CVE-2026-42534
MediumCVSS 5.3Exploitation Probability (EPSS)
Low risk40th percentile - higher than 40% of all known CVEs
Summary
A vulnerability in Unbound up to version 1.25.0 affects the jostle logic for replacing slow queries. Retransmits of the same query update its timestamp, preventing the logic from treating it as aged and replacing it with new queries. An attacker can exploit this to degrade resolution performance.
Risk Assessment
The risk is a potential DoS attack on the DNS service by deliberately slowing responses and sending duplicate queries, exhausting thread limits and degrading resolution performance for all users.
Recommendation
Immediately update Unbound to version 1.25.1 or later, which includes a fix that prevents updating the initial query timestamp.
Original NVD description (English source)
NLnet Labs Unbound up to and including version 1.25.0 has a vulnerability in the jostle logic that could defeat its purpose and degrade resolution performance. Retransmits of the same query could renew the age of slow running queries and not allow the jostle logic to see them as aged and potential targets for replacement with new queries. An adversary who can query a vulnerable Unbound and who can control a domain name server that replies slowly and/or maliciously to Unbound's queries can exploit the vulnerability and degrade the resolution performance of Unbound. When Unbound's 'num-queries-per-thread' reaches its limit, the jostle logic kicks in. When a new query comes in, half of the available queries that are also slow to resolve are candidates for replacement. The vulnerability then happens because duplicate queries that need resolution would skew the aging result by using the timestamp of the latest duplicate query instead of the original one that started the resolution effort. Cache and local data response performance remains unaffected. Coordinated attacks could raise this to a denial of resolution service. Unbound 1.25.1 contains a patch with a fix to attach an initial, non-updatable start time for incoming queries that allow the jostle logic to work as intended.