CVE-2026-21921
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 · uneditedA Use After Free vulnerability in the chassis daemon (chassisd) of Juniper Networks Junos OS and Junos OS Evolved allows a network-based attacker authenticated with low privileges to cause a Denial-of-Service (DoS). When telemetry collectors are frequently subscribing and unsubscribing to sensors continuously over a long period of time, telemetry-capable processes like chassisd, rpd or mib2d will crash and restart, which - depending on the process - can cause a complete outage until the system has recovered. This issue affects: Junos OS: * all versions before 22.4R3-S8, * 23.2 versions before 23.2R2-S5, * 23.4 versions before 23.4R2; Junos OS Evolved: * all versions before 22.4R3-S8-EVO, * 23.2 versions before 23.2R2-S5-EVO, * 23.4 versions before 23.4R2-EVO.
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 confidenceA Use After Free vulnerability in the Junos OS chassis daemon (chassisd) allows an authenticated low-privilege network attacker to cause denial of service by repeatedly subscribing and unsubscribing to telemetry sensors, which triggers memory corruption and crashes in chassisd and related processes (rpd, mib2d), potentially causing complete system outage.
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< 22.4= 22.4= 23.2= 23.4< 22.4= 22.4= 23.2= 23.4CVSS 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
- Low
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- None
- Integrity
- None
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
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 Junos OS versionRun 'show version' command on the device CLI to obtain the installed Junos versionAffected if The displayed version is < 22.4, = 22.4, = 23.2, or = 23.4
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Verify telemetry subscription capabilityCheck if the telemetry sensor subscription feature is available by reviewing user role permissions with 'show system privilegege' or consulting your access control configurationAffected if Authenticated users with low-privilege accounts can subscribe to telemetry sensors
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Identify low-privilege accounts with telemetry accessReview user accounts and their permissions, specifically looking for non-admin users with permissions to access the telemetry subscription API or gRPC serviceAffected if Low-privilege or non-administrative user accounts exist with permission to create or modify telemetry subscriptions
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Check for recent chassis daemon crashesRun 'show system core-dumps' and review /var/log/chassisd logs for any recent chassisd, rpd, or mib2d process crashes or restartsAffected if Recent crashes of chassisd or related processes (rpd, mib2d) are present in logs or core dumps
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Monitor for telemetry abuse patternsReview system logs and telemetry subscription activity for repeated rapid subscription/unsubscription events from the same sourceAffected if Evidence exists of repeated telemetry subscription and unsubscription cycles that could indicate exploitation attempts
Your environment is affected if you are running Junos OS or Junos OS Evolved version 22.4, 23.2, or 23.4 (or any version below 22.4) and low-privilege authenticated users have the ability to subscribe to telemetry sensors.
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 · scoped22.4
Apply vendor patches (22.4R3-S8/23.2R2-S5/23.4R2 or later for Junos OS; equivalent EVO versions) or restrict and monitor telemetry subscription access to prevent abuse.
22.4R3-S8, 23.2R2-S5, or 23.4R2 (or later for respective Junos OS/Evolved branches)
- 1. Identify the current Junos OS or Junos OS Evolved version running on the device using 'show version'
- 2. Determine the appropriate upgrade path based on current version (22.4, 23.2, or 23.4 series)
- 3. Download the required fixed release: For 22.4 branch use 22.4R3-S8 (or later), for 23.2 branch use 23.2R2-S5 or later, for 23.4 branch use 23.4R2 or later
- 4. For Junos OS Evolved, use corresponding EVO images: 22.4R3-S8-EVO, 23.2R2-S5-EVO, or 23.4R2-EVO
- 5. Upload the upgrade package to the device's /var/tmp directory
- 6. Verify the package integrity using 'request system software validate <package>'
- 7. Execute the upgrade with 'request system software add <package>' and appropriate options (e.g., reboot for minor upgrades)
- 8. After reboot, verify the new version is installed using 'show version'
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation3.0 h
- Implementation4.0 h
- Testing3.0 h
- Review / QA2.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-21921 — 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-2026-21921 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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- Version or environment caveats, and links to real fixes
- No weaponised exploit code, or anything meant to cause harm
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