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CVE-2025-21590

From Exploited flaws, HARD’s guide to the flaws being used in attacks · Checked
This article is about a flaw in Juniper Junos OS. For every flaw being used in attacks, see Exploited flaws; for Juniper’s, see Juniper flaws being used in attacks.

CVE-2025-21590 is a security flaw in Juniper Junos OS, the operating system on Juniper’s routers and firewalls. Someone who already has a way in can use Juniper Junos OS to run their own code. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 13 Mar 2025.[1]

Juniper Junos OS is business software or equipment. Most families won’t have it, but attackers break into it to get inside companies, and from there to what those companies hold about their customers, patients and staff. Fixed in version 21.2R3-S9, 21.4R3-S10, 22.2R3-S6 and 22.4R3-S6 and others of Junos OS.

Description

CISA calls it an improper isolation or compartmentalization in Juniper Junos OS. In its words: “Juniper Junos OS contains an improper isolation or compartmentalization vulnerability. This vulnerability could allows a local attacker with high privileges to inject arbitrary code.”[1]

The CVE record adds: “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.”[2]

The underlying weakness is CWE-653, in MITRE’s list of common software weaknesses.

It scores 4.4 out of 10 on the Common Vulnerability Scoring System, which rates it medium, scored by the organisation that published the CVE using CVSS 3.1.[2]

Affected versions

NVD’s record lists these fixed versions: in each line, everything before the version shown is affected.[2]

ProductFixed in
Junos OS21.2R3-S9
Junos OS21.4R3-S10
Junos OS22.2R3-S6
Junos OS22.4R3-S6
Junos OS23.2R2-S3
Junos OS23.4R2-S4
Junos OS24.2R1-S2, 24.2R2

Most families won’t have Juniper Junos OS; it’s for whoever looks after IT at work.

Exploitation

CISA added it to its catalog of known exploited vulnerabilities on 13 Mar 2025, within a day of it being made public. CISA lists a flaw only when it has reliable evidence of it being exploited, and gave US federal agencies until 3 Apr 2025 to fix it.[1]

FIRST’s Exploit Prediction Scoring System, which estimates how likely a flaw is to be used in attacks in the next 30 days from what it sees across the internet, puts it at 2%. That’s in the top 23% of the flaws it scores.[4]

CISA assesses it as not automatable: each attack takes some work.

CISA doesn’t say who was attacked, or how.

Fix

Juniper has published a fix or advice.[3] CISA’s 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.”[1]

  1. Check whether you use it.Juniper Junos OS is business software or equipment, so most families won’t have it. If you run a business, or work from home on company equipment, ask whoever looks after your IT.
  2. Send this page to your IT person.Ask them to confirm it’s fixed (fixed in version 21.2R3-S9, 21.4R3-S10, 22.2R3-S6 and 22.4R3-S6 and others of Junos OS), or not in use. CISA gave US government agencies until 3 Apr 2025 to fix it.
  3. If it can’t be fixed, take it offline.CISA’s advice when there’s no fix is to stop using the product, or take it off the internet, until there is one.

You can’t fix someone else’s server, but you can watch for what follows a break-in like this: a breach notice, a password reset you didn’t ask for, or a scam message that knows things it shouldn’t. HARD watches for those for you and your family.

Timeline

DateEvent
The flaw is made public in the National Vulnerability Database, with a severity score of 4.4.[2]
CISA adds it to its catalog of flaws known to be used in attacks.[1]
CISA’s deadline for US government agencies to fix it.[1]
FIRST puts the odds of it being used in attacks in the next 30 days at 2%.[4]

See also

Other Juniper Junos OS flaws

  • CVE-2023-36851: SRX series missing authentication for critical function ·
  • CVE-2023-36847: EX series missing authentication for critical function ·
  • CVE-2023-36846: SRX series missing authentication for critical function ·
  • CVE-2023-36845: EX series and SRX series PHP external variable modification ·
  • CVE-2023-36844: EX series PHP external variable modification ·
  • CVE-2020-1631: path traversal ·

Other Juniper flaws

  • CVE-2015-7755, Juniper ScreenOS: improper authentication ·

Added the same day

  • CVE-2025-24201, Apple devices: webKit out-of-bounds write ·

Every Juniper flaw on CISA’s list · Everything added in March 2025

References

  1. ^ Cybersecurity and Infrastructure Security Agency. “Juniper Junos OS Improper Isolation or Compartmentalization Vulnerability”. Known Exploited Vulnerabilities Catalog, added 13 Mar 2025.
  2. ^ National Institute of Standards and Technology. “CVE-2025-21590 Detail”. National Vulnerability Database, published 12 Mar 2025.
  3. ^ Juniper. Advisories and fixes: supportportal.juniper.net, supportportal.juniper.net.
  4. ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.017, percentile 77.9, 31 Oct 2026.
  5. CVE Program. “CVE-2025-21590”. The official CVE record.

This article is built from CISA’s catalog (version 2026.10.30), NVD’s record and FIRST’s scores, kept in HARD’s database and checked every morning. The plain English is HARD’s.