CVE-2026-56444: CWE-772: Missing Release of Resource after Effective Lifetime in NLnet Labs Unbound
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.
AI Analysis
Technical Summary
CVE-2026-56444 describes a resource management vulnerability (CWE-772) in NLnet Labs Unbound DNS resolver versions 1.20.0 through 1.25.1. When configured with 'serve-expired: yes' and with 'serve-expired-client-timeout' greater than 'discard-timeout' and both greater than zero—contrary to recommended settings—the discard-timeout branch drops aged client replies without decrementing the counter tracking reply addresses. This counter can then reach its maximum, causing new clients for duplicate in-flight queries to be silently dropped, degrading DNS resolution service. The default shipped values ('serve-expired-client-timeout: 1800' and 'discard-timeout: 1900') prevent this vulnerable code path from being executed. Exploitation requires a malicious actor to query the resolver for a slow-on-demand authoritative zone under their control to drive the counter past the threshold.
Potential Impact
The vulnerability does not compromise confidentiality or integrity but causes availability degradation by silently dropping new client queries when the internal counter reaches its limit. This can result in denial of service for DNS resolution under specific misconfigurations. There are no known exploits in the wild.
Mitigation Recommendations
No official patch or remediation level has been published yet. The vulnerability is only exploitable under non-default configurations where 'serve-expired-client-timeout' is set higher than 'discard-timeout' and both are greater than zero. The shipped default settings prevent this issue. Users should ensure that configuration values follow recommended defaults to avoid triggering the vulnerable code path. Monitor vendor advisories for any forthcoming patches or official guidance.
CVE-2026-56444: CWE-772: Missing Release of Resource after Effective Lifetime in NLnet Labs Unbound
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.
CVSS v3.1
Score 5.9medium
Affected software
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Weaknesses
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
CVE-2026-56444 describes a resource management vulnerability (CWE-772) in NLnet Labs Unbound DNS resolver versions 1.20.0 through 1.25.1. When configured with 'serve-expired: yes' and with 'serve-expired-client-timeout' greater than 'discard-timeout' and both greater than zero—contrary to recommended settings—the discard-timeout branch drops aged client replies without decrementing the counter tracking reply addresses. This counter can then reach its maximum, causing new clients for duplicate in-flight queries to be silently dropped, degrading DNS resolution service. The default shipped values ('serve-expired-client-timeout: 1800' and 'discard-timeout: 1900') prevent this vulnerable code path from being executed. Exploitation requires a malicious actor to query the resolver for a slow-on-demand authoritative zone under their control to drive the counter past the threshold.
Potential Impact
The vulnerability does not compromise confidentiality or integrity but causes availability degradation by silently dropping new client queries when the internal counter reaches its limit. This can result in denial of service for DNS resolution under specific misconfigurations. There are no known exploits in the wild.
Mitigation Recommendations
No official patch or remediation level has been published yet. The vulnerability is only exploitable under non-default configurations where 'serve-expired-client-timeout' is set higher than 'discard-timeout' and both are greater than zero. The shipped default settings prevent this issue. Users should ensure that configuration values follow recommended defaults to avoid triggering the vulnerable code path. Monitor vendor advisories for any forthcoming patches or official guidance.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- NLnet Labs
- Date Reserved
- 2026-06-22T12:35:21.345Z
- Cvss Version
- 3.1
- State
- PUBLISHED
- Remediation Level
- null
Threat ID: 6a60c4249c2644c7f80e926c
Added to database: 07/22/2026, 13:22:44 UTC
Last enriched: 07/22/2026, 13:37:59 UTC
Last updated: 07/23/2026, 02:07:13 UTC
Views: 11
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