CVE-2026-56444 (GCVE-0-2026-56444)

Vulnerability from cvelistv5 – Published: 2026-07-22 13:10 – Updated: 2026-07-22 13:58
VLAI
Title
Degradation of resolution service when 'discard-timeout' and 'serve-expired-client-timeout' are combined in unusual configuration
Summary
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.
Severity
5.9 (Medium)
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
SSVC
Exploitation: none Automatable: yes Technical Impact: partial
CISA Coordinator (v2.0.3)
CWE
  • CWE-772 - Missing Release of Resource after Effective Lifetime
Assigner
References
Impacted products
Vendor Product Version
NLnet Labs Unbound Affected: 1.20.0 , < 1.25.2 (semver)
Create a notification for this product.
Date Public
2026-07-22 00:00
Credits
Qifan Zhang (Palo Alto Networks) Xin Wang (Northwestern Polytechnical University) Jiapeng Li (Northwestern Polytechnical University) Jiajia Liu (Northwestern Polytechnical University) Haruki Oyama (Waseda University)
Show details on NVD website
To clipboard 

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  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
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  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.


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