CVE-2026-42534 Medium Severity in unbound Vulnerability Details & Analysis

Summary

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 exploi

Description

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.

📊 Relative Risk Intelligence

This CVE is Lower Risk - more severe than 19.2% of all 353,401 vulnerabilities in our database.

#285,722

Below average severity

Severity Percentile

🎯 CISA SSVC Assessment Updated: May 20, 2026

🔍 Exploitation Status

None

No known exploits

⚙️ Automatable

YES

Can be exploited automatically

💥 Technical Impact

Partial

Limited system impact

🏆 Discovered By

Qifan Zhang (Palo Alto Networks)

SSVC data provided by CISA

CVE ID	Severity	CVSS	EPSS	Summary	Published
CVE-2026-32665	⚠️ high	7.5	0.3	In NLnet Labs Unbound 1.22.0 up to and including 1.25.1, when downstream DNS-over-QUIC (DoQ) is enabled, the first two b...	2026-07-22
CVE-2026-40691	⚠️ high	7.5	0.3	In Unbound 1.9.0 up to and including 1.25.1, when a DNSCrypt query is received over TCP, the routine that encrypts the r...	2026-07-22
CVE-2026-41637	ℹ️ low	3.7	0.3	In NLnet Labs Unbound 1.22.0 up to and including 1.25.1, client terminated DNS-over-QUIC (DoQ) queries are not accounted...	2026-07-22
CVE-2026-42955	ℹ️ low	3.7	0.2	In NLnet Labs Unbound 1.16.2 up to and including 1.25.1, a similar vulnerability as with CVE-2026-40622 in the 'ghost do...	2026-07-22
CVE-2026-44621	🔶 medium	5.9	0.3	With NLnet Labs Unbound up to and including version 1.25.1, applications using libunbound and configured with 'unwanted-...	2026-07-22
CVE-2026-44687	ℹ️ low	3.7	0.2	In NLnet Labs Unbound 1.13.2 up to and including 1.25.1, stub or forward zones where the name is below an intermediate l...	2026-07-22

CVE-2026-42534

Summary

Description

📊 Relative Risk Intelligence

🎯 CISA SSVC Assessment Updated: May 20, 2026

📦 Affected Products 1

🔗 References 8

🔗 Related CVEs 6