Vulnerability · Published Jul 22, 2026 · Assigned by NLnet Labs
CVE-2026-56444Degradation of resolution service when 'discard-timeout' and 'serve-expired-client-timeout' are combined in unusual configuration
Medium 5.9
In plain English
A medium-severity security flaw in Unbound from NLnet Labs. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.4% (higher than 30% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official 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 packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Alpine:v3.22 | unbound | 1.25.2-r0 | ALPINE-CVE-2026-56444 |
| Alpine:v3.23 | unbound | 1.25.2-r0 | ALPINE-CVE-2026-56444 |
| Alpine:v3.24 | unbound | 1.25.2-r0 | ALPINE-CVE-2026-56444 |
| Debian:12 | unbound | no fix listed | DEBIAN-CVE-2026-56444 |
| Debian:13 | unbound | 1.26.1-0+deb13u1 | DEBIAN-CVE-2026-56444 |
| Debian:14 | unbound | 1.25.2-1 | DEBIAN-CVE-2026-56444 |
| Ubuntu:22.04:LTS | unbound | no fix listed | UBUNTU-CVE-2026-56444 |
| Ubuntu:24.04:LTS | unbound | no fix listed | UBUNTU-CVE-2026-56444 |
| Ubuntu:26.04:LTS | unbound | no fix listed | UBUNTU-CVE-2026-56444 |
| Ubuntu:26.10 | unbound | no fix listed | UBUNTU-CVE-2026-56444 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| NLnet Labs | Unbound | 1.20.0 – before 1.25.2 |
Weakness: CWE-772 · Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
References
- https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-56444.txt vendor-advisory