Possible cache poisoning vulnerability via a cache side-channel exposed by 'unwanted-reply-threshold'

MITRE NVD CVE.ORG JSON API

Summary

CVECVE-2026-109125
StatePUBLISHED
AssignerNLnet Labs
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-10-21 09:17:06 UTC
Updated2026-10-22 15:22:40 UTC
DescriptionIn NLnet Labs Unbound up to and including version 1.26.1, an observable cache-level side-channel can be exposed with 'unwanted-reply-threshold > 0' configurations (not a default configuration). If a response-like UDP datagram reaches the upstream source port currently used by a recursive query, but its TXID does not match any pending query, Unbound counts the packet as an unwanted reply. When the accumulated count reaches the configured threshold, Unbound clears the worker's RRset cache and message cache. This cache clear action does not destroy the current upstream UDP pending query. Therefore, an external observer can distinguish whether a candidate port matches the current upstream source port by checking whether an existing cached item has been cleared. This is a cache side-channel vulnerability that can be used to probe the upstream source port. Although the attack is not feasible on default configurations and the suggested value (10 million) makes it an ureliable attack on a public resolver, in scenarios where the attacker can keep a concurrent query window alive, the attacker can first filter the source port and then perform birthday-style guessing against the TXID, making cache poisoning a feasible attack path.

Risk And Classification

Primary CVSS: v3.1 5.3 MEDIUM from [email protected]

CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N

EPSS: 0.001340000 probability, percentile 0.026280000 (date 2026-10-22)

Problem Types: CWE-205 | CWE-205 CWE-205: Observable Behavioral Discrepancy

VersionSourceTypeScoreSeverityVector
3.1[email protected]Secondary5.3MEDIUMCVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N
3.1CNACVSS5.3MEDIUMCVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N

CVSS v3.1 Breakdown

Attack Vector
Adjacent
Attack Complexity
High
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
High
Availability
None

CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA NLnet Labs Unbound affected 1.26.2 semver Not specified

References

ReferenceSourceLinkTags
www.nlnetlabs.nl/downloads/unbound/CVE-2026-109125.txt [email protected] www.nlnetlabs.nl
CVE Program record CVE.ORG www.cve.org canonical
NVD vulnerability detail NVD nvd.nist.gov canonical, analysis

Vendor Comments And Credit

Discovery Credit

CNA: TaoFei Guo (Peking University) (en)

CNA: Yang Luo (Tsinghua University) (en)

CNA: JianJun Chen (Tsinghua University) (en)

Additional Advisory Data

SourceTimeEvent
CNA2026-09-28T00:00:00.000ZIssue reported by TaoFei Guo
CNA2026-10-05T00:00:00.000ZNLnet Labs shares patch
CNA2026-10-06T00:00:00.000ZTaoFei Guo verifies patch
CNA2026-10-21T00:00:00.000ZFixes released with version 1.26.2

Solutions

CNA: This issue is fixed starting with version 1.26.2

© CVE.report 2026

Use of this information constitutes acceptance for use in an AS IS condition. There are NO warranties, implied or otherwise, with regard to this information or its use. Any use of this information is at the user's risk. It is the responsibility of user to evaluate the accuracy, completeness or usefulness of any information, opinion, advice or other content. EACH USER WILL BE SOLELY RESPONSIBLE FOR ANY consequences of his or her direct or indirect use of this web site. ALL WARRANTIES OF ANY KIND ARE EXPRESSLY DISCLAIMED. This site will NOT BE LIABLE FOR ANY DIRECT, INDIRECT or any other kind of loss.

CVE, CWE, and OVAL are registred trademarks of The MITRE Corporation and the authoritative source of CVE content is MITRE's CVE web site. This site includes MITRE data granted under the following license.

Free CVE JSON API cve.report/api

CVE.report and Source URL Uptime Status status.cve.report