# CVE-2021-27861: L2 network filtering bypass using stacked VLAN0 and LLC/SNAP headers with invalid lengths

> Attackers can bypass Layer 2 network filtering and IPv6 RA guard by crafting packets with invalid LLC/SNAP header lengths and optional VLAN0 headers.

- **Severity:** Medium 4.7 (`CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:C/C:N/I:L/A:N`)
- **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:** 27 Sep 2022
- **Updated:** 4 Nov 2025
- **Also known as:** GHSA-wcc2-hc4j-m2wc

## What is CVE-2021-27861?

CVE-2021-27861 is a medium-severity vulnerability in IEEE 802.2, published on 27 Sep 2022. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

Layer 2 network filtering capabilities such as IPv6 RA guard can be bypassed using LLC/SNAP headers with invalid length (and optionally VLAN0 headers)

Weakness: CWE-290 (Authentication Bypass by Spoofing), CWE-130 (Improper Handling of Length Parameter Inconsistency)

## Which versions are affected by CVE-2021-27861?

It affects 2 products.


Products: Ieee Ieee 802.2, Ietf P802.1q.

## How do I fix CVE-2021-27861?

No fix has been published for CVE-2021-27861 yet. Limit exposure of the affected software until one is available.

## Is CVE-2021-27861 being exploited?

There is no known exploitation of CVE-2021-27861 in the wild and no public exploit code.

## Other vulnerabilities in IEEE 802.2

- [CVE-2021-27854](https://insights.sbomify.com/CVE-2021-27854/): L2 network filtering bypass using stacked VLAN0, LLC/SNAP headers, and Ethernet to Wifi frame translation (medium 4.7)
- [CVE-2021-27862](https://insights.sbomify.com/CVE-2021-27862/): Security feature bypass in 802.2 (medium 4.7)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-27861 and fix it if it is affected.

What it is: CVE-2021-27861 is a medium-severity vulnerability in IEEE 802.2, published on 27 Sep 2022. 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-2021-27861 in the wild and no public exploit code.

Affected product: IEEE 802.2. 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-2021-27861/ (Markdown: https://insights.sbomify.com/CVE-2021-27861/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-27861
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-27861
- OSV: https://osv.dev/vulnerability/CVE-2021-27861

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-27861/
Summaries are generated and may contain mistakes; the official description above is the record.