# What is CVE-2025-21590?

> CVE-2025-21590 is a medium-severity flaw (CVSS 4.4) in Junos from Juniper, published on 12 March 2025. It is on the list of flaws known to be exploited, so criminals are already using it. The latest forecast puts its chance of use in the next 30 days at 1.7%. If you run an affected version, patch it first.

Canonical page: https://www.precursorintelligence.com/what-is/cve-2025-21590

## At a glance

- **Verdict:** Fix this now. Attackers are using this flaw today. If you run an affected version, it goes to the top of your list.
- **Precursor Intelligence risk score:** 74 out of 100 (High); action: Emergency patch. Kept up to date as the evidence changes.
- **Severity:** Medium (CVSS 4.4 out of 10)
- **Known to be exploited:** Yes, it is on the government list of flaws known to be exploited
- **Chance of use in the next 30 days:** 1.7% (latest exploit-likelihood forecast)
- **Published:** 12 March 2025
- **Record last updated:** 24 October 2025
- **Main product:** Junos from Juniper
- **Affected products:** 1

## What is CVE-2025-21590? In plain terms

An attacker has to be already on the device with an administrator account, and nobody has to click anything. If it works, the attacker can change data.

### The official description

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.

### What kind of flaw is it?

- CWE-653: Improper Isolation or Compartmentalization

### How an attack on it works

- Where an attacker can be: Already on the device
- How hard it is to pull off: Easy: no special conditions
- Login needed first: An administrator account
- Does someone have to click: No, nobody has to do anything
- What an attacker gets: Can change data

Severity vector: `CVSS:3.1/AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:H/A:N`

## Is CVE-2025-21590 being exploited? Yes

Attackers are already using it. It is on the government list of flaws known to be exploited, and the latest forecast puts the chance of it being used in the next 30 days at 1.7%.

Exploit likelihood over the last 7 days: 1.7%, 1.7%, 1.7%, 1.7%, 1.7%, 1.7%, 1.7%.

### Timeline

- 12 March 2025: Published
- 24 October 2025: Record last updated

### How Precursor Intelligence ranks it

CVE-2025-21590 scores **74 out of 100 (High)**, with the action "Emergency patch".

Every published flaw gets a risk score from 0 to 100, kept up to date as the evidence changes. Four things go into the score, from the one that counts most to the ones that count less:

- Evidence it is being exploited: biggest factor
- How severe it is: strong factor
- How widely exposed it is: smaller factor
- Threat activity around it: smaller factor

85 and above is Critical. Anything confirmed as exploited is always marked for an emergency patch, whatever its score. When this flaw matches software you track, it lands in your list with its score and the reasons behind it.

## Who is affected by CVE-2025-21590?

The published record lists 1 product.

- [Junos](https://www.precursorintelligence.com/product/juniper/junos) by [Juniper](https://www.precursorintelligence.com/vendors/juniper) (Operating system): up to 21.2, 21.2-r1, 21.2-r1-s1, 21.2-r1-s2, 21.2-r2, 21.2-r2-s1, 21.2-r2-s2, 21.2-r3, 21.2-r3-s1, 21.2-r3-s2, 21.2-r3-s3, 21.2-r3-s4, 21.2-r3-s5, 21.2-r3-s6, 21.2-r3-s7, 21.2-r3-s8, 21.4, 21.4-r1, 21.4-r1-s1, 21.4-r1-s2, 21.4-r2, 21.4-r2-s1, 21.4-r2-s2, 21.4-r3, 21.4-r3-s1, 21.4-r3-s2, 21.4-r3-s3, 21.4-r3-s4, 21.4-r3-s5, 21.4-r3-s6, 21.4-r3-s7, 21.4-r3-s8, 21.4-r3-s9, 22.2, 22.2-r1, 22.2-r1-s1, 22.2-r1-s2, 22.2-r2, 22.2-r2-s1, 22.2-r2-s2 and 38 more

## How do I fix CVE-2025-21590?

1. **Check whether you run it.** Compare your versions with the affected products above. Not sure what you run? Precursor Intelligence can work it out from your public website, or you can pick it from a list.
2. **Apply the vendor’s fix.** Install the update named in the vendor’s advisory. We’ve put the advisory first in the list.
3. **If you can’t patch today.** Follow any mitigation the vendor publishes, and limit who can reach the affected system from the internet until the update is in.
4. **Check it wasn’t used before you patched.** Because criminals are already using this flaw, look for signs of a break-in on affected systems using the vendor’s guidance.

### References

- [supportportal.juniper.net](https://supportportal.juniper.net/JSA93446) (Vendor advisory)
- [cloud.google.com](https://cloud.google.com/blog/topics/threat-intelligence/china-nexus-espionage-targets-juniper-routers) (Third-party advisory)
- [cisa.gov](https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2025-21590) (Government resource)

## Questions

### What is CVE-2025-21590?

CVE-2025-21590 is a medium-severity flaw in Junos from Juniper, published on 12 March 2025. An attacker has to be already on the device with an administrator account, and nobody has to click anything. If it works, the attacker can change data.

### How serious is CVE-2025-21590?

It scores 4.4 out of 10 on the CVSS severity scale, which is medium. Our risk score for CVE-2025-21590 is 74 out of 100, which is High. Its action is "Emergency patch". Attackers are using this flaw today. If you run an affected version, it goes to the top of your list.

### Is CVE-2025-21590 being exploited?

Attackers are already using it. It is on the government list of flaws known to be exploited, and the latest forecast puts the chance of it being used in the next 30 days at 1.7%.

### Which versions does CVE-2025-21590 affect?

For Junos from Juniper: up to 21.2, 21.2-r1, 21.2-r1-s1, 21.2-r1-s2, 21.2-r2, 21.2-r2-s1 and 72 more.

### How do I fix CVE-2025-21590?

Install the update the vendor names in its advisory, linked first under "How do I fix" on this page. If you can't patch today, follow the vendor's mitigation and limit who can reach the affected system from the internet. Because criminals are already using it, also check affected systems for signs of a break-in.

## Does CVE-2025-21590 affect you?

Precursor Intelligence keeps checking for you:

- **Tell us what you run.** Pick your software from a list, or let us work it out from your website.
- **We match it to every flaw.** Over 400,000 published flaws, checked against your list.
- **Ranked by real-world risk.** Every flaw gets a 0 to 100 risk score, kept up to date, led by evidence criminals are using it.
- **Told as soon as we know.** An alert by email, in the app or by webhook to your own tools.

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