CVE-2025-21590: Juniper Junos OS Improper Isolation or Compartmentalization 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.

Exploitation evidence

First exploit evidence seen: 2025-03-13 (earliest date across the sources below).

Exploited in the wild

• CISA KEV: listed since 2025-03-13. Source

Sources: CISA KEV.

How to fix it

CISA required action

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

Fixed versions

• juniper junos: upgrade to 21.2 or later

Derived from the NVD CPE configuration (versionEndExcluding / versionEndIncluding). Always confirm with the vendor advisory.

Vendor advisories and patches

• https://supportportal.juniper.net/s/article/2025-03-Out-of-Cycle-Security-Bulletin-Junos-OS-A-local-attacker-with-shell-access-can-execute-arbitrary-code-CVE-2025-21590?language=en_US (CISA KEV notes)
• https://supportportal.juniper.net/JSA93446 (Vendor Advisory)

MITRE ATT&CK mitigations for this chain

• M1033 Limit Software Installation (for T1059) : Prevent user installation of unrequired command and scripting interpreters.
• M1045 Code Signing (for T1059) : Where possible, only permit execution of signed scripts.
• M1042 Disable or Remove Feature or Program (for T1059) : Disable or remove any unnecessary or unused shells or interpreters.
• M1038 Execution Prevention (for T1059, T1068) : Use application control where appropriate. For example, PowerShell Constrained Language mode can be used to restrict access to sensitive or otherwise dangerous language elements such as those used to execute arbitrary Windows APIs or files (e.g., `Add-Type`).
• M1049 Antivirus/Antimalware (for T1059) : Anti-virus can be used to automatically quarantine suspicious files.
• M1026 Privileged Account Management (for T1059) : When PowerShell is necessary, consider restricting PowerShell execution policy to administrators. Be aware that there are methods of bypassing the PowerShell execution policy, depending on environment configuration. PowerShell JEA (Just Enough Administration) may also be used to sandbox administration and limit what c
• M1047 Audit (for T1059) : Inventory systems for unauthorized command and scripting interpreter installations.
• M1021 Restrict Web-Based Content (for T1059) : Script blocking extensions can help prevent the execution of scripts and HTA files that may commonly be used during the exploitation process. For malicious code served up through ads, adblockers can help prevent that code from executing in the first place.
• M1040 Behavior Prevention on Endpoint (for T1059) : On Windows 10, enable Attack Surface Reduction (ASR) rules to prevent [Visual Basic](https://attack.mitre.org/techniques/T1059/005) and [JavaScript](https://attack.mitre.org/techniques/T1059/007) scripts from executing potentially malicious downloaded content .
• M1051 Update Software (for T1068) : Update software regularly by employing patch management for internal enterprise endpoints and servers.

Detection Generic guidance

• Execution: Watch for the service process spawning shells or script interpreters (cmd.exe, powershell, sh, bash, python) via EDR process-tree rules.
• Privilege Escalation: Alert on processes gaining SYSTEM/root unexpectedly, and on exploitation crashes of privileged services.

Response checklist Generic guidance

1. Inventory: find every instance of the affected product and version (include internet-facing and forgotten ones).
2. Patch or mitigate per the vendor advisory; if you cannot patch, remove internet exposure or apply the vendor workaround.
3. Assume compromise if the system was exposed before patching: hunt for the indicators in the advisory and review logs back to the disclosure date.
4. Rotate credentials and secrets stored on or used by the affected system if compromise cannot be ruled out.