# CVE-2026-21912: Junos OS: MX10k Series: 'show system firmware' CLI command may lead to LC480 or LC2101 line card reset

> A local low-privileged attacker can cause line card and chassis daemon crashes by repeatedly executing a specific CLI command due to a race condition.

- **Severity:** Medium 6.8 (`CVSS:4.0/AV:L/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/U:Amber`)
- **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:** 15 Jan 2026
- **Also known as:** GHSA-w5q5-7f2p-x4hm

## What is CVE-2026-21912?

CVE-2026-21912 is a medium-severity 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 Time-of-check Time-of-use (TOCTOU) Race Condition vulnerability in the method to collect FPC Ethernet firmware statistics of Juniper Networks Junos OS on MX10k Series allows a local, low-privileged attacker executing the 'show system firmware' CLI command to cause an LC480 or LC2101 line card to reset.

On MX10k Series systems with LC480 or LC2101 line cards, repeated execution of the 'show system firmware' CLI command can cause the line card to crash and restart. Additionally, some time after the line card crashes, chassisd may also crash and restart, generating a core dump.This issue affects Junos OS on MX10k Series: 

  *  all versions before 21.2R3-S10, 
  *  from 21.4 before 21.4R3-S9, 
  *  from 22.2 before 22.2R3-S7, 
  *  from 22.4 before 22.4R3-S6, 
  *  from 23.2 before 23.2R2-S2, 
  *  from 23.4 before 23.4R2-S3, 
  *  from 24.2 before 24.2R2.

Weakness: CWE-367 (Time-of-check Time-of-use (TOCTOU) Race Condition)

## Which versions are affected by CVE-2026-21912?

It affects 1 product.


Products: Juniper Junos.

## How do I fix CVE-2026-21912?

No fix has been published for CVE-2026-21912 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-21912 being exploited?

There is no known exploitation of CVE-2026-21912 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-2026-21912 and fix it if it is affected.

What it is: CVE-2026-21912 is a medium-severity 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-2026-21912 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-2026-21912/ (Markdown: https://insights.sbomify.com/CVE-2026-21912/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-21912
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-21912
- OSV: https://osv.dev/vulnerability/CVE-2026-21912

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-21912/
Summaries are generated and may contain mistakes; the official description above is the record.