# CVE-2026-21919: Race condition in Junos OS

> A race condition in Junos OS management daemon allows attackers to cause a denial of service by rapidly connecting and disconnecting NETCONF sessions.

- **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:N/AU:Y/R:U/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:** 9 Apr 2026
- **Updated:** 13 Apr 2026
- **Also known as:** GHSA-qv89-vxpv-h7vc

## What is CVE-2026-21919?

CVE-2026-21919 is a high-severity vulnerability in Juniper Junos, published on 9 Apr 2026. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

An Incorrect Synchronization vulnerability in the management daemon (mgd) of Juniper Networks Junos OS and Junos OS Evolved allows a network-based attacker with low privileges to cause a complete Denial-of-Service (DoS) of the management plane.

When NETCONF sessions are quickly established and disconnected, a locking issue causes mgd processes to hang in an unusable state. When the maximum number of mgd processes has been reached, no new logins are possible. This leads to the inability to manage the device and requires a power-cycle to recover.

This issue can be monitored by checking for mgd processes in lockf state in the output of 'show system processes extensive':

user@host> show system processes extensive | match mgd
<pid> root         20    0   501M  4640K lockf    1   0:01   0.00% mgd

If the system still can be accessed (either via the CLI or as root, which might still be possible as last resort as this won't invoke mgd), mgd processes in this state can be killed with 'request system process terminate <PID>' from the CLI or with 'kill -9 <PID>' from the shell. 

This issue affects:

Junos OS:

  *  23.4 versions before 23.4R2-S4,
  *  24.2 versions before 24.2R2-S1,
  *  24.4 versions before 24.4R1-S3, 24.4R2;

This issue does not affect Junos OS versions before 23.4R1;

Junos OS Evolved:

  *  23.4 versions before 23.4R2-S5-EVO,
  *  24.2 versions before 24.2R2-S1-EVO,
  *  24.4 versions before 24.4R1-S3-EVO, 24.4R2-EVO.

This issue does not affect Junos OS Evolved versions before 23.4R1-EVO;

Weakness: CWE-821 (Incorrect Synchronization)

## Which versions are affected by CVE-2026-21919?

It affects 2 products.


Products: Juniper Junos, Juniper Junos Os Evolved.

## How do I fix CVE-2026-21919?

No fix has been published for CVE-2026-21919 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-21919 being exploited?

There is no known exploitation of CVE-2026-21919 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-2026-21919 and fix it if it is affected.

What it is: CVE-2026-21919 is a high-severity vulnerability in Juniper Junos, published on 9 Apr 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-2026-21919 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-2026-21919/ (Markdown: https://insights.sbomify.com/CVE-2026-21919/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-21919
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-21919
- OSV: https://osv.dev/vulnerability/CVE-2026-21919

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-21919/
Summaries are generated and may contain mistakes; the official description above is the record.