# CVE-2026-21921: Use after free in Junos OS

> Authenticated attackers can cause a denial of service in Junos OS by triggering a use-after-free in chassisd via frequent telemetry subscriptions.

- **Severity:** High 7.1 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:L/AU:Y/R:A/RE:M`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 15 Jan 2026
- **Updated:** 15 Jan 2026
- **Also known as:** GHSA-fm5x-5pxq-6229

## What is CVE-2026-21921?

CVE-2026-21921 is a high-severity use after free vulnerability in Juniper Junos, published on 15 Jan 2026. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

A Use After Free vulnerability in the chassis daemon (chassisd) of Juniper Networks Junos OS and Junos OS Evolved allows a network-based attacker authenticated with low privileges to cause a Denial-of-Service (DoS).

When telemetry collectors are frequently subscribing and unsubscribing to sensors continuously over a long period of time, telemetry-capable processes like chassisd, rpd or mib2d will crash and restart, which - depending on the process - can cause a complete outage until the system has recovered.

This issue affects:

 Junos OS: 

  *  all versions before 22.4R3-S8,
  *  23.2 versions before 23.2R2-S5,
  *  23.4 versions before 23.4R2;

Junos OS Evolved:

  *  all versions before 22.4R3-S8-EVO,
  *  23.2 versions before 23.2R2-S5-EVO,
  *  23.4 versions before 23.4R2-EVO.

Weakness: CWE-416 (Use After Free)

## Which versions are affected by CVE-2026-21921?

It affects 2 products.


Products: Juniper Junos, Juniper Junos Os Evolved.

## How do I fix CVE-2026-21921?

No fix has been published for CVE-2026-21921 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-21921 being exploited?

There is no known exploitation of CVE-2026-21921 in the wild and no public exploit code.

## Other vulnerabilities in Juniper Junos

- [CVE-2023-36845](https://insights.sbomify.com/CVE-2023-36845/): Juniper Junos OS EX Series and SRX Series PHP External Variable Modification (critical 9.8, exploited in the wild)
- [CVE-2020-1631](https://insights.sbomify.com/CVE-2020-1631/): Juniper Junos OS Path Traversal (high 8.8, exploited in the wild)
- [CVE-2025-21590](https://insights.sbomify.com/CVE-2025-21590/): Juniper Junos OS Improper Isolation or Compartmentalization (medium 6.7, exploited in the wild)
- [CVE-2023-36851](https://insights.sbomify.com/CVE-2023-36851/): Juniper Junos OS SRX Series Missing Authentication for Critical Function (medium 5.3, exploited in the wild)
- [CVE-2023-36844](https://insights.sbomify.com/CVE-2023-36844/): Juniper Junos OS EX Series PHP External Variable Modification (medium 5.3, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-21921 and fix it if it is affected.

What it is: CVE-2026-21921 is a high-severity use after free vulnerability in Juniper Junos, published on 15 Jan 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-21921 in the wild and no public exploit code.

Affected product: Juniper Junos. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-21921/ (Markdown: https://insights.sbomify.com/CVE-2026-21921/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-21921
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-21921
- OSV: https://osv.dev/vulnerability/CVE-2026-21921

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-21921/
Summaries are generated and may contain mistakes; the official description above is the record.