CVE-2026-44621: CWE-754: Improper Check for Unusual or Exceptional Conditions in NLnet Labs Unbound
With NLnet Labs Unbound up to and including version 1.25.1, applications using libunbound and configured with 'unwanted-reply-threshold', could eventually be abruptly terminated if the threshold is reached and libunbound needs to call 'libworker_alloc_cleanup' since the function is absent from the function call allow list. When an application using libunbound sets 'unwanted-reply-threshold' to any non-zero value and the iterator queries an authoritative that replies with enough wrong-transaction-ID UDP datagrams to cross the threshold, the 'libworker_alloc_cleanup' will eventually be called. Since the function is absent from the function call allow list, this leads to a fatal exit of libunbound and eventual termination of the embedding application.Unbound itself is not affected since its relevant function 'worker_alloc_cleanup' is registed in the allow list and proceeds to perform the documented cache flush.
AI Analysis
Technical Summary
CVE-2026-44621 describes a medium severity vulnerability in NLnet Labs Unbound's libunbound library up to version 1.25.1. When configured with a non-zero 'unwanted-reply-threshold', if an authoritative server replies with enough incorrect UDP datagrams to exceed this threshold, libunbound calls 'libworker_alloc_cleanup'. This function is absent from the function call allow list, resulting in a fatal exit of libunbound and termination of the embedding application. The Unbound daemon is not affected because its 'worker_alloc_cleanup' function is registered in the allow list and handles cache flushes as expected. The CVSS 3.1 score is 5.9 (medium), reflecting network attack vector, high attack complexity, no privileges required, no user interaction, unchanged scope, no confidentiality or integrity impact, but high availability impact.
Potential Impact
The vulnerability causes applications embedding libunbound and configured with 'unwanted-reply-threshold' to potentially terminate abruptly when the threshold is exceeded due to malformed or incorrect UDP responses. This results in a denial of service (availability impact) for those applications. The Unbound daemon itself is not impacted by this issue.
Mitigation Recommendations
Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. Until a fix is available, users embedding libunbound with 'unwanted-reply-threshold' configured should consider disabling this setting or avoiding configurations that could trigger the threshold. Monitor vendor communications for updates and official patches.
CVE-2026-44621: CWE-754: Improper Check for Unusual or Exceptional Conditions in NLnet Labs Unbound
Description
With NLnet Labs Unbound up to and including version 1.25.1, applications using libunbound and configured with 'unwanted-reply-threshold', could eventually be abruptly terminated if the threshold is reached and libunbound needs to call 'libworker_alloc_cleanup' since the function is absent from the function call allow list. When an application using libunbound sets 'unwanted-reply-threshold' to any non-zero value and the iterator queries an authoritative that replies with enough wrong-transaction-ID UDP datagrams to cross the threshold, the 'libworker_alloc_cleanup' will eventually be called. Since the function is absent from the function call allow list, this leads to a fatal exit of libunbound and eventual termination of the embedding application.Unbound itself is not affected since its relevant function 'worker_alloc_cleanup' is registed in the allow list and proceeds to perform the documented cache flush.
CVSS v3.1
Score 5.9medium
Weaknesses
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
CVE-2026-44621 describes a medium severity vulnerability in NLnet Labs Unbound's libunbound library up to version 1.25.1. When configured with a non-zero 'unwanted-reply-threshold', if an authoritative server replies with enough incorrect UDP datagrams to exceed this threshold, libunbound calls 'libworker_alloc_cleanup'. This function is absent from the function call allow list, resulting in a fatal exit of libunbound and termination of the embedding application. The Unbound daemon is not affected because its 'worker_alloc_cleanup' function is registered in the allow list and handles cache flushes as expected. The CVSS 3.1 score is 5.9 (medium), reflecting network attack vector, high attack complexity, no privileges required, no user interaction, unchanged scope, no confidentiality or integrity impact, but high availability impact.
Potential Impact
The vulnerability causes applications embedding libunbound and configured with 'unwanted-reply-threshold' to potentially terminate abruptly when the threshold is exceeded due to malformed or incorrect UDP responses. This results in a denial of service (availability impact) for those applications. The Unbound daemon itself is not impacted by this issue.
Mitigation Recommendations
Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. Until a fix is available, users embedding libunbound with 'unwanted-reply-threshold' configured should consider disabling this setting or avoiding configurations that could trigger the threshold. Monitor vendor communications for updates and official patches.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- NLnet Labs
- Date Reserved
- 2026-06-22T10:11:10.524Z
- Cvss Version
- 3.1
- State
- PUBLISHED
- Remediation Level
- null
Threat ID: 6a60c4219c2644c7f80e9134
Added to database: 07/22/2026, 13:22:41 UTC
Last enriched: 07/22/2026, 13:41:11 UTC
Last updated: 07/22/2026, 21:49:27 UTC
Views: 4
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