# CVE-2026-21910: Junos OS: EX4k Series, QFX5k Series: In an EVPN-VXLAN configuration link flaps cause Inter-VNI traffic drop

> An unauthenticated attacker can cause a denial of service by flapping interfaces in EVPN-VXLAN configurations on specific Juniper switches.

- **Severity:** High 7.1 (`CVSS:4.0/AV:A/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:L/AU:Y/R:U/RE:M/U:Green`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **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-fhh6-4pm2-6vqv

## What is CVE-2026-21910?

CVE-2026-21910 is a high-severity denial of service 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

An Improper Check for Unusual or Exceptional Conditions vulnerability in the packet forwarding engine (PFE) of Juniper Networks Junos OS on EX4k Series and QFX5k Series platforms allows an unauthenticated network-adjacent attacker flapping an interface to cause traffic between VXLAN Network Identifiers (VNIs) to drop, leading to a Denial of Service (DoS).

On all EX4k and QFX5k platforms, a link flap in an

EVPN-VXLAN configuration Link Aggregation Group (LAG)
results in Inter-VNI traffic dropping when there are multiple load-balanced next-hop routes for the same destination.

This issue is only applicable to systems that support EVPN-VXLAN Virtual Port-Link Aggregation Groups (VPLAG), such as the QFX5110, QFX5120, QFX5200, EX4100, EX4300, EX4400, and EX4650.

Service can only be restored by restarting the affected FPC via the 'request chassis fpc restart slot <slot-number>' command.

This issue affects Junos OS 

on EX4k and QFX5k Series: 

  *  all versions before 21.4R3-S12, 
  *  all versions of 22.2
  *  from 22.4 before 22.4R3-S8, 
  *  from 23.2 before 23.2R2-S5, 
  *  from 23.4 before 23.4R2-S5, 
  *  from 24.2 before 24.2R2-S3,
  *  from 24.4 before 24.4R2.

Weakness: CWE-754 (Improper Check for Unusual or Exceptional Conditions)

## Which versions are affected by CVE-2026-21910?

It affects 1 product.


Products: Juniper Junos.

## How do I fix CVE-2026-21910?

No fix has been published for CVE-2026-21910 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-21910 being exploited?

There is no known exploitation of CVE-2026-21910 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-2026-21910 and fix it if it is affected.

What it is: CVE-2026-21910 is a high-severity denial of service 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-2026-21910 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-2026-21910/ (Markdown: https://insights.sbomify.com/CVE-2026-21910/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-21910
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-21910
- OSV: https://osv.dev/vulnerability/CVE-2026-21910

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-21910/
Summaries are generated and may contain mistakes; the official description above is the record.