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.
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Fixes
Solution
This issue is fixed starting with version 1.25.2
Workaround
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References
History
Wed, 22 Jul 2026 14:30:00 +0000
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Wed, 22 Jul 2026 13:30:00 +0000
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| 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. | |
| Title | Degradation of resolution service when 'discard-timeout' and 'serve-expired-client-timeout' are combined in unusual configuration | |
| Weaknesses | CWE-772 | |
| References |
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| Metrics |
cvssV3_1
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Status: PUBLISHED
Assigner: NLnet Labs
Published:
Updated: 2026-07-22T13:58:09.702Z
Reserved: 2026-06-22T12:35:21.345Z
Link: CVE-2026-56444
Updated: 2026-07-22T13:57:57.751Z
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Weaknesses