CVE-2026-42534
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedNLnet 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.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceUnbound DNS resolver contains a flaw in its jostle logic that manages query replacement when the per-thread query limit is reached. When duplicate or retransmitted queries arrive, they update the timestamp to the latest duplicate rather than preserving the original query's start time. This prevents the jostle mechanism from recognizing aged slow queries as replacement candidates, allowing an attacker controlling a slow DNS server to degrade resolution performance.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data< 1.25.1CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- None
- Integrity
- None
- Availability
- Low
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Check Unbound versionRun `unbound -V` or `unbound-control version` to retrieve the installed version numberAffected if version is below 1.25.1 (e.g., 1.25.0, 1.24.0, 1.22.0, etc.)
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Locate Unbound configuration fileCommon paths are /etc/unbound/unbound.conf, /etc/unbound.conf, or check `unbound-checkconf` output for the active config pathAffected if config file exists and is in use
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Verify jostle-timeout settingInspect the configuration file for a `jostle-timeout` directive using grep or by viewing the file; check if it is set to a non-zero value (e.g., 200, 500, 1000)Affected if jostle-timeout is present and set to a value greater than 0, enabling the jostle logic where the flaw applies
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Confirm per-thread query limit configurationCheck for `num-queries-per-thread` in the config (default is 1024); this is the limit that triggers jostle behavior when reachedAffected if the query limit setting exists and is reachable (any non-zero value enables the condition that can trigger the vulnerable jostle logic)
You are affected if Unbound version is below 1.25.1 AND jostle-timeout is configured to a non-zero value, allowing the flawed query replacement logic to trigger when the per-thread query limit is reached.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
dbcve · scoped1.25.1
Upgrade Unbound to version 1.25.1 or later, which implements a non-updatable start time stamp for incoming queries to restore proper jostle logic functionality.
Unbound 1.25.1
- 1. Identify current Unbound version by running 'unbound -V' or checking the package manager
- 2. Obtain Unbound version 1.25.1 or later from the official NLnet Labs repository (www.nlnetlabs.nl) or your distribution's package manager
- 3. Back up current Unbound configuration files (typically in /etc/unbound/)
- 4. Upgrade Unbound using your package manager or by compiling from source
- 5. Verify the new version is running with 'unbound -V'
- 6. Test DNS resolution functionality to ensure normal operation
- 7. Monitor resolution performance to confirm the jostle logic is functioning correctly
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation1.0 h
- Implementation2.0 h
- Testing2.0 h
- Review / QA1.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-42534 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2026-42534 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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