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JunosOperating system · Juniper

CVE-2026-21903

MEDIUM · 6.5 CVSS v3.1 Published 2026-01-15
Fix available
A fix is available. Upgrade to 22.4 or later.
See remediation →
71/100
Remediation priority · Elevated
Remotely reachable Zero-click

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 · unedited
A Stack-based Buffer Overflow vulnerability in the Packet Forwarding Engine (pfe) of Juniper Networks Junos OS allows a network-based attacker, authenticated with low privileges to cause a Denial-of-Service (DoS). Subscribing to telemetry sensors at scale causes all FPC connections to drop, resulting in an FPC crash and restart. The issue was not seen when YANG packages for the specific sensors were installed. This issue affects Junos OS:  * all versions before 22.4R3-S7, * 23.2 version before 23.2R2-S4, * 23.4 versions before 23.4R2.

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 · moderate confidence

A stack-based buffer overflow in the Packet Forwarding Engine (pfe) of Juniper Networks Junos OS can be triggered when authenticated low-privilege users subscribe to telemetry sensors at scale, causing all FPC connections to drop and resulting in FPC crash and restart.

MitigationInstall the appropriate Junos OS version补丁 (22.4R3-S7, 23.2R2-S4, 23.4R2 or later) or install YANG packages for the specific sensors before subscribing at scale.

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
JunosOperating system
Affected:< 22.4= 22.4= 23.2= 23.4

CVSS 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 checks

Work through these to decide whether this CVE applies to you.

  1. Identify Junos OS version
    Run 'show version' command and locate the Junos OS version string (e.g., 22.4R1, 23.2R1, etc.)
    Affected if The installed version is < 22.4, = 22.4, = 23.2, or = 23.4 (check against the exact version numbers in your output)
  2. Verify telemetry sensor subscription configuration
    Run 'show configuration | display set | match sensor' or 'show configuration system services analytics' to identify any configured telemetry sensor subscriptions
    Affected if Any telemetry sensor subscriptions are configured and enabled on the device
  3. Check for FPC crash logs
    Run 'show log messages | match FPC' and look for crash, restart, or connection drop events in the recent logs
    Affected if Recent logs show FPC connections dropping, FPC crashes, or FPC restarts occurring on the device
  4. Identify active user sessions subscribing to sensors
    Run 'show system subscriber-management interface' or check authentication logs for low-privilege users who may be subscribing to telemetry sensors
    Affected if Authenticated low-privilege users have active sessions involving telemetry sensor subscriptions

You are affected if your Junos OS version falls within < 22.4, 22.4, 23.2, or 23.4 AND you have telemetry sensor subscriptions configured with FPC crashes occurring in your logs.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

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 · scoped
Upgrade available Upgrade to 22.4 or later
Fixed in 22.4
Interim mitigation

Install the appropriate Junos OS version补丁 (22.4R3-S7, 23.2R2-S4, 23.4R2 or later) or install YANG packages for the specific sensors before subscribing at scale.

Recommended fix High confidence

22.4R3-S7, 23.2R2-S4, or 23.4R2

  1. Identify the current Junos OS version running on the device using 'show version'
  2. Determine which release train is appropriate for your environment (22.4R3-S7, 23.2R2-S4, or 23.4R2)
  3. Backup the current configuration using 'request system backup' or 'save' command
  4. Schedule a maintenance window for the upgrade
  5. Upgrade Junos OS to one of the fixed versions: 22.4R3-S7, 23.2R2-S4, or 23.4R2
  6. After upgrade, verify FPC status using 'show chassis fpc' to confirm stability
  7. If using telemetry sensors, consider installing YANG packages for the specific sensors to prevent recurrence
  8. Monitor system logs for any FPC crashes or restarts
Caveat Standard Junos OS upgrade considerations apply - verify hardware compatibility and review release notes for any behavior changes

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Junos Scoped from the published advisory
  • Consultation6.0 h
  • Implementation12.0 h
  • Testing8.0 h
  • Review / QA4.0 h
30.0 hours of engineering $5,280
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-21903 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data