# CVE-2026-21911: Junos OS Evolved: Flapping management interface causes MAC learning on label-switched interfaces to stop

> An unauthenticated attacker can cause high CPU usage and stop MAC learning by flapping the management interface on Junos OS Evolved devices.

- **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:A/V:C/RE:M/U:Amber`)
- **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-6fvr-r66p-5w4v

## What is CVE-2026-21911?

CVE-2026-21911 is a high-severity denial of service vulnerability in Juniper Junos Os Evolved, 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 Incorrect Calculation vulnerability in the Layer 2 Control

Protocol 

 Daemon (l2cpd) of Juniper Networks Junos OS Evolved allows an unauthenticated network-adjacent attacker flapping the management interface to cause the learning of new MACs over label-switched interfaces (LSI) to stop while generating a flood of logs, resulting in high CPU usage.

When the issue is seen, the following log message will be generated:

op:1 flag:0x6 mac:xx:xx:xx:xx:xx:xx bd:2 ifl:13302 reason:0(REASON_NONE) i-op:6(INTRNL_OP_HW_FORCE_DELETE) status:10 lstatus:10 err:26(GETIFBD_VALIDATE_FAILED) err-reason 4(IFBD_VALIDATE_FAIL_EPOCH_MISMATCH) hw_wr:0x4 ctxsync:0 fwdsync:0  rtt-id:51 p_ifl:0  fwd_nh:0 svlbnh:0 event:- smask:0x100000000 dmask:0x0 mplsmask 0x1 act:0x5800 extf:0x0 pfe-id 0 hw-notif-ifl 13302 programmed-ifl 4294967295 pseudo-vtep underlay-ifl-idx 0 stack:GET_MAC, ALLOCATE_MAC, GET_IFL, GET_IFF, GET_IFBD, STOP, 

This issue affects Junos OS Evolved: 

  *  all versions before 21.4R3-S7-EVO, 
  *  from 22.2 before 22.2R3-S4-EVO, 
  *  from 22.3 before 22.3R3-S3-EVO, 
  *  from 22.4 before 22.4R3-S2-EVO, 
  *  from 23.2 before 23.2R2-S1-EVO, 
  *  from 23.4 before 23.4R1-S2-EVO, 23.4R2-EVO.

Weakness: CWE-682 (Incorrect Calculation)

## Which versions are affected by CVE-2026-21911?

It affects 1 product.


Products: Juniper Junos Os Evolved.

## How do I fix CVE-2026-21911?

No fix has been published for CVE-2026-21911 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-21911 being exploited?

There is no known exploitation of CVE-2026-21911 in the wild and no public exploit code.

## Other vulnerabilities in Juniper Junos Os Evolved

- [CVE-2026-21902](https://insights.sbomify.com/CVE-2026-21902/): Junos OS Evolved: PTX Series: A vulnerability allows a unauthenticated, network-based attacker to execute code as root (critical 9.3)
- [CVE-2023-28983](https://insights.sbomify.com/CVE-2023-28983/): Junos OS Evolved: Shell Injection vulnerability in the gNOI server (high 8.8)
- [CVE-2025-21599](https://insights.sbomify.com/CVE-2025-21599/): Denial of service in Junos OS Evolved (high 8.7)
- [CVE-2024-47502](https://insights.sbomify.com/CVE-2024-47502/): Junos OS Evolved: TCP session state is not always cleared on the Routing Engine leading to DoS (high 8.7)
- [CVE-2024-39531](https://insights.sbomify.com/CVE-2024-39531/): Junos OS Evolved: ACX 7000 Series: Protocol specific DDoS configuration affects other protocols (high 8.7)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-21911 and fix it if it is affected.

What it is: CVE-2026-21911 is a high-severity denial of service vulnerability in Juniper Junos Os Evolved, 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-2026-21911 in the wild and no public exploit code.

Affected product: Juniper Junos Os Evolved. 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-2026-21911/ (Markdown: https://insights.sbomify.com/CVE-2026-21911/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-21911
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-21911
- OSV: https://osv.dev/vulnerability/CVE-2026-21911

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-21911/
Summaries are generated and may contain mistakes; the official description above is the record.