.28.1 | SUSE-SU-2026:3884-1 |
| SLE HPC 15 SP5 | unbound | 1.25.2-150100.10.28.1 | SUSE-SU-2026:3884-1 |
| SLE HPC 15 SP4 | unbound | 1.25.2-150100.10.28.1 | SUSE-SU-2026:3884-1 |
openSUSE
Security tracker ↗| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| openSUSE Tumbleweed | unbound | 1.25.2-1.1 | openSUSE-SU-2026:11380-1 |
| openSUSE Leap 16.0 | unbound | 1.25.2-160000.1.1 | openSUSE-SU-2026:21550-1 |
Alpine
Security tracker ↗| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| Alpine v3.24 | unbound | 1.25.2-r0 | — |
| Alpine v3.23 | unbound | 1.25.2-r0 | — |
| Alpine v3.22 | unbound | 1.25.2-r0 | — |
Azure Linux
| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| Azure Linux 3 | unbound | 1.25.2-1 | — |
Advisories 14
- RLSA-2026:97497 Moderate: unbound security, bug fix, and enhancement update Rocky Linux Security Advisories · Nov 13, 2026
- RLSA-2026:97121 Moderate: unbound security, bug fix, and enhancement update Rocky Linux Security Advisories · Nov 13, 2026
- WID-SEC-2026-2492 Unbound: Mehrere Schwachstellen BSI CERT-Bund (WID) · Nov 10, 2026
- RHSA-2026:43588 Red Hat Security Advisory: Red Hat Hardened Images RPMs bug fix and enhancement update Red Hat Security Advisories (RHSA) · Oct 20, 2026
- SUSE-SU-2026:3885-1 Security update for unbound SUSE / openSUSE Security Updates · Sep 10, 2026
- SUSE-SU-2026:3884-1 Security update for unbound SUSE / openSUSE Security Updates · Sep 10, 2026
- SUSE-SU-2026:23360-1 Security update for unbound SUSE / openSUSE Security Updates · Sep 10, 2026
- SUSE-SU-2026:23349-1 Security update for unbound SUSE / openSUSE Security Updates · Sep 10, 2026
- SUSE-SU-2026:23226-1 Security update for unbound SUSE / openSUSE Security Updates · Aug 27, 2026
- SUSE-SU-2026:23215-1 Security update for unbound SUSE / openSUSE Security Updates · Aug 27, 2026
- WID-SEC-2026-3038 Meinberg LANTIME: Mehrere Schwachstellen BSI CERT-Bund (WID) · Aug 26, 2026
- openSUSE-SU-2026:21550-1 Security update for unbound SUSE / openSUSE Security Updates · Aug 12, 2026
- SUSE-SU-2026:23050-1 Security update for unbound SUSE / openSUSE Security Updates · Aug 12, 2026
- openSUSE-SU-2026:11380-1 libunbound8-1.25.2-1.1 on GA media SUSE / openSUSE Security Updates · Jul 27, 2026
References
- https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-56444.txt vendor-advisory