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CVE-2026-42534 is a denial-of-service and performance degradation vulnerability in NLnet Labs Unbound affecting versions up to and including 1.25.0. The flaw is in Unbound's jostle logic, which is intended to replace older slow-running queries when the per-thread outstanding query limit is reached. Due to incorrect aging behavior, retransmitted duplicate queries can refresh the apparent age of an already slow resolution attempt by using the timestamp of the latest duplicate rather than preserving the original start time. This prevents the jostle mechanism from correctly identifying stale slow queries as replacement candidates. An attacker who can send queries to a vulnerable resolver and control an authoritative name server that responds slowly or maliciously can exploit this behavior to keep expensive slow queries resident, reducing resolver efficiency and impairing recursive resolution performance. Cache hits and responses from local data are not affected. Unbound 1.25.1 fixes the issue by attaching an initial, non-updatable start time to incoming queries so the jostle logic can age and evict slow queries as intended.
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