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Actively exploited

CVE-2025-21590 Juniper Junos OS Improper Isolation or Compartmentalization Vulnerability

Juniper Junos OS contains an improper isolation or compartmentalization vulnerability. This vulnerability could allows a local attacker with high privileges to inject arbitrary code.

From the CISA KEV catalogue version 2026.11.06, with Cybergen analysis. Updated 6 Nov 2026.

Our Assessment

Should You Worry?

Patch urgentlyCybergen assessment
  • CISA confirmed exploitation in the wild and added it to the Known Exploited Vulnerabilities catalogue on 13 Mar 2025.
  • Rated Medium severity (CVSS 4.4, scored by Vendor).
  • It has been in the catalogue for 603 days. Systems still unpatched now have almost certainly been scanned for it.
The Vulnerability

What It Is

An Improper Isolation or Compartmentalization vulnerability in the kernel of Juniper Networks Junos OS allows a local attacker with high privileges to compromise the integrity of the device. A local attacker with access to the shell is able to inject arbitrary code which can compromise an affected device. This issue is not exploitable from the Junos CLI. This issue affects Junos OS:  * All versions before 21.2R3-S9, * 21.4 versions before 21.4R3-S10,  * 22.2 versions before 22.2R3-S6,  * 22.4 versions before 22.4R3-S6,  * 23.2 versions before 23.2R2-S3,  * 23.4 versions before 23.4R2-S4, * 24.2 versions before 24.2R1-S2, 24.2R2.

Affected Versions

ProductVersions
Juniper Networks Junos OSbefore 21.2R3-S9
21.4 to before 21.4R3-S10
22.2 to before 22.2R3-S6
22.4 to before 22.4R3-S6
23.2 to before 23.2R2-S3
23.4 to before 23.4R2-S4

From the public CVE record, as supplied by the vendor. Always confirm against the vendor advisory.

From Watch to Action

What To Do Now

  1. 1

    Find it

    Search your asset inventory and external attack surface for Juniper Junos OS. Include cloud images, appliances and anything a supplier runs for you.

    Attack Surface Management
  2. 2

    Fix it

    Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.

    Vulnerability Management
  3. 3

    Hunt for compromise

    If an affected system was reachable before you patched, assume it may have been hit. Check logs, new accounts, web shells and outbound connections from the date of exposure.

    Incident Response
  4. 4

    Prove it is fixed

    Rescan or test the system after patching. Upgrades that fail silently and forgotten secondary instances are common.

    Penetration Testing

Vendor Advisories

Regulation

The Compliance Clock

Cyber EssentialsCyber Essentials requires high and critical updates (CVSS 7.0 or above) within 14 days of release. Treat any exploited flaw the same way.NIS2If exploitation causes a significant incident, essential and important entities owe their CSIRT an early warning within 24 hours and a notification within 72 hours.DORAFinancial entities must classify ICT incidents and send an initial notification for a major incident within strict deadlines, with exploited flaws a common cause.US federal (BOD 22-01)Federal civilian agencies had until 3 Apr 2025 to remediate. A sensible benchmark for everyone else.

General guidance, not legal advice. Your obligations depend on your sector, scope and jurisdiction.

Stay Ahead

More Exploited Juniper Flaws

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CVE-2015-7755Juniper ScreenOS Improper Authentication VulnerabilityAdded 2 Oct 2025CVE-2023-36844Juniper Junos OS EX Series PHP External Variable Modification VulnerabilityAdded 13 Nov 2023CVE-2023-36845Juniper Junos OS EX Series and SRX Series PHP External Variable Modification VulnerabilityAdded 13 Nov 2023CVE-2023-36846Juniper Junos OS SRX Series Missing Authentication for Critical Function VulnerabilityAdded 13 Nov 2023CVE-2023-36847Juniper Junos OS EX Series Missing Authentication for Critical Function VulnerabilityAdded 13 Nov 2023CVE-2023-36851Juniper Junos OS SRX Series Missing Authentication for Critical Function VulnerabilityAdded 13 Nov 2023

All 8 exploited Juniper vulnerabilities

Not Sure If You Are Exposed?

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Evidence

Sources

  1. CISA Known Exploited Vulnerabilities catalogue
  2. NVD: CVE-2025-21590
  3. CVE record: CVE-2025-21590
  4. FIRST EPSS
  5. supportportal.juniper.net/s/article/2025-03-Out-of-Cycle-Security-Bulletin-Junos-OS-A-local-attacker
  6. supportportal.juniper.net/JSA93446
  7. cloud.google.com/blog/topics/threat-intelligence/china-nexus-espionage-targets-juniper-routers