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JunosOperating system · Juniper

CVE-2026-21909

MEDIUM · 6.5 CVSS v3.1 Published 2026-01-15
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
70/100
Remediation priority · Elevated
No privileges 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 Missing Release of Memory after Effective Lifetime vulnerability in the routing protocol daemon (rpd) Juniper Networks Junos OS and Junos OS Evolved allows an unauthenticated attacker controlling an adjacent IS-IS neighbor to send a specific update packet causing a memory leak. Continued receipt and processing of these packets will exhaust all available memory, crashing rpd and creating a Denial of Service (DoS) condition. Memory usage can be monitored through the use of the 'show task memory detail' command. For example: user@junos> show task memory detail | match ted-infra   TED-INFRA-COOKIE           25   1072     28   1184     229 user@junos> show task memory detail | match ted-infra   TED-INFRA-COOKIE           31   1360     34   1472     307 This issue affects: Junos OS:  * from 23.2 before 23.2R2,  * from 23.4 before 23.4R1-S2, 23.4R2,  * from 24.1 before 24.1R2;  Junos OS Evolved:  * from 23.2 before 23.2R2-EVO,  * from 23.4 before 23.4R1-S2-EVO, 23.4R2-EVO,  * from 24.1 before 24.1R2-EVO. This issue does not affect Junos OS versions before 23.2R1 or Junos OS Evolved versions before 23.2R1-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 confidence

A missing memory release vulnerability in the routing protocol daemon (rpd) of Juniper Junos OS and Junos OS Evolved allows an unauthenticated attacker controlling an adjacent IS-IS neighbor to send specially crafted update packets that cause a memory leak. Repeated packets exhaust available memory, crash rpd, and create a denial of service condition.

MitigationUpgrade Junos OS to version 23.2R2, 23.4R1-S2/23.4R2, or 24.1R2 (or respective Evolved versions) per the vendor advisory. Additionally, restrict IS-IS adjacency to trusted neighbors via authentication or interface filtering to reduce attack surface.

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:= 23.2= 23.4= 24.1
Junos Os EvolvedOperating system
Affected:= 23.2= 23.4= 24.1

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
Adjacent
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS:3.1/AV:A/AC:L/PR:N/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. Confirm Junos version is in affected range
    Run 'show version' or 'show version detail' to display the installed Junos OS version. Compare the version string (e.g., 23.2, 23.4, 24.1) against the affected versions: 23.2, 23.4, 24.1.
    Affected if The installed version matches exactly 23.2, 23.4, or 24.1.
  2. Verify IS-IS protocol is configured
    Run 'show isis interface' to list interfaces where IS-IS is enabled. Run 'show configuration protocols isis' to view the IS-IS protocol configuration.
    Affected if IS-IS is enabled on one or more interfaces.
  3. Check for established IS-IS neighbors
    Run 'show isis adjacency' to display active IS-IS neighbor adjacencies.
  4. Monitor rpd memory usage
    Run 'show system processes extensive' or 'show system memory' to observe rpd process memory consumption over time.
    Affected if rpd process shows continuously increasing memory usage, indicating a potential memory leak.

You are affected if your Junos device runs version 23.2, 23.4, or 24.1 AND has IS-IS protocol enabled with active adjacent neighbors.

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
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Upgrade Junos OS to version 23.2R2, 23.4R1-S2/23.4R2, or 24.1R2 (or respective Evolved versions) per the vendor advisory. Additionally, restrict IS-IS adjacency to trusted neighbors via authentication or interface filtering to reduce attack surface.

Recommended fix High confidence

23.2R2 or later; 23.4R1-S2/23.4R2 or later; 24.1R2 or later (use -EVO suffix for Junos OS Evolved)

  1. 1. Identify current Junos OS or Junos OS Evolved version using 'show version'
  2. 2. Based on current version, plan upgrade to the minimum fixed release: For 23.2.x upgrade to 23.2R2 (or 23.2R2-EVO for Evolved), for 23.4.x upgrade to 23.4R1-S2 or 23.4R2 (or corresponding -EVO versions), for 24.1.x upgrade to 24.1R2 (or 24.1R2-EVO)
  3. 3. Back up current configuration using 'request system configuration rescue save'
  4. 4. Download the appropriate upgrade package from Juniper Networks support portal
  5. 5. Upgrade the device using 'request system software add <package>' with appropriate options
  6. 6. Reboot the device after upgrade completes
  7. 7. Verify upgrade success and check IS-IS neighbor stability
  8. 8. Monitor memory usage using 'show task memory detail | match ted-infra' to confirm leak is resolved
Caveat Standard Junos upgrade precautions apply - test in lab first, review release notes for compatibility, ensure configuration compatibility

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

Fix this in Junos Scoped from the published advisory
  • Consultation3.0 h
  • Implementation6.0 h
  • Testing4.0 h
  • Review / QA2.0 h
15.0 hours of engineering $2,640
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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-2026-21909 in production — separate from our analysis above.

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What this is

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What belongs here
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  • Version or environment caveats, and links to real fixes
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