# CVE-2026-21908: Use after free in Junos OS

> An authenticated attacker can crash the 802.1X daemon or execute code by flapping a port, requiring 802.1X to be enabled.

- **Severity:** High 7.5 (`CVSS:4.0/AV:A/AC:L/AT:P/PR:L/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:L/RE:M/U:Green`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 15 Jan 2026
- **Updated:** 26 Feb 2026
- **Also known as:** GHSA-h976-gqwv-338w

## What is CVE-2026-21908?

CVE-2026-21908 is a high-severity use after free vulnerability in Juniper Junos, published on 15 Jan 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

A Use After Free vulnerability was identified in the 802.1X authentication daemon (dot1xd) of Juniper Networks Junos OS and Junos OS Evolved that could allow an authenticated, network-adjacent attacker flapping a port to crash the dot1xd process, leading to a Denial of Service (DoS), or potentially execute arbitrary code within the context of the process running as root.

The issue is specific to the processing of a change in authorization (CoA) when a port bounce occurs. A pointer is freed but was then referenced later in the same code path. Successful exploitation is outside the attacker's direct control due to the specific timing of the two events required to execute the vulnerable code path.

This issue affects systems with 802.1X authentication port-based network access control (PNAC) enabled.
This issue affects:

Junos OS: 

  *  from 23.2R2-S1 before 23.2R2-S5, 
  *  from 23.4R2 before 23.4R2-S6, 
  *  from 24.2 before 24.2R2-S3, 
  *  from 24.4 before 24.4R2-S1, 
  *  from 25.2 before 25.2R1-S2, 25.2R2; 

Junos OS Evolved: 

  *  from 23.2R2-S1 before 23.2R2-S5-EVO, 
  *  from 23.4R2 before 23.4R2-S6-EVO, 
  *  from 24.2 before 24.2R2-S3-EVO, 
  *  from 24.4 before 24.4R2-S1-EVO, 
  *  from 25.2 before 25.2R1-S2-EVO, 25.2R2-EVO.

Weakness: CWE-416 (Use After Free)

## Which versions are affected by CVE-2026-21908?

It affects 2 products.


Products: Juniper Junos, Juniper Junos Os Evolved.

## How do I fix CVE-2026-21908?

No fix has been published for CVE-2026-21908 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-21908 being exploited?

There is no known exploitation of CVE-2026-21908 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-2026-21908 and fix it if it is affected.

What it is: CVE-2026-21908 is a high-severity use after free vulnerability in Juniper Junos, published on 15 Jan 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-21908 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-2026-21908/ (Markdown: https://insights.sbomify.com/CVE-2026-21908/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-21908
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-21908
- OSV: https://osv.dev/vulnerability/CVE-2026-21908

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-21908/
Summaries are generated and may contain mistakes; the official description above is the record.