# CVE-2021-27853: L2 network filtering can be bypassed using stacked VLAN0 and LLC/SNAP headers

> Attackers can bypass Layer 2 network filtering mechanisms like IPv6 RA guard or ARP inspection by using stacked VLAN0 and LLC/SNAP 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-v8jg-p3fp-g968

## What is CVE-2021-27853?

CVE-2021-27853 is a medium-severity vulnerability in Cisco Ios Xe, 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 or ARP inspection can be bypassed using combinations of VLAN 0 headers and LLC/SNAP headers.

Weakness: CWE-290 (Authentication Bypass by Spoofing)

## Which versions are affected by CVE-2021-27853?

It affects 96 products.


Products: Cisco Catalyst 6503-E Firmware, Cisco Catalyst 6504-E Firmware, Cisco Catalyst 6506-E Firmware, Cisco Catalyst 6509-E Firmware, Cisco Catalyst 6509-Neb-a Firmware, Cisco Catalyst 6509-v-E Firmware, Cisco Catalyst 6513-E Firmware, Cisco Catalyst 6800ia Firmware, Cisco Catalyst 6807-Xl Firmware, Cisco Catalyst 6840-X Firmware, Cisco Catalyst 6880-X Firmware, Cisco Catalyst C6816-X-Le Firmware, Cisco Catalyst C6824-X-Le-40g Firmware, Cisco Catalyst C6832-X-Le Firmware, Cisco Catalyst C6840-X-Le-40g Firmware, Cisco Ios Xe, Cisco Meraki Ms210 Firmware, Cisco Meraki Ms225 Firmware, Cisco Meraki Ms250 Firmware, Cisco Meraki Ms350 Firmware and 76 more.

## How do I fix CVE-2021-27853?

No fix has been published for CVE-2021-27853 yet. Limit exposure of the affected software until one is available.

## Is CVE-2021-27853 being exploited?

There is no known exploitation of CVE-2021-27853 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-27853 and fix it if it is affected.

What it is: CVE-2021-27853 is a medium-severity vulnerability in Cisco Ios Xe, 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-2021-27853 in the wild and no public exploit code.

Affected product: Cisco Ios Xe. 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-2021-27853/ (Markdown: https://insights.sbomify.com/CVE-2021-27853/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-27853
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-27853
- OSV: https://osv.dev/vulnerability/CVE-2021-27853

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-27853/
Summaries are generated and may contain mistakes; the official description above is the record.