# CVE-2026-42534: Jostle logic bypass degrades resolution performance > An attacker can degrade DNS resolution performance or cause denial of service by exploiting a flaw in Unbound's query aging logic. - **Severity:** Medium 6.9 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/U:Amber`) - **Exploited in the wild:** Not known - **Login needed:** Not needed - **Attackable:** Over the internet - **Victim action needed:** Not needed - **Fix available:** Yes - **Published:** 20 May 2026 - **Updated:** 1 Sep 2026 - **Also known as:** ALPINE-CVE-2026-42534, AZL-87033, CLSA-2026-1779789234, CLSA-2026-1780949133, DEBIAN-CVE-2026-42534, ECHO-cd79-ba54-3567, GHSA-xfcv-gp55-3wpf, UBUNTU-CVE-2026-42534 ## What is CVE-2026-42534? CVE-2026-42534 is a medium-severity denial of service vulnerability in Nlnetlabs Unbound, published on 20 May 2026. An attacker can exploit it over the internet without logging in, with no action from the victim. ### Official 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. Weakness: CWE-911 (Improper Update of Reference Count), CWE-440 (Expected Behavior Violation) ## Which versions are affected by CVE-2026-42534? It affects 1 product. Products: Nlnetlabs Unbound. ## How do I fix CVE-2026-42534? Upgrade to a fixed version. ## Is CVE-2026-42534 being exploited? There is no known exploitation of CVE-2026-42534 in the wild and no public exploit code. ## Other vulnerabilities in Nlnetlabs Unbound - [CVE-2026-81642](https://insights.sbomify.com/CVE-2026-81642/): Heap buffer overflow and possible Remote Code Execution when digesting DNSKEY (critical 9.1) - [CVE-2026-33278](https://insights.sbomify.com/CVE-2026-33278/): Possible arbitrary code execution during DNSSEC validation (critical 9.1) - [CVE-2026-42944](https://insights.sbomify.com/CVE-2026-42944/): Heap overflow with multiple NSID, COOKIE, PADDING EDNS options (high 8.7) - [CVE-2026-42959](https://insights.sbomify.com/CVE-2026-42959/): Crash during DNSSEC validation of malicious content (high 8.7) - [CVE-2026-82717](https://insights.sbomify.com/CVE-2026-82717/): CNAME synthesis could lead to heap corruption (high 8.4) ## Fix prompt for AI coding agents ```text Check this codebase for CVE-2026-42534 and fix it if it is affected. What it is: CVE-2026-42534 is a medium-severity denial of service vulnerability in Nlnetlabs Unbound, published on 20 May 2026. An attacker can exploit it over the internet without logging in, with no action from the victim. Exploitation: There is no known exploitation of CVE-2026-42534 in the wild and no public exploit code. Affected product: Nlnetlabs Unbound. No package-level version data is available, so check the vendor advisory linked from the reference page. Task: 1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies. 2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it. 3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead. 4. Run the tests and summarise what changed and why. 5. If nothing in the project is affected, say so and change nothing. Reference: https://insights.sbomify.com/CVE-2026-42534/ (Markdown: https://insights.sbomify.com/CVE-2026-42534/index.md) ``` ## Sources - CVE record: https://www.cve.org/CVERecord?id=CVE-2026-42534 - NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-42534 - OSV: https://osv.dev/vulnerability/CVE-2026-42534 --- From sbomify Insights: https://insights.sbomify.com/CVE-2026-42534/ Summaries are generated and may contain mistakes; the official description above is the record.