What’s Vulnerable › Vulnerabilities
CVE-2026-56444: Degradation of resolution service when 'discard-timeout' and 'serve-expired-client-timeout' are combined in unusual configuration
- Severity: medium (CVSS 5.9 v3.1)
- Exploited in the wild: not listed in CISA KEV
- EPSS: 0.4% probability of exploitation in the next 30 days
- Public exploit or detection template: none known
- Published: 2026-07-22 · updated 2026-10-24
- Weakness: CWE-772
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
- NLnet Labs Unbound: from 1.20.0 before 1.25.2 — fixed in 1.25.2
How to fix
Upgrade to a fixed version: NLnet Labs Unbound 1.25.2. Check the vendor advisory for mitigations.
References
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