Unbound 1.201.25.1: Serve-Expired Branch Causes Silent Client Drops
CVE-2026-56444 Published on July 22, 2026
Degradation of resolution service when 'discard-timeout' and 'serve-expired-client-timeout' are combined in unusual configuration
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.
Timeline
Issue reported by Qifan Zhang
NLnet Labs shares patch with Qifan Zhang 24 days later.
Qifan Zhang verifies patch 1 day later.
Issue reported by Haruki Oyama 8 days later.
NLnet Labs shares patch with Haruki Oyama 2 days later.
Haruki Oyama verifies patch
Issue reported by Xin Wang 19 days later.
NLnet Labs shares patch with Xin Wang 1 day later.
Xin Wang verifies patch
Fixes released with version 1.25.2 26 days later.
Weakness Type
Missing Release of Resource after Effective Lifetime
The software does not release a resource after its effective lifetime has ended, i.e., after the resource is no longer needed. When a resource is not released after use, it can allow attackers to cause a denial of service by causing the allocation of resources without triggering their release. Frequently-affected resources include memory, CPU, disk space, power or battery, etc.
Products Associated with CVE-2026-56444
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Affected Versions
NLnet Labs Unbound:- Version 1.20.0 and below 1.25.2 is affected.